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Farm-level costs of settling basins for treatment of effluents from levee-style catfish ponds
Aquacultural Engineering 28 (2003) 171 /199 www.elsevier.com/locate/aqua-online
Farm-level costs of settling basins for treatment of effluents from levee-style catfish ponds Carole R. Engle *, Diego Valderrama 1 Aquaculture/Fisheries Center, University of Arkansas at Pine Bluff, Mail Stop 4912, 1200 North University Drive, Pine Bluff, AR 71601, USA Received 1 December 2002; accepted 22 March 2003
Abstract Compelled by pending regulatory rule changes, settling basins have been proposed as a treatment alternative for catfish pond effluents, but the associated costs to catfish farmers have not been estimated. Economic engineering techniques were used to design 160 scenarios as a basis for estimating total investment and total annual costs. For static-water, levee-style catfish pond facilities, sizing of settling basins is controlled by factors such as type of effluent to be treated, pond layout, size of the largest foodfish pond, number of drainage directions, scope of regulations governing effluents, and the availability of land. Regulations that require settling basins on catfish farms would increase total investment cost on catfish farms by $126 / 2990 ha 1 and total annual per-ha costs by $19 /367 ha 1. More numerous drainage directions on farms resulted in the greatest increase in costs. While both investment and operating costs increased with larger sizes of foodfish ponds, costs per ha were relatively greater on smaller than on larger farms. For farms on which existing fish ponds would have to be converted to settling basins, over half of the cost was due to the production foregone and annual fixed costs of the pond. Requiring catfish farmers to construct settling basins would impose a disproportionately greater financial burden on smaller farms. The magnitude of the increased costs associated with settling basins was too high relative to market prices of catfish for this technology to be economically feasible. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Catfish effluents; Catfish economics; Effluent treatment
* Corresponding author. Tel.: /1-870-543-8537; fax: /1-870-543-8129. E-mail address: [email protected] (C.R. Engle). 1 Present address. University of Rhode Island, Rhode Island, USA. 0144-8609/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0144-8609(03)00027-X
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1. Introduction The aquaculture industry has been one of the fastest-growing agribusinesses in the southeastern United States over the past two decades. US aquaculture has grown from a farm-gate value of $192 million in 1980 to nearly $1 billion in 1999 (USDA, 2000). Catfish production has demonstrated the most rapid increase in US aquaculture, from less than 32 000 MT in 1980 to over 271 000 MT of foodfish in 2001 (USDA, 2002). A lawsuit filed by the Natural Resources Defense Council against the Environmental Protection Agency in 1989 resulted in the inclusion of aquaculture in the effluent limitation guidelines (ELG) rulemaking process in December, 1999. The ELG process will result in a rule for the industrial classification of ‘‘aquaculture’’ and may or may not distinguish between effluents from flow-through and static systems. Moreover, any treatment technology guidelines selected for the rule may be applied to all aquaculture. A proposed rule was published by EPA in the Federal Register after 12 September 2002, but will not be finalized until 30 June 2004. The intervening 2 years will be a period of refining and adjusting the rule based on continued comments and input from both environmental and industry groups. From an economics perspective, there is a trade-off between the cost to society of poor environmental quality resulting from pollution and the costs of controlling or reducing pollution to maintain a comparatively high level of environmental quality (Engle and Valderrama, 2002). This paper will present detailed estimates of costs associated with implementing one treatment option (settling basins) that has been discussed by EPA for catfish farms. Settling basins have been used to treat effluents from flow-through systems. Much is known about the design, implementation, and costs associated with settling basins for trout raceways (Henderson and Bromage, 1988; Hinshaw et al., 1990; State of Idaho, 1997; Fornshell, 2001; Dunning and Sloan, 1997). Boyd et al. (1998) recommended an 8-h retention time for Alabama soils, but indicated that a settling time of 24 h or less was adequate for effective reduction of multiple parameter concentration levels. Boyd and Queiroz (2001) used the 8-h retention time, spread over a 48-h draining period to calculate settling basin volumes and areas for flow settling on catfish farms. The Boyd and Queiroz (2001) study did not evaluate the effect of various farm-specific factors associated with integrating settling basin technologies on farms with different drainage patterns, pond layouts, nor for varying regulatory options. Moreover, there do not appear to be estimates of farm-wide costs associated with the use of settling basins for static pond catfish production in the scientific literature. Several previous studies have evaluated costs associated with a variety of other technologies to treat effluents from catfish ponds. Hebicha (1989) evaluated dilution, dilution plus wetlands, intermittent sand filters, and caged fish culture as treatment options. None were found to be feasible and intermittent sand filters were identified to have particularly high costs. Schmittou (1991), in an evaluation of the consequences of catfish effluent control, showed that small farms would be more adversely affected than larger farms. Cerezo (1993) confirmed that increased costs
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associated with additional pollution control would force more small-scale producers to exit the industry. Kouka and Engle (1996) evaluated costs of (1) land application of catfish pond effluents for irrigation; (2) recycling water through constructed wetlands; and (3) recycling water from a catfish pond through a pond stocked with filter-feeding fish with both enterprise budget and whole-farm mathematical programming models. Catfish production costs were found to increase from $0 to 0.11 kg 1. While the use of water for crop irrigation resulted in no additional cost in the analysis, this alternative may not be feasible due to timing and application conflicts on most farms (Engle and Valderrama, 2002). Moreover, the Kouka and Engle (1996) study corroborated earlier findings by Cerezo (1993) that effluent treatment technologies imposed on catfish farms may have a disproportionately negative impact on smallerscale businesses. Posadas (2001) showed that, in spite of the improvements in water quality associated with constructed wetlands to treat catfish pond effluents, marketable yield of catfish did not increase. Constructed wetlands systems increased both investment and operating costs. Catfish production costs increased by $0.089, 0.127, and 0.165 kg 1 for constructed wetlands that represented 15, 25, and 35% of the pond area, respectively. The primary objective of the present study was to conduct an economic analysis of the on-farm micro-level costs associated with the use of settling basins to treat effluents from levee-style catfish ponds. Specific objectives included the estimation of total investment and annual variable and fixed costs of the construction and operation of settling basins on (a) new land and (b) conversion of existing ponds to serve as settling basins. The effect of farm size, pond layout, drainage patterns, and type of effluent (pond draining vs. storm overflow) on farm costs was evaluated.
1.1. Catfish pond effluents In the early years of the catfish industry, ponds were ‘‘flushed’’ with water pumped from wells. However, McGee and Boyd (1983) demonstrated that ‘‘flushing’’ at the 5% rates possible in commercial levee culture ponds is generally not beneficial. Flushing is no longer a routine practice in commercial catfish production largely due to the costs involved. Current management practices for catfish foodfish production involve draining ponds only once every 5/10 years without draining between multiple fish crops. The major parameters that have potential to be of concern in effluents from catfish ponds are nitrogen, phosphorus, organic matter, and settleable solids. These components are derived from feeds fed to catfish in the ponds. Approximately 30% of the nitrogen and phosphorus added to ponds in feed are recovered in the catfish that are harvested (Tucker et al., 2000). The remaining nutrients are released into the pond ecosystem as fish waste products that, in turn, stimulate the growth and production of phytoplankton. Nitrogen and phosphorus compounds and organic matter are present in pond water during growout and represent potential pollutants if discharged.
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Tucker and van der Ploeg (1993) demonstrated that the quality of potential effluents was poorest (highest concentrations of solids, organic matter, total phosphorus, and total nitrogen) in the summer. Most of the organic matter in channel catfish pond water consists of living phytoplankton cells and phytoplankton-derived detritus. Most of the nitrogen and phosphorus in catfish pond waters are present in particulate organic matter, primarily within phytoplankton cells (Tucker et al., 2000). Levee ponds are drained only to re-work pond bottoms and move eroded levee material back on to interior levee slopes. While nutrient concentrations and organic matter increase over the summer months, there is little or no water discharged from ponds during that time period (Tucker and Lloyd, 1985). Tucker and Lloyd (1985) demonstrated, from samples of 25 commercial catfish ponds and 24 receiving streams in west-central Mississippi, that, on average, concentrations of nitrate, nitrite, and soluble reactive phosphorus were higher in streams than in pond waters. Watershed ponds may have more frequent discharge depending on rainfall and if there is a stream flowing through the watershed pond. Boyd et al. (2000) collected water samples upstream and downstream of catfish farms on eight streams in WestCentral Alabama. While comparative values varied, there were approximately equal numbers of cases in which water quality below catfish farms were improved and degraded, suggesting that catfish farm effluents did not have a consistently adverse impact on stream water quality. In spite of these findings, EPA has considered settling basins as a possible treatment technology for catfish ponds.
2. Materials and methods 2.1. Factors affecting the design of settling basins to treat effluents from catfish farms A variety of factors affect the design of settling basins if these would be mandated for catfish ponds. Hydraulic residence time (HRT) will affect both the number and area of settling basins. Whether treatment will be required for draining discharges or storm event overflow will affect both the composition and timing of effluent discharge, and these will affect the number and area of settling basins. Moreover, the pattern of gravity-flow drainages on farms will affect the number of basins required. 2.1.1. Hydraulic residence time The cost of construction of settling basins is primarily related to the required sedimentation area. Hydraulic residence time is critical to determining whether the same settling basin can be used for treatment of multiple ponds within a farm or whether multiple numbers of settling basins will be required. With a short HRT, the same settling basin may be used for the treatment of multiple ponds, while a long HRT will require a higher number of settling basins if a large volume of effluents needs treatment within a short period of time. Stoke’s law (Boyd, 1995) allows the calculation of settling velocities for mineral soil particles and dead organic material. The equation follows below:
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vs
g(rr rw )dp2 18m
175
(1)
where vs is the velocity of particle (m s 1); g, the gravitation acceleration (m s 2); rr is the density of particles (kg m 3); rw , the density of water (kg m 3); dp , the diameter of particle (m) and m is the dynamic viscosity of water (kg m 1 s 1). According to Stokes’ law, settling velocity is primarily a function of particle size and density. HRT of effluent discharges will depend upon the settling velocity of particles in the effluent. The most prevalent type of particle in effluents from a catfish pond would be phytoplankton cells (50 mm in diameter) (Boney, 1975) and mineral soil particles (1 /5 mm in diameter, depending on soil type) (Boyd, 1995; USDA/AAES, 1967). Phytoplankton cells represent the majority of particles suspended in overflow effluents and the initial stages of pond draining effluents. However, live phytoplankton possess a range of floating mechanisms that considerably slows down their sinking rates (Boney, 1975). While phytoplankton is the principal constituent of pond effluents, the last 20% of pond draining discharges in watershed ponds may also contain an appreciable load of mineral soil particles due to the re-suspension of solid particles by the fish and seining crew (Schwartz and Boyd, 1994). As such, the concentration of total suspended solids (TSS) in the last portion of effluents may be several times higher than that of regular pond water. Boyd et al. (2000) reported TSS concentrations of over 1000 mg l 1 in the final drawdown of watershed ponds in Alabama, while the concentration of TSS in the initial drawdown was only 69 mg l 1. However, due to differences in harvesting techniques, final discharges from levee-style ponds in delta regions do not exhibit this same increase in TSS concentrations (Tucker et al., 2000). However, if a single regulation were mandated for both watershed and levee ponds, the rule potentially could specify treatment of the last 20% of effluents discharged. The particle-size distribution of TSS is heavily dependent on soil composition. The average size of clay particles is 1 mm, while silt and sand particles are larger. Clayey soils have a larger percentage of smaller particles; catfish ponds are typically built in soils with 30/60% clay (USDA/AAES, 1967). In areas with lower concentrations of clay soil particles, 75% reduction in TSS could be achieved with an HRT of 1 day or less (Boyd and Queiroz, 2001). However, catfish ponds in areas with high percentages of fine clays in the soil, such as in the delta region of the US, would have higher HRTs. A soil with 100% clay particles would require a maximum HRT of about 20 days, based on Eq. (1). An HRT of 1 day (minimum calculated HRT was 19 h) would remove soil particles larger than 5 mm (silt and sand particles), while an HRT of 20 days (minimum calculated HRT was 476 h) would be required to remove clay soil particles (average size of 1 mm). Eq. (1) was used to calculate sinking velocities and then HRT for settling basins. It is important to note that most of the phytoplankton in the discharge is living. For settling to occur, some type of herbicide application would be required. For this analysis, the average size of phytoplankton cells of 50 mm (Boney, 1975) was used to calculate HRT. For this average size of phytoplankton, the HRT to effectively settle
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Table 1 Particle density (rr ) and particle diameter (dp ) values used in the calculation of terminal settling velocities (vs ) and HRTs for suspended phytoplankton organisms and mineral soil particles Type of particle
rr (kg m 3)
dp (m)
vs (m s 1)
HRT (h)
Dead phytoplankton cell Mineral soil particle Mineral soil particle
1050 2500 2500
5 /10 5 5 /10 6 1 /10 6
6.91 /10 5 2.00 /10 5 8.01 /10 7
6 19 476
Average depth of the settling basin is 1.37 m (4.5 ft) (based on typical catfish pond depths (Tucker and Robinson, 1991)).
out dead phytoplankton cells is only 6 h because of the comparatively large size of the particles (Table 1). These estimated HRTs approximate other reports in the literature (Boyd et al., 1998; Teichert-Coddington et al., 1999). Boyd et al. (1998) recommended an 8-h retention time based on a laboratory study with Alabama soil samples. However, these same authors refer to a settling time of 24 h or less for effective reduction of multiple parameters. From a practical farm management perspective, farmers would likely schedule water movement from a production pond to a settling basin on a daily, not hourly, basis due to the high number of other duties and tasks performed daily by farm managers. Thus, for this analysis, retention times of 1 and 20 days were analyzed. 2.1.2. Storm event overflow and drainage discharges Important differences exist with respect to effluent composition and timing of occurrence between storm event overflow and pond draining discharges. Overflow effluents are composed primarily of phytoplankton cells and occur more frequently during the winter/spring months when the dilution capacity of receiving streams is higher. This is also the time of year when biological activity is at a minimum. Levee ponds have relatively little overflow due to the minimal watershed (excluding the pond surface). In contrast, pond draining effluents may be released any time of year, but more commonly at the end of the summer. Pond draining effluents typically contain a higher load of mineral soil particles. However, most modern catfish harvesting is conducted without water drawdown. Complete draining of grow-out ponds takes place only after 5 /10 years of continued operation and is done primarily for inventory purposes and renovation of pond bottoms. Overflow effluents are primarily composed of phytoplankton cells and are released during the coldest months of the year when the dilution capacity of streams is higher. This is also the time of year when biological activity is at a minimum. 2.1.3. Patterns of gravity-flow drainage on farms Observations on commercial catfish farms in the delta confirm that the patterns of gravity-flow drainage vary widely from farm to farm. While water discharged from some farms may flow in only one direction, others may flow in as many as four or
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five different directions. The number of settling basins required to treat pond effluents will increase with the number of drainage directions on the farm.
2.2. Design of settling basin treatment systems for model farm scenarios
2.2.1. Model farm scenarios Three farm sizes, corresponding to the 64, 128, and 256 land-ha scenarios budgeted by Keenum and Waldrop (1988) and updated by Engle and Killian (1997) and two pond sizes, 4 and 6 ha, were considered in the analysis (Table 2). Land areas represented 56, 114, and 228 water ha, respectively. Fingerling ponds were assumed to be 2 ha each and to occupy 10% of the farm area. Scenarios were developed for all farm and pond sizes for: (1) HRT’s of both 1 and 20 days; (2) treatment of either all effluents or only the last 20% of effluents; and (3) both draining and storm overflow effluents. Additional scenarios were developed to evaluate (1) new construction of settling basins in cases where land was available and (2) conversion of existing ponds into settling basins where additional land was not available. In all, 160 scenarios were developed for cost analysis.
2.2.2. Model farm drainage patterns Two principal farm drainage patterns were evaluated: (1) all ponds drain toward one side of the farm where one main drainage ditch collects all discharges for conveyance to a receiving body of water by gravity flow; and (2) a farm with two main drainage canals located on opposite sides of the farm in which each one receives the discharge of approximately 50% of the ponds (Fig. 1). Sensitivity analyses were conducted to estimate costs for up to four drainage directions. Table 2 Farm-size scenarios used for the cost analysis of settling basins for the treatment of effluents from catfish ponds Farm size/pond type
4-ha pond
6-ha pond
Ponds (number)
Total area (ha)
Ponds (number)
Total area (ha)
64-ha farm Fingerling ponds Foodfish ponds
3 12
6 48
3 8
6 48
128-ha farm Fingerling ponds Foodfish ponds
5 26
10 104
5 17
10 102
256-ha farm Fingerling ponds Foodfish ponds
12 50
24 200
12 50
24 200
Fingerling ponds are 2 ha each.
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Fig. 1. Pond layout of catfish farms designed to use settling basins to treat effluent discharges. The number of fingerling and foodfish ponds for each farm layout corresponds to the scenarios described in Table 2. Drainage pipes and canals and the location of constructed or retrofitted settling basins are shown for each diagram. (A) A 64-ha farm with 12 4-ha foodfish ponds, single drainage system, and a constructed settling basin; (B) a 256-ha farm with 34 6-ha ponds, double drainage system, and two excavated sedimentation basins; (C) a 128-ha farm with 17 6-ha ponds and double drainage system. Two foodfish ponds have been retrofitted. Arrows denote direction of water flow.
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2.2.3. Construction of settling basins Settling basins were assumed to be constructed by excavation at the end of the main drainage canal(s) on the lowest part of the farm (Fig. 1) while levee ponds are typically constructed by both cutting and filling. However, excavation would be necessary to allow water to flow by gravity into the settling basin area. This design does not require pumping except to empty the basin. Land was assumed to be available for these scenarios. Settling basin depth was assumed to be the same as that of a catfish production pond, and ponds were assumed to be drained on an average of once every 6 years. Boyd and Queiroz (2001) propose a formula to calculate settling basin volume that includes dividing HRT by pond draining time. This method requires prior knowledge of pond volumes and draining times. Many farmers do not have accurate measures of these parameters nor can the typical farm manager monitor filling the settling basin to ensure that all water is exposed to the appropriate HRT. The analysis presented in this paper adopts a more conservative approach of using average pond volumes and an HRT that is initiated when all water from the pond being drained has entered the settling basin. An arbitrary assumption was made to model multiple settling basins, but limit the size of any individual settling basin to 8 ha to avoid large (24 ha) unmanageable settling basins. Large basins would present additional problems of erosion due to turbulence from the wind and other factors. Total settling basin area ranged from 0.8 to 24 ha (Table 3). Regardless of farm size, increases in the number of drainage directions on a farm increases the number of settling basins required (Table 3). Farms with two drainage directions need twice the number of basins as compared with farms with a single drainage direction. Farms with ponds that drain in three, four, or five different directions require three, four, or five settling basins, respectively. Each block of ponds in a different location will require a separate settling basin regardless of the number or size of ponds in each block. The size of each individual settling basin remained constant across different numbers of drainage directions with a few exceptions. This is because the size of an individual basin must match the size of the ponds to be drained at the same time. Thus, the size of the largest individual pond or group of ponds to be drained at the same time dictates the size of the settling area. Longer HRT periods required larger and, in some cases, more settling basins (Table 3). In those farm scenarios with an HRT of 1 day, sedimentation basins were sized according to the total discharge of a single foodfish pond. It was assumed in this analysis that an individual foodfish pond would be drained once every 6 year and that only one foodfish pond would be drained at a time. Thus, 8 and 24-h settling times would not require additional settling area. However, fingerling ponds were assumed to be drained every year. It was further assumed that no fingerling ponds would be drained at the same time that a foodfish pond was being drained and that the farmer could delay drainage of the fingerling pond for 1 day. For example, if the average pond size in a 64-ha farm is 4 ha and all effluents are to be treated, then the settling basin must have an area of 4 ha.
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Pond size (ha)
Draining layout
HRT (days)
Portion to be treated (%)
Settling basinsa 64-ha farm
4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
Single Single Single Single Double Double Double Double Single Single Single Single Double Double Double Double a b c d e
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
Last All Last All Last All Last All Last All Last All Last All Last All
20% 20% 20% 20% 20% 20% 20% 20%
128-ha farm
256-ha farm
Areaa (ha)
Number
Area (ha)
Number
Area (ha)
Number
0.8 4 1.2 6 0.8 4 0.8 4 1.2 6 1.2 6 1.2 6 1.2 6
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
0.8 4 2 5 0.8 4 1.2 6b 1.2 6 2 5c 1.2 6 1.2 6d
1 1 1 2 2 2 2 2 1 1 1 2 2 2 2 2
0.8 4 4.8 8 0.8 4 2.4 6 1.2 6 4.8 8e 1.2 6 2.4 6
1 1 1 3 2 2 2 4 1 1 1 3 2 2 2 4
Area of an individual settling basin. There are five fingerling ponds. Three will drain in the same direction, requiring a 6-ha basin. 10 ha of fingerling ponds require 10 ha of settling area, but pond size is limited to 8 ha; to have two 5-ha ponds used. 6-ha foodfish pond will contain the largest effluent volume to be treated at one time. Requires 24 ha of settling area, but individual pond size limited to 8 ha; thus, three 8-ha ponds.
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Table 3 Area and number of settling basins that would be required for 64, 128, and 256-ha farm scenarios that include different drainage directions, hydraulic retention times (HRT), and the proportion of the pond volume to be treated
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For an HRT of 20 days, foodfish ponds would still be drained serially once per 6 year. However, the farmer needs to have the flexibility to simultaneously drain all fingerling ponds for management purposes. Thus, for HRT’s of 20 days, the size of the settling basin was determined by either the size of a foodfish pond or the total fingerling hectareage, whichever was largest. For the 64-ha farm with 4-ha foodfish ponds and three 2-ha fingerling ponds with single drainage, one 6 ha settling basin could treat all the effluents (1.2 ha would be required to treat the last 20%). These 6 ha correspond to the total fingerling hectareage that would need to be treated simultaneously. However, if the farm has a double drainage system, two 4-ha settling basins are needed, one on each side of the farm. Since there are three fingerling ponds, two would drain to one side (requiring 4 ha of settling area). The other side would only require 2 ha of settling for the remaining fingerlings but would need to be sized at 4 ha to accommodate draining a 4-ha foodfish pond. A pumping unit (either portable or stationary) would be needed to pump treated effluents back into the discharge canal. Stationary re-lift pumps with electric motors and pumping capacities of 114, 227, and 454 l s 1 were selected for the 64, 128, and 256-ha farms, respectively. Pumps with higher capacities were selected for the larger farms because of the larger effluent volume from fingerling ponds. The costs of pumps were $4300, 5225, and 7170, respectively. Additional detail on calculations of pumping cost are included in Appendix A. 2.2.4. Converting existing growout ponds to settling basins Additional scenarios were developed for farms that would need to convert existing ponds if land were not available for new construction. In these cases, the last production pond draining into the main ditch on the farm would be taken out of production (Fig. 1). Large effluent volumes require converting several adjacent production ponds linked with culverts. The only feasible design is to convert ponds in a continuous row. For the scenarios analyzed, from 2 to 21% of the water surface area were removed from production to provide adequate settling area. Required settling basin area is the same as that calculated for constructing new basins (Table 3). However, in those instances in which the required settling area exceeds the size of existing ponds, additional foodfish ponds were taken out of production and linked (Fig. 1). Contingent upon the volume of effluents, up to six foodfish ponds may need to be converted and interconnected to provide adequate settling area. For farms with ponds that drain in multiple directions, several pond arrangements were tested to ensure the most effective conveyance of effluents. Ponds located in a continuous row resulted in the most effective design configuration because it minimized culvert and pumping distance. The culverts used to convey water among basins were assumed to be 14.5 m long, but size of the culvert varied with both the pumping rate from the drainage canal and the number of interconnected basins. Culverts would be located in the deep end of the basins (about 1.45 m) through the inner levee. Flow through the culverts must be large enough to avoid overflow from pumping in the first settling basin. Flow velocity is a direct function of available head, which is
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determined by the difference in water levels between the linked ponds. Culverts should be provided with standpipes at the inlet end, which would allow the first pond to be filled completely before releasing water to the second pond, thereby increasing effective head pressure, maximizing flow velocity, and reducing filling times. If several ponds need to be linked, each pond should be allowed to fill before transmitting water to the next pond. Calculations of culvert flow rate are detailed in Appendix B. Spreadsheet models were developed to simulate culvert flow through systems of two, three, four, and six linked basins. Culvert sizes were adjusted to allow sufficient flow speed to minimize the chances of overflow in the first basin. Table 4 summarizes the selection of culvert diameters for each system of linked basins. Conversion of growout ponds into settling basins is complicated by the need to construct an earthen dam in each principal drainage ditch. While other designs were considered, this design involves the least amount of earth work and drainpipe installation. The earthen dam is used to retain discharges from other production ponds to be pumped to the settling basin via a stationary re-lift pump. To minimize cost, this dam should be located close to the last foodfish pond (the one to be converted to a settling basin). Solids in the effluents are allowed to settle out in the basin for the appropriate period of time before final release of discharges. A new drainpipe is installed in the settling basin to convey treated effluents towards the receiving body of water (Fig. 1c). Stationary re-lift pumps equipped with water-level sensors were selected over portable PTO-driven pumps to relieve the farm manager from monitoring the pumping. Capacities of the selected pumps were 114, 257, and 454 l s 1 for the small, medium, and large farm scenarios, respectively. Farms discharging in two directions (double drainage) require two pumps.
2.2.5. Storm overflow discharge versus draining discharge Separate scenarios were developed for unintentional overflow discharges during heavy storm events. Settling basin area for storm overflow was based on the sum of the effluent volume generated by a 7.6-cm per day maximum annual storm event Table 4 Summary of culvert sizes and corresponding installation costs for interconnected settling basins Number of linked basins
Culvert diameter (cm)
Culvert cost ($ per linear m)
Pumping rate from drainage canal (l s 1)
Scenarios requiring interconnected basins (ha)
2 2 2 2 3 3 4 6
5.5 6.3 6.3 7.9 6.3 7.1 9.4 10.2
93.11 99.67 99.67 117.70 99.67 109.51 132.46 148.85
113.58 189.30 227.16 454.32 189.30 227.16 454.32 454.32
64 and 128 128 256 256 128 256 256 256
Scenarios for which linked systems were required.
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(National Weather Service Forecast Office, 2001) in addition to the settling area required for draining effluents. In the event of a large storm coinciding with pond draining, the settling basin would have to be large enough to treat both volumes simultaneously. Converting ponds is not feasible to treat storm overflow effluents. Operation of settling basins that have been linked requires monitoring that may not be available during rainfall events.
2.3. Cost analysis
2.3.1. Estimating costs for new settling basin construction Economic engineering techniques were used to determine construction and operating costs associated with the use of settling basins to treat effluents from catfish ponds. Investment costs included the excavation of settling basins and the installation of stationary re-lift pumps to drain effluents out of the excavated basins. Annual fixed costs include depreciation costs of basins, pumps and interest on the investment. Depreciation costs for settling basins and pumping equipment were estimated with a useful life of 20 and 10 years, respectively. Interest on investment was calculated with a 10% interest rate and a loan horizon of 20 and 7 years for the construction of basins and the purchase of pumps, respectively. Annual operating costs consisted of copper sulfate applications (to promote settling of phytoplankton cells) and the annual cost of pumping. Copper sulfate was applied at 2 mg l 1 at a cost of $1.32 kg 1. Pumping rate was adjusted for flow rates of effluents discharged into the main drainage canal for the different pond sizes. Settling basins were assumed to remain filled with water to prevent levee erosion in empty basins.
2.3.2. Estimating costs to convert existing growout ponds for use as settling basins Investment costs included the purchase of pumps and installation of new drainlines and culverts. Cost of drainpipe installation is dependent on pipe diameter and length of the installation. For example, a 4-ha basin needs about 274 m of 25.4cm drainpipe, which is installed at a cost of $17.39 per linear m (total cost is $1590). In contrast, a draining system for six 6-ha basins requires 678 m of 30.5-cm drainpipe installed at a cost of $12.96 per linear m, for a total cost of $8789. Annual fixed costs consisted of pump depreciation and interest on investment in the pumping equipment. Annual operating costs included copper sulfate applications and decreased revenue from catfish production lost by taking ponds out of production. Published enterprise budgets (Engle, 1998; Engle and Killian, 1997) were used to obtain estimates of lost revenue. Pond yields of 5682 kg ha 1 were assumed in each case.
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Table 5 Total investment and total annual operating costs of newly-constructed settling basins for 64, 128, and 256-ha catfish farms, 4-ha ponds, single drainage direction Farm size
Annual costa ($)
Investment cost ($) Total $
$ ha 1
Total $
$ ha 1
$ kg 1
64-ha All effluents Last 20% of effluents
127 624 /191 346 29 078 /41 092
1994 /2990 454 /642
15 950 /23 507 3885 /5388
249 /367 61 /84
0.05 /0.08 0.01 /0.02
128-ha All effluents Last 20% of effluents
128 642 /313 246 30 096 /66 993
1005 /2447 235 /523
16 273 /38 871 4208 /8725
127 /304 33 /68
0.02 /0.07 0.01 /0.02
256-ha All effluents Last 20% of effluents
130 781 /747 015 32 235 /155 463
511 /2918 126 /607
16 947 /92 393 4882 /19 969
66 /361 19 /78
0.01 /0.07 0.01 /0.02
a Includes both operating costs and annual fixed costs that include depreciation and interest on the investment.
3. Results and discussion 3.1. Costs of constructing settling basins on new land Table 5 indicates that large amounts of investment capital are needed to construct settling basins. The amount of investment capital is heavily dependent on the drainage layout of the farm and the scope of regulations governing the release of effluents. For instance, investment costs in a 256-ha catfish farm may range from $32 235 to 747 015 ($126 /2918 ha 1), depending on what proportion of the effluent volume is treated. Total investment costs were somewhat lower for smaller farms but higher on a per-ha basis. For example, on the 64-ha farm, total investment cost ranged from $29 078 to 191 346 per farm ($454 /2990 ha 1), indicating a disproportionately larger financial burden on the smaller farm to acquire the investment capital required to install settling basins. As a percentage of the total investment cost of the entire catfish farm, these costs range from 5 to 32% for the 64ha farm and 2/33% for the 256-ha farm. It is clear that the cost of constructing settling basins is not a function of farm size. It is a relatively constant amount that represents a relatively greater per-ha cost on smaller farms than on larger farms. Total annual operating costs ranged from $61 to 367 ha 1 for the 64-ha farm and decreased to $19/361 ha 1 for the largest farm size considered. Total annual fixed costs comprised 83/92% of total annual costs. These estimates of total annual operating costs for newly constructed settling basins were relatively low because the design criteria were set for water to flow by gravity into the settling basin. There are farm situations in which this may not be possible; the addition of pumping stations would require additional investment and operating costs. Costs of producing catfish would increase by at least $0.01 /0.08 kg 1 if farmers would be required to construct new settling basins if their entire farm drained in the
C.R. Engle, D. Valderrama / Aquacultural Engineering 28 (2003) 171 /199
185
Fig. 2. Total investment and total annual operating costs of newly-constructed settling basins for a 128-ha catfish farm with ponds that drain in one, two, three, or four different directions, an HRT of 1 day and treatment of only the last 20% of effluents.
same direction (Table 5). Costs kg 1 will be much higher for the farms that have more than one drainage. Given concerns over price competitiveness with low-priced imports, any cost increase will reduce the competitive position of catfish farmers. Moreover, this cost estimate assumes that financing for the increased capital will be readily available. Caps on lending by bank examiners may prevent easy access to the required capital. The most dramatic impact on the costs of settling basins was the number of drainage directions (Fig. 2). The number of required settling basins increased with the number of drainage directions and the investment cost also increased in direct proportion to the number of drainage directions. Total annual costs increased at the same percentage rate as the increase in investment cost. Since the number of drainage directions depends upon the location and topography associated with each farm, it is independent of farm size. Even small farms of 64 ha or less can have ponds that drain in three or more different directions. Fig. 2 presents increasing costs for increasing numbers of drainage directions only for conservative scenarios of a 1-day HRT and treatment of only the last 20% of effluents. Nevertheless, the same pattern of increasing costs with increasing numbers of drainage directions holds true for the other scenarios. Total costs of constructing and operating settling basins were slightly higher on larger than on smaller farms (Fig. 3). However, on a per-ha basis, settling basin costs decreased with larger farm sizes. Thus, requiring settling basins would impose a disproportionately greater financial burden on smaller farms as opposed to larger farms. Both investment and annual operating costs increased with larger pond size (Fig. 3). Costs increased faster with larger pond sizes than with larger farm sizes. This is because the size of the settling basin is dependent upon the size of the largest pond to
186
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Fig. 3. Total investment and total annual operating costs of newly-constructed settling basins for 64, 128, and 256-ha catfish farms with either 4 or 6-ha ponds, HRT of 1 day with ponds draining in one direction, and treatment only of the last 20% of effluents.
be drained, not the size of the farm. Catfish farmers do not drain all their ponds every year nor do they drain them all at the same time. The larger size of settling basin required to capture the water draining from larger ponds requires additional excavation and earthmoving costs. Moreover, the potential for re-suspension of clay particles due to wind turbulence and erosion in the basin would likely increase with the size of the settling basin. Total investment and total annual operating costs both increased more than fourfold by requiring that all effluents from the pond be treated in settling basins as compared with requiring that only the last 20% of effluents be treated (Fig. 4). These same cost parameters increased by approximately 40% as HRT increased from 1 to 20 days. 3.2. Cost of converting existing growout ponds into settling basins Converting existing production ponds into settling basins resulted in a more narrow range of investment costs than those of newly-constructed settling basins.
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187
Fig. 4. Total investment and total annual operating costs of newly-constructed settling basins for HRT’s of 1 and 20 days for draining all effluents or only the last 20% of effluents for a 128-ha catfish farm with 4ha ponds that drain in two different directions.
These costs ranged from $5837 to 12 039 for the 64-ha farm, $6762 /18 886 for the 128-ha farm, and $8707 /30 574 for the 256-ha farm (Table 6) (see Appendix D for itemized costs). Total capital invested in the business increased by 1.0 /2.0, 0.6 /1.7, and 0.4 /1.4% for the 64, 128, and 256-ha farms, respectively. Decreasing costs per ha on larger farms result from economies of scale and illustrate the greater economic impact that would occur on smaller farms. Total annual costs for converting production ponds into settling basins ranged from $10 935 to 31 663 for the 64-ha farm to a range of $12 603/68 463 for the 256ha farm. This represented $171/495 ha 1 for the 64-ha farm to $49 /267 ha 1 for the larger farm size. Over half of these costs are related to the lost revenue and continued fixed costs associated with ponds taken out of production. These additional operating costs represent an increase of 2.6 /7.6, 1.3 /4.9, and 0.8 /4.2% in total annual operating costs for the 64, 128, and 256-ha farms, respectively. Production costs would increase by $0.01 /0.14 kg 1, depending upon the scenario (Table 6). This magnitude of cost increase would not be economically feasible even for the single and double drainage scenarios included in Table 6. In addition to the increased capital costs, the system must be designed and monitored carefully, especially when several converted ponds are interconnected. If pumping rates from the drainage canal exceed the conveyance capacity of the system, the first basin will overflow. A more logical and simple approach may be to use each production pond as its own settling basin as suggested by Tucker et al. (2000). Certain good management practices, such as harvesting ponds without ever draining the last 20% of water (for watershed ponds), or holding this volume of water in
188
Pond size (ha)
Draining layout
HRT (days)
Percentage to be treated (%)
Farm size 64-ha farm
4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
Single Single Single Single Double Double Double Double Single Single Single Single Double Double Double Double a
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
Last All Last All Last All Last All Last All Last All Last All Last All
20% 20% 20% 20% 20% 20% 20% 20%
128-ha farm
256-ha farm
Investment ($)
Annuala ($)
$ kg 1 Investment ($) ($)
Annual ($)
$ kg 1 Investment ($) ($)
Annual ($)
$ kg 1 ($)
5837 5837 5837 8656 10 891 10 891 10 891 10 891 6413 6413 6413 6413 12 039 12 039 12 039 12 039
10 935 10 935 10 935 21 135 21 625 21 625 21 625 21 625 15 955 15 955 15 955 15 955 31 663 31 663 31 663 31 663
0.04 0.04 0.04 0.09 0.09 0.09 0.09 0.09 0.06 0.06 0.06 0.06 0.14 0.14 0.14 0.14
10 949 10 949 10 949 30 872 21 526 21 526 21 526 41 438 15 819 15 819 15 819 30 675 31 278 31 278 31 278 31 278
0.02 0.02 0.02 0.05 0.03 0.03 0.03 0.07 0.02 0.02 0.02 0.05 0.05 0.05 0.05 0.05
12 603 12 603 23 773 67 882 24 471 24 471 24 471 68 463 17 976 17 976 17 976 67 287 35 227 35 227 35 227 68 015
0.01 0.01 0.02 0.05 0.02 0.02 0.02 0.05 0.01 0.01 0.01 0.05 0.03 0.03 0.03 0.05
6762 6762 6762 13 012 12 741 12 741 12 741 18 886 7338 7338 7338 11 154 13 889 13 889 13 889 13 889
Includes both operating costs and annual fixed costs that include depreciation and interest on the investment.
8707 8707 13 914 29 044 16 631 16 631 16 631 30 574 9283 9283 9283 23 168 17 779 17 779 17 779 26 198
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Table 6 Investment and annual costs of converting growout ponds into settling basins for 64, 128, and 256-ha farm scenarios by the number of drainage directions, HRT, and proportions of pond volume to be treated
Foodfish pond size (ha)
64-ha farm 4 4 6 6
Draining layout
HRT (days)
Overflow caused Required sediby a 7.6 cm storm mentation area event (m3) (ha)
Basins constructed (number)
Area of indivi- Total investdual settling ba- ment costs sins (ha) ($)a
Annual costs
Operating costs ($)b
Fixed costs ($)c
Total costs
Single Double Single Double
1 1 1 1
41 634 41 634 41 634 41 634
3 3 3 3
1 2 1 2
3 1.5 3 1.5
96 356 100 314 96 356 100 314
1046 1046 1046 1046
11 086 11 740 11 086 11 740
12 132 12 786 12 132 12 786
128-ha farm 4 Single 4 Double 6 Single 6 Double
1 1 1 1
87 894 87 894 86 352 86 352
6.33 6.33 6.22 6.22
1 2 1 2
6.33 3.17 6.22 3.11
200 091 204 920 196 756 201 468
2185 2186 2148 2148
22 849 23 643 22 472 23 253
25 034 25 829 24 620 25 401
256-ha farm 4 Single 4 Double 6 Single 6 Double
1 1 1 1
172 704 172 704 175 788 175 788
12.44 12.44 12.67 12.67
2 2 2 2
6.22 6.22 6.33 6.33
390 856 397 402 397 779 404 072
4246 4247 4322 4321
44 495 45 576 45 278 46 329
48 741 49 823 49 600 50 650
a b c
Includes construction costs of basins and acquisition of pumps. Includes copper sulfate and pumping costs. Includes depreciation and interest on investment.
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Table 7 Total costs of construction of settling basins for overflow effluents from catfish farms by farm size
189
190
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existing farm ditches prior to final discharge, may be implemented to achieve reductions in solid and nutrient loads comparable to what could be accomplished with the construction of independent and expensive structures.
3.3. Costs of constructing settling basins for the treatment of overflow effluents Total investment costs associated with construction of settling basins to treat overflow effluents from catfish farms ranged from $96 356 to 100 314 for 64-ha farms, $196 756/204 920 for 128-ha farms, and $390 856/404 072 for 256-ha farms (Table 7) (see Appendix E for itemized costs). These costs represent increases of 16 / 17, 18, and 18% above investment costs for the 64, 128, and 256-ha farms, respectively, without the installation of settling basins.
4. Conclusions Costs associated with the implementation of settling basins on static levee-style pond catfish farms are dependent on the size and number of basins, and whether sufficient land is available for basin construction or if existing production ponds must be converted. The size of settling basins on catfish farms is controlled by factors such as the type of effluent to be treated, pond layout, size of the largest pond, number of drainage canals, and scope of regulations governing the release of aquacultural effluents. Moreover, the effectiveness of settling basins in practical operation on farms is unknown. There is potential for re-suspension of particles due to wind turbulence and erosion in the basins. Costs increase dramatically as the number of different farm drainage directions increases. Requirements to treat all effluents as compared with only the last 20% also increase costs sharply. Larger pond sizes result in greater treatment costs due to the necessity of constructing larger settling basins. Costs are higher on larger farms, but treatment costs on a per-ha basis are lower for larger farms than for smaller farms. The cost for small farms is particularly burdensome, increasing investment costs by 5/13%. Increasing HRT from 1 to 20 days did not increase costs by as much as the number of drainage directions or pond sizes. However, increasing HRT increases costs rapidly if all effluents are required to be treated. The costs estimated in this study do not cover all possible scenarios. Farms that drain in more than five directions and those that require pumping instead of gravity flow, will have much higher costs than those estimated in this analysis. Increased production costs of the magnitude of those estimated in this study cannot be passed on to processors. Many catfish farmers have long-term financing at maximum credit levels with little or no credit reserve and cannot borrow additional capital. The financial burden is particularly high for smaller-scale farms. The costs associated with the use of settling basins to treat effluents from catfish ponds are too high to be economically feasible.
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191
Acknowledgements The authors gratefully acknowledge the helpful review comments and suggestions of Nathan Stone, Steve Lochmann, and Bartholomew Green. The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 99-38500-7375 from the United States Department of Agriculture.
Appendix A Hourly pumping costs were calculated using the following equation (Yoo and Boyd, 1994): C
Q TDH 0:746 R 3960 Ep Ed Em
(3)
where C is the pumping cost per hour of operation ($ h 1); Q is the discharge rate (l s 1); TDH, total dynamic head (m); R is cost of electricity ($ kW 1 h 1); Ep is pump efficiency (decimal); Ed is drive train efficiency in decimal and Em is the motor efficiency (decimal). Total dynamic head (TDH ) is 3.66 m while assumed cost of electricity is $0.065 kW 1 h 1. Manufacturer specifications were consulted to obtain values for the remaining items in the equation. The annual number of pumping hours was estimated for each farm scenario, taking into account that foodfish ponds are drained only once every 6 years, while fingerling ponds are drained annually. It should be noted that although only a portion of effluents may be subject to treatment, the total volume of effluents needs to circulate through and be pumped out of the basin. Annual pumping costs were calculated based on the estimated costs of operation per hour and total number of hours.
Appendix B The following equation was used to estimate culvert flow rates (Yoo and Boyd, 1994): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2gH Q A (4) 1 km kc L where Q is the flow rate (cm s 1); A is the cross-sectional area of culvert flow (m2); g is the gravity acceleration (32.2 m s 2); H is the effective head causing flow in culvert (fm); km is the minor-loss coefficient caused by culvert entrance. Assumed to be 0.8. kc is the friction loss coefficient for culvert flowing full; and L is the culvert length (m).
192
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In turn, kc is calculated using the equation: kc
29:16n2 d 4=3
(5)
where n is the Manning’s roughness coefficient of culvert. Assumed to be 0.016 for plastic tubing and d is the culvert diameter (cm). Calculation of effective head is dependent on flow conditions at the culvert outlet. For a free-flow culvert, effective head is equivalent to water depth above culvert inlet in the head-pond minus 0.6 times the culvert diameter. When a sufficient volume of water has been transmitted to the receiving pond, both ends of the culvert are submerged and efficient head is expressed as the difference in water levels between the two ponds. The difficulty with using Eq. (4) is that it only provides an estimate of flow rate at a specific head. However, effective head is at a maximum at the initial stage of flow, but it constantly declines as the differences in level between headwater and tailwater drops. In order to estimate average flow rate during the entire period of time required to fill the receiving pond, one must obtain several estimates of Q at successive time intervals, which should be sufficiently short (e.g. 1 s). Effective head (H) is recalculated for each time interval because of the change in water level caused by the culvert flow. Successive estimates of Q are obtained until the tail-pond has been completely filled, then average flow rate is simply calculated as the arithmetic mean of the various Q estimates. Besides verifying water level in the tail-pond after each time interval, water level must also be monitored in the head-pond since water is constantly being pumped from the main drainage canal at an average rate and overflow needs to be avoided. The only variable in Eq. (4) that can be manipulated to regulate culvert flow is culvert diameter (A ).
Appendix C Table 8
Appendix D Table 9
Appendix E Table 10
Table 8 Itemized costs of newly-constructed settling basins for all scenarios evaluated Farm/settling Drainages basin size (ha) (number)
HRTa Volume treated (%)
Investment
Pump installa- Total investtion ($) ment ($)
Total operating costs
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
24 348 122 894 36 362 186 616 48 696 245 787 48 696 245 787 36 632 184 616 36 632 184 616 73 264 369 232 73 264 369 232
4730 4730 4730 4730 9460 9460 9460 9460 4730 4730 4730 4730 9460 9460 9460 9460
29 078 127 624 41 092 191 346 58 156 254 647 58 156 255 247 41 362 189 346 41 362 189 346 82 724 378 692 82 724 378 692
3442 14 569 4828 21 540 6884 29 138 6884 29 138 4828 21 540 4828 21 540 9657 43 081 9657 43 081
235 1173 352 1759 469 2345 469 2345 352 1759 352 1759 704 3517 704 3517
208 208 208 208 208 208 208 208 208 208 208 208 208 208 208 208
443 1381 560 1967 677 2553 677 2553 560 1967 560 1967 912 3725 912 3725
3885 15 950 5388 23 507 7561 31 691 7561 31 691 5388 23 507 5388 23 507 10 569 46 806 10 569 46 806
128-ha 4 4 4 4 4 4 4 4
1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100
24 348 122 894 61 245 307 498 48 696 245 788 73 264 369 232
5748 5748 5748 5748 11 495 11 495 11 495 11 495
30 096 128 642 66 993 313 246 60 191 257 283 84 759 380 727
3592 14 719 7757 35 559 7185 29 439 9958 43 381
234 1172 586 2931 469 2345 703 3517
381 381 381 381 381 381 381 381
615 1553 967 3312 850 2726 1084 3898
4208 16 273 8725 38 871 8035 32 164 11 042 47 280
193
64-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
C.R. Engle, D. Valderrama / Aquacultural Engineering 28 (2003) 171 /199
Excavation ($)
Annual fixed costs ($)
194
Table 8 (Continued ) Farm/settling Drainages basin size (ha) (number)
HRTa Volume treated (%)
Investment
Annual fixed costs ($) Pump installa- Total investtion ($) ment ($)
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
6 6 6 6 6 6 6 6
1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100
36 632 184 616 61 245 307 498 73 264 369 232 73 264 369 232
5748 5748 5748 5748 11 495 11 495 11 495 11 495
42 380 190 364 66 993 313 246 84 759 380 727 84 759 380 727
4979 21 691 7757 35 559 9958 43 381 9958 43 381
352 1759 587 2931 704 3518 704 3518
376 376 376 376 376 376 376 376
728 2135 963 3307 1080 3894 1080 3894
5707 23 826 8720 38 866 11 037 47 275 11 037 47 275
256-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
24 348 122 894 147 576 739 128 48 696 245 788 147 130 738 464 36 632 184 616 147 576 739 128 73 264 369 232 147 130 738 464
7887 7887 7887 7887 15 774 15 774 15 774 15 774 7887 7887 7887 7887 15 774 15 774 15 774 15 774
32 235 130 781 155 463 747 015 64 470 261 562 162 904 754 238 44 519 192 503 155 463 747 015 89 038 385 006 162 904 754 238
3909 15 035 17 823 84 619 7817 30 071 18 929 85 697 5295 22 007 17 823 84 619 10 590 44 014 18 929 85 697
234 1172 1407 7034 469 2345 1407 7034 351 1758 1406 7034 703 3517 1406 7034
739 739 739 739 739 739 739 739 748 748 748 748 748 748 748 748
973 1911 2146 7773 1208 3084 2146 7773 1099 2506 2154 7782 1451 4265 2154 7782
4882 16 947 19 969 92 393 9025 33 155 21 075 93 470 6394 24 513 19 978 92 401 12 041 48 279 21 084 93 479
a
Hydraulic residence time.
C.R. Engle, D. Valderrama / Aquacultural Engineering 28 (2003) 171 /199
Excavation ($)
Total operating costs
Table 9 Itemized costs of converting existing settling basins for all scenarios evaluated Farm/settling ba- Drainages sin size (ha) (number)
Total ($)
Annual fixed costsc ($)
Total operating costs
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
4300 4300 4300 4300 8600 8600 8600 8600 4300 4300 4300 4300 8600 8600 8600 8600
1537 1537 1537 4356 2291 2291 2291 2291 2113 2113 2113 2113 3439 3439 3439 3439
5837 5837 5837 8656 10 891 10 891 10 891 10 891 6413 6413 6413 6413 12 039 12 039 12 039 12 039
9893 9893 9893 19 214 19 707 19 707 19 707 19 707 14 470 14 470 14 470 14 470 28 862 28 862 28 862 28 862
879 879 879 1758 1758 1758 1758 1758 1319 1319 1319 1319 2638 2638 2638 2638
163 163 163 163 159 159 159 159 167 167 167 167 163 163 163 163
1042 1042 1042 1921 1918 1918 1918 1918 1485 1485 1485 1485 2801 2801 2801 2801
10 935 10 935 10 935 21 135 21 625 21 625 21 625 21 625 15 955 15 955 15 955 15 955 31 663 31 663 31 663 31 663
128-ha 4 4 4 4 4 4 4 4 6
1 1 1 1 2 2 2 2 1
1 1 20 20 1 1 20 20 1
20 100 20 100 20 100 20 100 20
5225 5225 5225 5225 10 450 10 450 10 450 10 450 5225
1537 1537 1537 7787 2291 2291 2291 8436 2113
6762 6762 6762 13 012 12 741 12 741 12 741 18 886 7338
9773 9773 9773 27 937 19 468 19 468 19 468 37 622 14 215
879 879 879 2638 1758 1758 1758 3517 1319
297 297 297 297 299 299 299 299 285
1176 1176 1176 2935 2058 2058 2058 3816 1604
10 949 10 949 10 949 30 872 21 526 21 526 21 526 41 438 15 819
195
64-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
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HRTa Volume trea- Investment ted (%) Pumpsb Drains/cul($) verts ($)
196
Table 9 (Continued ) Farm/settling ba- Drainages sin size (ha) (number)
Total ($)
Annual fixed costsc ($)
Total operating costs
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
6 6 6 6 6 6 6
1 1 1 2 2 2 2
1 20 20 1 1 20 20
100 20 100 20 100 20 100
5225 5225 5225 10 450 10 450 10 450 10 450
2113 2113 5929 3439 3439 3439 3439
7338 7338 11 154 13 889 13 889 13 889 13 889
14 215 14 215 27 752 28 353 28 353 28 353 28 353
1319 1319 2638 2638 2638 2638 2638
285 285 285 288 288 288 288
1604 1604 2923 2925 2925 2925 2925
15 819 15 819 30 675 31 278 31 278 31 278 31 278
256-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
7170 7170 7170 7170 14 340 14 340 14 340 14 340 7170 7170 7170 7170 14 340 14 340 14 340 14 340
1537 1537 6744 21 874 2291 2291 2291 16 234 2113 2113 2113 15 998 3439 3439 3439 11 858
8707 8707 13 914 29 044 16 631 16 631 16 631 30 574 9283 9283 9283 23 168 17 779 17 779 17 779 26 198
11 089 11 089 21 379 61 972 22 100 22 100 22 100 62 575 16 031 16 031 16 031 61 385 31 985 31 985 31 985 62 136
879 879 1758 5275 1758 1758 1758 5275 1319 1319 1319 5275 2638 2638 2638 5275
635 635 635 635 613 613 613 613 627 627 627 627 604 604 604 604
1514 1514 2394 5910 2371 2371 2371 5888 1945 1945 1945 5902 3242 3242 3242 5879
12 603 12 603 23 773 67 882 24 471 24 471 24 471 68 463 17 976 17 976 17 976 67 287 35 227 35 227 35 227 68 015
a b c
Hydraulic residence time. Includes installation. Includes the revenue lost due to removing pond(s) from production, depreciation on pumps, pipes, and culverts, and interest on the investment.
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HRTa Volume trea- Investment ted (%) Pumpsb Drains/cul($) verts ($)
Farm/settling ba- Drainages sin size (ha) (number)
HRTa Investment Excavation ($)
Annual fixed costs ($) Pump installation ($)
Total investment ($)
Total operating costs CuSO4 ($)
Pumping ($)
Total annual costs ($) Total ($)
64-ha 4 4 6 6
1 2 1 2
1 1 1 1
81 432 82 199 81 432 82 199
4730 9460 4730 9460
92 056 45 857 92 056 45 857
11 086 11 740 11 086 11 740
223 223 223 223
879 440 879 440
1102 1102 1102 1102
12 189 12 842 12 189 12 842
128-ha 4 4 6 6
1 2 1 2
1 1 1 1
171 193 171 903 168 285 168 890
5748 11 496 5748 11 496
194 866 97 235 191 531 95 509
22 849 23 643 22 472 23 253
441 441 434 434
1856 929 1824 912
2297 2298 2258 2258
25 146 25 941 24 730 25 511
256-ha 4 4 6 6
1 2 1 2
1 1 1 1
168 285 336 570 171 193 342 385
7887 15 774 7887 15 774
383 686 191 531 390 609 194 866
44 495 45 576 45 278 46 329
802 802 816 816
3647 1824 3713 1856
4449 4451 4529 4528
48 945 50 027 49 807 50 858
a
Hydraulic residence time.
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Table 10 Itemized costs of newly-constructed settling basins for overflow effluents
197
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References Boney, A.O., 1975. Phytoplankton. Edward Arnold Publishers, London, UK. Boyd, C.E., 1995. Bottom Soils, Sediment, and Pond Aquaculture. Chapman and Hall, New York. Boyd, C.E., Queiroz, J.F., 2001. Feasibility of retention structures, settling basins, and best management practices in effluent regulation for Alabama channel catfish farming. Reviews in Fisheries Science 9 (2), 43 /67. Boyd, C.E., Gross, A., Rowan, M., 1998. Laboratory study of sedimentation for improving quality of pond effluents. Journal of Applied Aquaculture 8, 39 /48. Boyd, C.E., Queiroz, J., Whitis, G., Rowan, M., Lee, J.-Y., Gross, A., 2000. Environmental assessment of channel catfish farming in Alabama. Journal of the World Aquaculture Society 31 (4), 511 /544. Cerezo, G.A., 1993. Catfish pond effluent control: an economic analysis into wetlands, water recycling, and the effect of taxation of effluents. Unpublished Master’s thesis, Auburn University, AL. Dunning, R., Sloan, D., 1997. Aquaculture in North Carolina, rainbow trout: inputs, outputs, and economics. North Carolina Department of Agriculture and Consumer Services, Division of Aquaculture and Natural Resources, Franklin, NC, USA. Engle, C.R., 1998. Annual costs and returns of raising bighead carp in commercial catfish ponds. FSA 9078. Cooperative Extension Program, University of Arkansas at Pine Bluff, Pine Bluff, AR. Engle, C.R., Killian, H.S., 1997. Costs of producing catfish on commercial farms in levee ponds in Arkansas. STB 252. Cooperative Extension Program, University of Arkansas at Pine Bluff, Pine Bluff, AR, USA. Engle, C.R., Valderrama, D., 2002. The economic impact of aquaculture in the United States. In: Tomasso, J. (Ed.), The Environmental Impact of Aquaculture in the United States. The US Aquaculture Society, Baton Rouge, LA. Fornshell, G., 2001. Settling basin design. Western Regional Aquaculture Center. WRAC Publication No. 106. University of Washington, Seattle, WA, USA. Hebicha, H., 1989. Catfish pond harvesting pollution control and costs for West Alabama. Unpublished Ph.D. Dissertation, Auburn University, Alabama. Henderson, J.P., Bromage, N.R., 1988. Optimising the removal of suspended solids from aquacultural effluents in settlement lakes. Aquacultural Engineering 7, 167 /181. Hinshaw, J.M., Rogers, L.E., Easley, J.E., 1990. Budgets for trout production: estimated costs and returns for trout farming in the south. Southern Regional Aquaculture Center Publication No. 221, Stoneville, MS, USA. Keenum, M.E., Waldrop, J., 1988. Economic analysis of farm-raised catfish production in Mississippi. Mississippi Agricultural Forestry Experiment Station, Technical Bulletin 155, Mississippi State University, Mississippi. Kouka, P.J., Engle, C.R., 1996. Economic implications of treating effluents from catfish production. Aquaculture Engineering 15 (4), 273 /290. McGee, M.V., Boyd, C.E., 1983. Evaluation of the influence of water exchange in channel catfish ponds. Transaction of the American Fisheries Society 112, 557 /560. National Weather Service Forecast Office, 2001. Southern Region web site. http://www.srh.noaa.gov/lzk/ html/wxcntl3.htm. Posadas, B.C., 2001. Comparative economic analysis of using different sizes of constructed wetlands in recirculating catfish pond production. Journal of Applied Aquaculture 11 (3), 1 /19. Schmittou, A.T., 1991. Economic consequences of catfish pond effluent control in Alabama. Masters thesis, Auburn University, Auburn, AL. Schwartz, M., Boyd, C., 1994. Effluent quality during harvest of channel catfish from watershed ponds. The Progressive Fish-Culturist 56, 25 /32. State of Idaho, 1997. Idaho waste management guidelines for aquaculture operations. Division of Environmental Quality, Boise, ID, USA. Teichert-Coddington, D.R., Rouse, D.B., Potts, A., Boyd, C.E., 1999. Treatment of harvest discharge from intensive shrimp ponds by settling. Aquaculture Engineering 19, 147 /161.
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Tucker, C.S., Lloyd, S.W., 1985. Water quality in streams and channel catfish Ictalurus punctatus ponds in West-Central Mississippi. Technical Bulletin 129, Mississippi Agricultural and Forestry Experiment Station, Mississippi State, Mississippi, USA. Tucker, C.S., Robinson, E.H., 1991. Channel Catfish Farming Handbook. Van Nostrand Reinhold, New York. Tucker, C.S., van der Ploeg, M., 1993. Seasonal changes in water quality in commercial channel catfish culture ponds in Mississippi. Journal of the World Aquaculture Society 24, 473 /481. Tucker, C.S., Boyd, C., Hargreaves, J., Stone, N., Romaire, R., 2000. Effluents from channel catfish aquaculture ponds. Technical Subgroup for Catfish Production in Ponds, Joint Subcommittee on Aquaculture, Aquaculture Effluents Task Force, Washington, DC. USDA, 2000. Census of aquaculture. National Agricultural Statistics Services, Washington, DC. USDA, 2002. Aquaculture situation and outlook report. Economic Research Service, Washington, DC. USDA/Arkansas Agricultural Experiment Station, 1967. Soil survey of Chicot County, Arkansas. US Government Printing Office, Washington, DC. Yoo, K.H., Boyd, C., 1994. Hydrology and Water Supply for Aquaculture. Chapman and Hall, New York.
Farm-level costs of settling basins for treatment of effluents from levee-style catfish ponds Carole R. Engle *, Diego Valderrama 1 Aquaculture/Fisheries Center, University of Arkansas at Pine Bluff, Mail Stop 4912, 1200 North University Drive, Pine Bluff, AR 71601, USA Received 1 December 2002; accepted 22 March 2003
Abstract Compelled by pending regulatory rule changes, settling basins have been proposed as a treatment alternative for catfish pond effluents, but the associated costs to catfish farmers have not been estimated. Economic engineering techniques were used to design 160 scenarios as a basis for estimating total investment and total annual costs. For static-water, levee-style catfish pond facilities, sizing of settling basins is controlled by factors such as type of effluent to be treated, pond layout, size of the largest foodfish pond, number of drainage directions, scope of regulations governing effluents, and the availability of land. Regulations that require settling basins on catfish farms would increase total investment cost on catfish farms by $126 / 2990 ha 1 and total annual per-ha costs by $19 /367 ha 1. More numerous drainage directions on farms resulted in the greatest increase in costs. While both investment and operating costs increased with larger sizes of foodfish ponds, costs per ha were relatively greater on smaller than on larger farms. For farms on which existing fish ponds would have to be converted to settling basins, over half of the cost was due to the production foregone and annual fixed costs of the pond. Requiring catfish farmers to construct settling basins would impose a disproportionately greater financial burden on smaller farms. The magnitude of the increased costs associated with settling basins was too high relative to market prices of catfish for this technology to be economically feasible. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Catfish effluents; Catfish economics; Effluent treatment
* Corresponding author. Tel.: /1-870-543-8537; fax: /1-870-543-8129. E-mail address: [email protected] (C.R. Engle). 1 Present address. University of Rhode Island, Rhode Island, USA. 0144-8609/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0144-8609(03)00027-X
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1. Introduction The aquaculture industry has been one of the fastest-growing agribusinesses in the southeastern United States over the past two decades. US aquaculture has grown from a farm-gate value of $192 million in 1980 to nearly $1 billion in 1999 (USDA, 2000). Catfish production has demonstrated the most rapid increase in US aquaculture, from less than 32 000 MT in 1980 to over 271 000 MT of foodfish in 2001 (USDA, 2002). A lawsuit filed by the Natural Resources Defense Council against the Environmental Protection Agency in 1989 resulted in the inclusion of aquaculture in the effluent limitation guidelines (ELG) rulemaking process in December, 1999. The ELG process will result in a rule for the industrial classification of ‘‘aquaculture’’ and may or may not distinguish between effluents from flow-through and static systems. Moreover, any treatment technology guidelines selected for the rule may be applied to all aquaculture. A proposed rule was published by EPA in the Federal Register after 12 September 2002, but will not be finalized until 30 June 2004. The intervening 2 years will be a period of refining and adjusting the rule based on continued comments and input from both environmental and industry groups. From an economics perspective, there is a trade-off between the cost to society of poor environmental quality resulting from pollution and the costs of controlling or reducing pollution to maintain a comparatively high level of environmental quality (Engle and Valderrama, 2002). This paper will present detailed estimates of costs associated with implementing one treatment option (settling basins) that has been discussed by EPA for catfish farms. Settling basins have been used to treat effluents from flow-through systems. Much is known about the design, implementation, and costs associated with settling basins for trout raceways (Henderson and Bromage, 1988; Hinshaw et al., 1990; State of Idaho, 1997; Fornshell, 2001; Dunning and Sloan, 1997). Boyd et al. (1998) recommended an 8-h retention time for Alabama soils, but indicated that a settling time of 24 h or less was adequate for effective reduction of multiple parameter concentration levels. Boyd and Queiroz (2001) used the 8-h retention time, spread over a 48-h draining period to calculate settling basin volumes and areas for flow settling on catfish farms. The Boyd and Queiroz (2001) study did not evaluate the effect of various farm-specific factors associated with integrating settling basin technologies on farms with different drainage patterns, pond layouts, nor for varying regulatory options. Moreover, there do not appear to be estimates of farm-wide costs associated with the use of settling basins for static pond catfish production in the scientific literature. Several previous studies have evaluated costs associated with a variety of other technologies to treat effluents from catfish ponds. Hebicha (1989) evaluated dilution, dilution plus wetlands, intermittent sand filters, and caged fish culture as treatment options. None were found to be feasible and intermittent sand filters were identified to have particularly high costs. Schmittou (1991), in an evaluation of the consequences of catfish effluent control, showed that small farms would be more adversely affected than larger farms. Cerezo (1993) confirmed that increased costs
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associated with additional pollution control would force more small-scale producers to exit the industry. Kouka and Engle (1996) evaluated costs of (1) land application of catfish pond effluents for irrigation; (2) recycling water through constructed wetlands; and (3) recycling water from a catfish pond through a pond stocked with filter-feeding fish with both enterprise budget and whole-farm mathematical programming models. Catfish production costs were found to increase from $0 to 0.11 kg 1. While the use of water for crop irrigation resulted in no additional cost in the analysis, this alternative may not be feasible due to timing and application conflicts on most farms (Engle and Valderrama, 2002). Moreover, the Kouka and Engle (1996) study corroborated earlier findings by Cerezo (1993) that effluent treatment technologies imposed on catfish farms may have a disproportionately negative impact on smallerscale businesses. Posadas (2001) showed that, in spite of the improvements in water quality associated with constructed wetlands to treat catfish pond effluents, marketable yield of catfish did not increase. Constructed wetlands systems increased both investment and operating costs. Catfish production costs increased by $0.089, 0.127, and 0.165 kg 1 for constructed wetlands that represented 15, 25, and 35% of the pond area, respectively. The primary objective of the present study was to conduct an economic analysis of the on-farm micro-level costs associated with the use of settling basins to treat effluents from levee-style catfish ponds. Specific objectives included the estimation of total investment and annual variable and fixed costs of the construction and operation of settling basins on (a) new land and (b) conversion of existing ponds to serve as settling basins. The effect of farm size, pond layout, drainage patterns, and type of effluent (pond draining vs. storm overflow) on farm costs was evaluated.
1.1. Catfish pond effluents In the early years of the catfish industry, ponds were ‘‘flushed’’ with water pumped from wells. However, McGee and Boyd (1983) demonstrated that ‘‘flushing’’ at the 5% rates possible in commercial levee culture ponds is generally not beneficial. Flushing is no longer a routine practice in commercial catfish production largely due to the costs involved. Current management practices for catfish foodfish production involve draining ponds only once every 5/10 years without draining between multiple fish crops. The major parameters that have potential to be of concern in effluents from catfish ponds are nitrogen, phosphorus, organic matter, and settleable solids. These components are derived from feeds fed to catfish in the ponds. Approximately 30% of the nitrogen and phosphorus added to ponds in feed are recovered in the catfish that are harvested (Tucker et al., 2000). The remaining nutrients are released into the pond ecosystem as fish waste products that, in turn, stimulate the growth and production of phytoplankton. Nitrogen and phosphorus compounds and organic matter are present in pond water during growout and represent potential pollutants if discharged.
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Tucker and van der Ploeg (1993) demonstrated that the quality of potential effluents was poorest (highest concentrations of solids, organic matter, total phosphorus, and total nitrogen) in the summer. Most of the organic matter in channel catfish pond water consists of living phytoplankton cells and phytoplankton-derived detritus. Most of the nitrogen and phosphorus in catfish pond waters are present in particulate organic matter, primarily within phytoplankton cells (Tucker et al., 2000). Levee ponds are drained only to re-work pond bottoms and move eroded levee material back on to interior levee slopes. While nutrient concentrations and organic matter increase over the summer months, there is little or no water discharged from ponds during that time period (Tucker and Lloyd, 1985). Tucker and Lloyd (1985) demonstrated, from samples of 25 commercial catfish ponds and 24 receiving streams in west-central Mississippi, that, on average, concentrations of nitrate, nitrite, and soluble reactive phosphorus were higher in streams than in pond waters. Watershed ponds may have more frequent discharge depending on rainfall and if there is a stream flowing through the watershed pond. Boyd et al. (2000) collected water samples upstream and downstream of catfish farms on eight streams in WestCentral Alabama. While comparative values varied, there were approximately equal numbers of cases in which water quality below catfish farms were improved and degraded, suggesting that catfish farm effluents did not have a consistently adverse impact on stream water quality. In spite of these findings, EPA has considered settling basins as a possible treatment technology for catfish ponds.
2. Materials and methods 2.1. Factors affecting the design of settling basins to treat effluents from catfish farms A variety of factors affect the design of settling basins if these would be mandated for catfish ponds. Hydraulic residence time (HRT) will affect both the number and area of settling basins. Whether treatment will be required for draining discharges or storm event overflow will affect both the composition and timing of effluent discharge, and these will affect the number and area of settling basins. Moreover, the pattern of gravity-flow drainages on farms will affect the number of basins required. 2.1.1. Hydraulic residence time The cost of construction of settling basins is primarily related to the required sedimentation area. Hydraulic residence time is critical to determining whether the same settling basin can be used for treatment of multiple ponds within a farm or whether multiple numbers of settling basins will be required. With a short HRT, the same settling basin may be used for the treatment of multiple ponds, while a long HRT will require a higher number of settling basins if a large volume of effluents needs treatment within a short period of time. Stoke’s law (Boyd, 1995) allows the calculation of settling velocities for mineral soil particles and dead organic material. The equation follows below:
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vs
g(rr rw )dp2 18m
175
(1)
where vs is the velocity of particle (m s 1); g, the gravitation acceleration (m s 2); rr is the density of particles (kg m 3); rw , the density of water (kg m 3); dp , the diameter of particle (m) and m is the dynamic viscosity of water (kg m 1 s 1). According to Stokes’ law, settling velocity is primarily a function of particle size and density. HRT of effluent discharges will depend upon the settling velocity of particles in the effluent. The most prevalent type of particle in effluents from a catfish pond would be phytoplankton cells (50 mm in diameter) (Boney, 1975) and mineral soil particles (1 /5 mm in diameter, depending on soil type) (Boyd, 1995; USDA/AAES, 1967). Phytoplankton cells represent the majority of particles suspended in overflow effluents and the initial stages of pond draining effluents. However, live phytoplankton possess a range of floating mechanisms that considerably slows down their sinking rates (Boney, 1975). While phytoplankton is the principal constituent of pond effluents, the last 20% of pond draining discharges in watershed ponds may also contain an appreciable load of mineral soil particles due to the re-suspension of solid particles by the fish and seining crew (Schwartz and Boyd, 1994). As such, the concentration of total suspended solids (TSS) in the last portion of effluents may be several times higher than that of regular pond water. Boyd et al. (2000) reported TSS concentrations of over 1000 mg l 1 in the final drawdown of watershed ponds in Alabama, while the concentration of TSS in the initial drawdown was only 69 mg l 1. However, due to differences in harvesting techniques, final discharges from levee-style ponds in delta regions do not exhibit this same increase in TSS concentrations (Tucker et al., 2000). However, if a single regulation were mandated for both watershed and levee ponds, the rule potentially could specify treatment of the last 20% of effluents discharged. The particle-size distribution of TSS is heavily dependent on soil composition. The average size of clay particles is 1 mm, while silt and sand particles are larger. Clayey soils have a larger percentage of smaller particles; catfish ponds are typically built in soils with 30/60% clay (USDA/AAES, 1967). In areas with lower concentrations of clay soil particles, 75% reduction in TSS could be achieved with an HRT of 1 day or less (Boyd and Queiroz, 2001). However, catfish ponds in areas with high percentages of fine clays in the soil, such as in the delta region of the US, would have higher HRTs. A soil with 100% clay particles would require a maximum HRT of about 20 days, based on Eq. (1). An HRT of 1 day (minimum calculated HRT was 19 h) would remove soil particles larger than 5 mm (silt and sand particles), while an HRT of 20 days (minimum calculated HRT was 476 h) would be required to remove clay soil particles (average size of 1 mm). Eq. (1) was used to calculate sinking velocities and then HRT for settling basins. It is important to note that most of the phytoplankton in the discharge is living. For settling to occur, some type of herbicide application would be required. For this analysis, the average size of phytoplankton cells of 50 mm (Boney, 1975) was used to calculate HRT. For this average size of phytoplankton, the HRT to effectively settle
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Table 1 Particle density (rr ) and particle diameter (dp ) values used in the calculation of terminal settling velocities (vs ) and HRTs for suspended phytoplankton organisms and mineral soil particles Type of particle
rr (kg m 3)
dp (m)
vs (m s 1)
HRT (h)
Dead phytoplankton cell Mineral soil particle Mineral soil particle
1050 2500 2500
5 /10 5 5 /10 6 1 /10 6
6.91 /10 5 2.00 /10 5 8.01 /10 7
6 19 476
Average depth of the settling basin is 1.37 m (4.5 ft) (based on typical catfish pond depths (Tucker and Robinson, 1991)).
out dead phytoplankton cells is only 6 h because of the comparatively large size of the particles (Table 1). These estimated HRTs approximate other reports in the literature (Boyd et al., 1998; Teichert-Coddington et al., 1999). Boyd et al. (1998) recommended an 8-h retention time based on a laboratory study with Alabama soil samples. However, these same authors refer to a settling time of 24 h or less for effective reduction of multiple parameters. From a practical farm management perspective, farmers would likely schedule water movement from a production pond to a settling basin on a daily, not hourly, basis due to the high number of other duties and tasks performed daily by farm managers. Thus, for this analysis, retention times of 1 and 20 days were analyzed. 2.1.2. Storm event overflow and drainage discharges Important differences exist with respect to effluent composition and timing of occurrence between storm event overflow and pond draining discharges. Overflow effluents are composed primarily of phytoplankton cells and occur more frequently during the winter/spring months when the dilution capacity of receiving streams is higher. This is also the time of year when biological activity is at a minimum. Levee ponds have relatively little overflow due to the minimal watershed (excluding the pond surface). In contrast, pond draining effluents may be released any time of year, but more commonly at the end of the summer. Pond draining effluents typically contain a higher load of mineral soil particles. However, most modern catfish harvesting is conducted without water drawdown. Complete draining of grow-out ponds takes place only after 5 /10 years of continued operation and is done primarily for inventory purposes and renovation of pond bottoms. Overflow effluents are primarily composed of phytoplankton cells and are released during the coldest months of the year when the dilution capacity of streams is higher. This is also the time of year when biological activity is at a minimum. 2.1.3. Patterns of gravity-flow drainage on farms Observations on commercial catfish farms in the delta confirm that the patterns of gravity-flow drainage vary widely from farm to farm. While water discharged from some farms may flow in only one direction, others may flow in as many as four or
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five different directions. The number of settling basins required to treat pond effluents will increase with the number of drainage directions on the farm.
2.2. Design of settling basin treatment systems for model farm scenarios
2.2.1. Model farm scenarios Three farm sizes, corresponding to the 64, 128, and 256 land-ha scenarios budgeted by Keenum and Waldrop (1988) and updated by Engle and Killian (1997) and two pond sizes, 4 and 6 ha, were considered in the analysis (Table 2). Land areas represented 56, 114, and 228 water ha, respectively. Fingerling ponds were assumed to be 2 ha each and to occupy 10% of the farm area. Scenarios were developed for all farm and pond sizes for: (1) HRT’s of both 1 and 20 days; (2) treatment of either all effluents or only the last 20% of effluents; and (3) both draining and storm overflow effluents. Additional scenarios were developed to evaluate (1) new construction of settling basins in cases where land was available and (2) conversion of existing ponds into settling basins where additional land was not available. In all, 160 scenarios were developed for cost analysis.
2.2.2. Model farm drainage patterns Two principal farm drainage patterns were evaluated: (1) all ponds drain toward one side of the farm where one main drainage ditch collects all discharges for conveyance to a receiving body of water by gravity flow; and (2) a farm with two main drainage canals located on opposite sides of the farm in which each one receives the discharge of approximately 50% of the ponds (Fig. 1). Sensitivity analyses were conducted to estimate costs for up to four drainage directions. Table 2 Farm-size scenarios used for the cost analysis of settling basins for the treatment of effluents from catfish ponds Farm size/pond type
4-ha pond
6-ha pond
Ponds (number)
Total area (ha)
Ponds (number)
Total area (ha)
64-ha farm Fingerling ponds Foodfish ponds
3 12
6 48
3 8
6 48
128-ha farm Fingerling ponds Foodfish ponds
5 26
10 104
5 17
10 102
256-ha farm Fingerling ponds Foodfish ponds
12 50
24 200
12 50
24 200
Fingerling ponds are 2 ha each.
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Fig. 1. Pond layout of catfish farms designed to use settling basins to treat effluent discharges. The number of fingerling and foodfish ponds for each farm layout corresponds to the scenarios described in Table 2. Drainage pipes and canals and the location of constructed or retrofitted settling basins are shown for each diagram. (A) A 64-ha farm with 12 4-ha foodfish ponds, single drainage system, and a constructed settling basin; (B) a 256-ha farm with 34 6-ha ponds, double drainage system, and two excavated sedimentation basins; (C) a 128-ha farm with 17 6-ha ponds and double drainage system. Two foodfish ponds have been retrofitted. Arrows denote direction of water flow.
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2.2.3. Construction of settling basins Settling basins were assumed to be constructed by excavation at the end of the main drainage canal(s) on the lowest part of the farm (Fig. 1) while levee ponds are typically constructed by both cutting and filling. However, excavation would be necessary to allow water to flow by gravity into the settling basin area. This design does not require pumping except to empty the basin. Land was assumed to be available for these scenarios. Settling basin depth was assumed to be the same as that of a catfish production pond, and ponds were assumed to be drained on an average of once every 6 years. Boyd and Queiroz (2001) propose a formula to calculate settling basin volume that includes dividing HRT by pond draining time. This method requires prior knowledge of pond volumes and draining times. Many farmers do not have accurate measures of these parameters nor can the typical farm manager monitor filling the settling basin to ensure that all water is exposed to the appropriate HRT. The analysis presented in this paper adopts a more conservative approach of using average pond volumes and an HRT that is initiated when all water from the pond being drained has entered the settling basin. An arbitrary assumption was made to model multiple settling basins, but limit the size of any individual settling basin to 8 ha to avoid large (24 ha) unmanageable settling basins. Large basins would present additional problems of erosion due to turbulence from the wind and other factors. Total settling basin area ranged from 0.8 to 24 ha (Table 3). Regardless of farm size, increases in the number of drainage directions on a farm increases the number of settling basins required (Table 3). Farms with two drainage directions need twice the number of basins as compared with farms with a single drainage direction. Farms with ponds that drain in three, four, or five different directions require three, four, or five settling basins, respectively. Each block of ponds in a different location will require a separate settling basin regardless of the number or size of ponds in each block. The size of each individual settling basin remained constant across different numbers of drainage directions with a few exceptions. This is because the size of an individual basin must match the size of the ponds to be drained at the same time. Thus, the size of the largest individual pond or group of ponds to be drained at the same time dictates the size of the settling area. Longer HRT periods required larger and, in some cases, more settling basins (Table 3). In those farm scenarios with an HRT of 1 day, sedimentation basins were sized according to the total discharge of a single foodfish pond. It was assumed in this analysis that an individual foodfish pond would be drained once every 6 year and that only one foodfish pond would be drained at a time. Thus, 8 and 24-h settling times would not require additional settling area. However, fingerling ponds were assumed to be drained every year. It was further assumed that no fingerling ponds would be drained at the same time that a foodfish pond was being drained and that the farmer could delay drainage of the fingerling pond for 1 day. For example, if the average pond size in a 64-ha farm is 4 ha and all effluents are to be treated, then the settling basin must have an area of 4 ha.
180
Pond size (ha)
Draining layout
HRT (days)
Portion to be treated (%)
Settling basinsa 64-ha farm
4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
Single Single Single Single Double Double Double Double Single Single Single Single Double Double Double Double a b c d e
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
Last All Last All Last All Last All Last All Last All Last All Last All
20% 20% 20% 20% 20% 20% 20% 20%
128-ha farm
256-ha farm
Areaa (ha)
Number
Area (ha)
Number
Area (ha)
Number
0.8 4 1.2 6 0.8 4 0.8 4 1.2 6 1.2 6 1.2 6 1.2 6
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
0.8 4 2 5 0.8 4 1.2 6b 1.2 6 2 5c 1.2 6 1.2 6d
1 1 1 2 2 2 2 2 1 1 1 2 2 2 2 2
0.8 4 4.8 8 0.8 4 2.4 6 1.2 6 4.8 8e 1.2 6 2.4 6
1 1 1 3 2 2 2 4 1 1 1 3 2 2 2 4
Area of an individual settling basin. There are five fingerling ponds. Three will drain in the same direction, requiring a 6-ha basin. 10 ha of fingerling ponds require 10 ha of settling area, but pond size is limited to 8 ha; to have two 5-ha ponds used. 6-ha foodfish pond will contain the largest effluent volume to be treated at one time. Requires 24 ha of settling area, but individual pond size limited to 8 ha; thus, three 8-ha ponds.
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Table 3 Area and number of settling basins that would be required for 64, 128, and 256-ha farm scenarios that include different drainage directions, hydraulic retention times (HRT), and the proportion of the pond volume to be treated
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For an HRT of 20 days, foodfish ponds would still be drained serially once per 6 year. However, the farmer needs to have the flexibility to simultaneously drain all fingerling ponds for management purposes. Thus, for HRT’s of 20 days, the size of the settling basin was determined by either the size of a foodfish pond or the total fingerling hectareage, whichever was largest. For the 64-ha farm with 4-ha foodfish ponds and three 2-ha fingerling ponds with single drainage, one 6 ha settling basin could treat all the effluents (1.2 ha would be required to treat the last 20%). These 6 ha correspond to the total fingerling hectareage that would need to be treated simultaneously. However, if the farm has a double drainage system, two 4-ha settling basins are needed, one on each side of the farm. Since there are three fingerling ponds, two would drain to one side (requiring 4 ha of settling area). The other side would only require 2 ha of settling for the remaining fingerlings but would need to be sized at 4 ha to accommodate draining a 4-ha foodfish pond. A pumping unit (either portable or stationary) would be needed to pump treated effluents back into the discharge canal. Stationary re-lift pumps with electric motors and pumping capacities of 114, 227, and 454 l s 1 were selected for the 64, 128, and 256-ha farms, respectively. Pumps with higher capacities were selected for the larger farms because of the larger effluent volume from fingerling ponds. The costs of pumps were $4300, 5225, and 7170, respectively. Additional detail on calculations of pumping cost are included in Appendix A. 2.2.4. Converting existing growout ponds to settling basins Additional scenarios were developed for farms that would need to convert existing ponds if land were not available for new construction. In these cases, the last production pond draining into the main ditch on the farm would be taken out of production (Fig. 1). Large effluent volumes require converting several adjacent production ponds linked with culverts. The only feasible design is to convert ponds in a continuous row. For the scenarios analyzed, from 2 to 21% of the water surface area were removed from production to provide adequate settling area. Required settling basin area is the same as that calculated for constructing new basins (Table 3). However, in those instances in which the required settling area exceeds the size of existing ponds, additional foodfish ponds were taken out of production and linked (Fig. 1). Contingent upon the volume of effluents, up to six foodfish ponds may need to be converted and interconnected to provide adequate settling area. For farms with ponds that drain in multiple directions, several pond arrangements were tested to ensure the most effective conveyance of effluents. Ponds located in a continuous row resulted in the most effective design configuration because it minimized culvert and pumping distance. The culverts used to convey water among basins were assumed to be 14.5 m long, but size of the culvert varied with both the pumping rate from the drainage canal and the number of interconnected basins. Culverts would be located in the deep end of the basins (about 1.45 m) through the inner levee. Flow through the culverts must be large enough to avoid overflow from pumping in the first settling basin. Flow velocity is a direct function of available head, which is
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determined by the difference in water levels between the linked ponds. Culverts should be provided with standpipes at the inlet end, which would allow the first pond to be filled completely before releasing water to the second pond, thereby increasing effective head pressure, maximizing flow velocity, and reducing filling times. If several ponds need to be linked, each pond should be allowed to fill before transmitting water to the next pond. Calculations of culvert flow rate are detailed in Appendix B. Spreadsheet models were developed to simulate culvert flow through systems of two, three, four, and six linked basins. Culvert sizes were adjusted to allow sufficient flow speed to minimize the chances of overflow in the first basin. Table 4 summarizes the selection of culvert diameters for each system of linked basins. Conversion of growout ponds into settling basins is complicated by the need to construct an earthen dam in each principal drainage ditch. While other designs were considered, this design involves the least amount of earth work and drainpipe installation. The earthen dam is used to retain discharges from other production ponds to be pumped to the settling basin via a stationary re-lift pump. To minimize cost, this dam should be located close to the last foodfish pond (the one to be converted to a settling basin). Solids in the effluents are allowed to settle out in the basin for the appropriate period of time before final release of discharges. A new drainpipe is installed in the settling basin to convey treated effluents towards the receiving body of water (Fig. 1c). Stationary re-lift pumps equipped with water-level sensors were selected over portable PTO-driven pumps to relieve the farm manager from monitoring the pumping. Capacities of the selected pumps were 114, 257, and 454 l s 1 for the small, medium, and large farm scenarios, respectively. Farms discharging in two directions (double drainage) require two pumps.
2.2.5. Storm overflow discharge versus draining discharge Separate scenarios were developed for unintentional overflow discharges during heavy storm events. Settling basin area for storm overflow was based on the sum of the effluent volume generated by a 7.6-cm per day maximum annual storm event Table 4 Summary of culvert sizes and corresponding installation costs for interconnected settling basins Number of linked basins
Culvert diameter (cm)
Culvert cost ($ per linear m)
Pumping rate from drainage canal (l s 1)
Scenarios requiring interconnected basins (ha)
2 2 2 2 3 3 4 6
5.5 6.3 6.3 7.9 6.3 7.1 9.4 10.2
93.11 99.67 99.67 117.70 99.67 109.51 132.46 148.85
113.58 189.30 227.16 454.32 189.30 227.16 454.32 454.32
64 and 128 128 256 256 128 256 256 256
Scenarios for which linked systems were required.
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(National Weather Service Forecast Office, 2001) in addition to the settling area required for draining effluents. In the event of a large storm coinciding with pond draining, the settling basin would have to be large enough to treat both volumes simultaneously. Converting ponds is not feasible to treat storm overflow effluents. Operation of settling basins that have been linked requires monitoring that may not be available during rainfall events.
2.3. Cost analysis
2.3.1. Estimating costs for new settling basin construction Economic engineering techniques were used to determine construction and operating costs associated with the use of settling basins to treat effluents from catfish ponds. Investment costs included the excavation of settling basins and the installation of stationary re-lift pumps to drain effluents out of the excavated basins. Annual fixed costs include depreciation costs of basins, pumps and interest on the investment. Depreciation costs for settling basins and pumping equipment were estimated with a useful life of 20 and 10 years, respectively. Interest on investment was calculated with a 10% interest rate and a loan horizon of 20 and 7 years for the construction of basins and the purchase of pumps, respectively. Annual operating costs consisted of copper sulfate applications (to promote settling of phytoplankton cells) and the annual cost of pumping. Copper sulfate was applied at 2 mg l 1 at a cost of $1.32 kg 1. Pumping rate was adjusted for flow rates of effluents discharged into the main drainage canal for the different pond sizes. Settling basins were assumed to remain filled with water to prevent levee erosion in empty basins.
2.3.2. Estimating costs to convert existing growout ponds for use as settling basins Investment costs included the purchase of pumps and installation of new drainlines and culverts. Cost of drainpipe installation is dependent on pipe diameter and length of the installation. For example, a 4-ha basin needs about 274 m of 25.4cm drainpipe, which is installed at a cost of $17.39 per linear m (total cost is $1590). In contrast, a draining system for six 6-ha basins requires 678 m of 30.5-cm drainpipe installed at a cost of $12.96 per linear m, for a total cost of $8789. Annual fixed costs consisted of pump depreciation and interest on investment in the pumping equipment. Annual operating costs included copper sulfate applications and decreased revenue from catfish production lost by taking ponds out of production. Published enterprise budgets (Engle, 1998; Engle and Killian, 1997) were used to obtain estimates of lost revenue. Pond yields of 5682 kg ha 1 were assumed in each case.
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Table 5 Total investment and total annual operating costs of newly-constructed settling basins for 64, 128, and 256-ha catfish farms, 4-ha ponds, single drainage direction Farm size
Annual costa ($)
Investment cost ($) Total $
$ ha 1
Total $
$ ha 1
$ kg 1
64-ha All effluents Last 20% of effluents
127 624 /191 346 29 078 /41 092
1994 /2990 454 /642
15 950 /23 507 3885 /5388
249 /367 61 /84
0.05 /0.08 0.01 /0.02
128-ha All effluents Last 20% of effluents
128 642 /313 246 30 096 /66 993
1005 /2447 235 /523
16 273 /38 871 4208 /8725
127 /304 33 /68
0.02 /0.07 0.01 /0.02
256-ha All effluents Last 20% of effluents
130 781 /747 015 32 235 /155 463
511 /2918 126 /607
16 947 /92 393 4882 /19 969
66 /361 19 /78
0.01 /0.07 0.01 /0.02
a Includes both operating costs and annual fixed costs that include depreciation and interest on the investment.
3. Results and discussion 3.1. Costs of constructing settling basins on new land Table 5 indicates that large amounts of investment capital are needed to construct settling basins. The amount of investment capital is heavily dependent on the drainage layout of the farm and the scope of regulations governing the release of effluents. For instance, investment costs in a 256-ha catfish farm may range from $32 235 to 747 015 ($126 /2918 ha 1), depending on what proportion of the effluent volume is treated. Total investment costs were somewhat lower for smaller farms but higher on a per-ha basis. For example, on the 64-ha farm, total investment cost ranged from $29 078 to 191 346 per farm ($454 /2990 ha 1), indicating a disproportionately larger financial burden on the smaller farm to acquire the investment capital required to install settling basins. As a percentage of the total investment cost of the entire catfish farm, these costs range from 5 to 32% for the 64ha farm and 2/33% for the 256-ha farm. It is clear that the cost of constructing settling basins is not a function of farm size. It is a relatively constant amount that represents a relatively greater per-ha cost on smaller farms than on larger farms. Total annual operating costs ranged from $61 to 367 ha 1 for the 64-ha farm and decreased to $19/361 ha 1 for the largest farm size considered. Total annual fixed costs comprised 83/92% of total annual costs. These estimates of total annual operating costs for newly constructed settling basins were relatively low because the design criteria were set for water to flow by gravity into the settling basin. There are farm situations in which this may not be possible; the addition of pumping stations would require additional investment and operating costs. Costs of producing catfish would increase by at least $0.01 /0.08 kg 1 if farmers would be required to construct new settling basins if their entire farm drained in the
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Fig. 2. Total investment and total annual operating costs of newly-constructed settling basins for a 128-ha catfish farm with ponds that drain in one, two, three, or four different directions, an HRT of 1 day and treatment of only the last 20% of effluents.
same direction (Table 5). Costs kg 1 will be much higher for the farms that have more than one drainage. Given concerns over price competitiveness with low-priced imports, any cost increase will reduce the competitive position of catfish farmers. Moreover, this cost estimate assumes that financing for the increased capital will be readily available. Caps on lending by bank examiners may prevent easy access to the required capital. The most dramatic impact on the costs of settling basins was the number of drainage directions (Fig. 2). The number of required settling basins increased with the number of drainage directions and the investment cost also increased in direct proportion to the number of drainage directions. Total annual costs increased at the same percentage rate as the increase in investment cost. Since the number of drainage directions depends upon the location and topography associated with each farm, it is independent of farm size. Even small farms of 64 ha or less can have ponds that drain in three or more different directions. Fig. 2 presents increasing costs for increasing numbers of drainage directions only for conservative scenarios of a 1-day HRT and treatment of only the last 20% of effluents. Nevertheless, the same pattern of increasing costs with increasing numbers of drainage directions holds true for the other scenarios. Total costs of constructing and operating settling basins were slightly higher on larger than on smaller farms (Fig. 3). However, on a per-ha basis, settling basin costs decreased with larger farm sizes. Thus, requiring settling basins would impose a disproportionately greater financial burden on smaller farms as opposed to larger farms. Both investment and annual operating costs increased with larger pond size (Fig. 3). Costs increased faster with larger pond sizes than with larger farm sizes. This is because the size of the settling basin is dependent upon the size of the largest pond to
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Fig. 3. Total investment and total annual operating costs of newly-constructed settling basins for 64, 128, and 256-ha catfish farms with either 4 or 6-ha ponds, HRT of 1 day with ponds draining in one direction, and treatment only of the last 20% of effluents.
be drained, not the size of the farm. Catfish farmers do not drain all their ponds every year nor do they drain them all at the same time. The larger size of settling basin required to capture the water draining from larger ponds requires additional excavation and earthmoving costs. Moreover, the potential for re-suspension of clay particles due to wind turbulence and erosion in the basin would likely increase with the size of the settling basin. Total investment and total annual operating costs both increased more than fourfold by requiring that all effluents from the pond be treated in settling basins as compared with requiring that only the last 20% of effluents be treated (Fig. 4). These same cost parameters increased by approximately 40% as HRT increased from 1 to 20 days. 3.2. Cost of converting existing growout ponds into settling basins Converting existing production ponds into settling basins resulted in a more narrow range of investment costs than those of newly-constructed settling basins.
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Fig. 4. Total investment and total annual operating costs of newly-constructed settling basins for HRT’s of 1 and 20 days for draining all effluents or only the last 20% of effluents for a 128-ha catfish farm with 4ha ponds that drain in two different directions.
These costs ranged from $5837 to 12 039 for the 64-ha farm, $6762 /18 886 for the 128-ha farm, and $8707 /30 574 for the 256-ha farm (Table 6) (see Appendix D for itemized costs). Total capital invested in the business increased by 1.0 /2.0, 0.6 /1.7, and 0.4 /1.4% for the 64, 128, and 256-ha farms, respectively. Decreasing costs per ha on larger farms result from economies of scale and illustrate the greater economic impact that would occur on smaller farms. Total annual costs for converting production ponds into settling basins ranged from $10 935 to 31 663 for the 64-ha farm to a range of $12 603/68 463 for the 256ha farm. This represented $171/495 ha 1 for the 64-ha farm to $49 /267 ha 1 for the larger farm size. Over half of these costs are related to the lost revenue and continued fixed costs associated with ponds taken out of production. These additional operating costs represent an increase of 2.6 /7.6, 1.3 /4.9, and 0.8 /4.2% in total annual operating costs for the 64, 128, and 256-ha farms, respectively. Production costs would increase by $0.01 /0.14 kg 1, depending upon the scenario (Table 6). This magnitude of cost increase would not be economically feasible even for the single and double drainage scenarios included in Table 6. In addition to the increased capital costs, the system must be designed and monitored carefully, especially when several converted ponds are interconnected. If pumping rates from the drainage canal exceed the conveyance capacity of the system, the first basin will overflow. A more logical and simple approach may be to use each production pond as its own settling basin as suggested by Tucker et al. (2000). Certain good management practices, such as harvesting ponds without ever draining the last 20% of water (for watershed ponds), or holding this volume of water in
188
Pond size (ha)
Draining layout
HRT (days)
Percentage to be treated (%)
Farm size 64-ha farm
4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
Single Single Single Single Double Double Double Double Single Single Single Single Double Double Double Double a
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
Last All Last All Last All Last All Last All Last All Last All Last All
20% 20% 20% 20% 20% 20% 20% 20%
128-ha farm
256-ha farm
Investment ($)
Annuala ($)
$ kg 1 Investment ($) ($)
Annual ($)
$ kg 1 Investment ($) ($)
Annual ($)
$ kg 1 ($)
5837 5837 5837 8656 10 891 10 891 10 891 10 891 6413 6413 6413 6413 12 039 12 039 12 039 12 039
10 935 10 935 10 935 21 135 21 625 21 625 21 625 21 625 15 955 15 955 15 955 15 955 31 663 31 663 31 663 31 663
0.04 0.04 0.04 0.09 0.09 0.09 0.09 0.09 0.06 0.06 0.06 0.06 0.14 0.14 0.14 0.14
10 949 10 949 10 949 30 872 21 526 21 526 21 526 41 438 15 819 15 819 15 819 30 675 31 278 31 278 31 278 31 278
0.02 0.02 0.02 0.05 0.03 0.03 0.03 0.07 0.02 0.02 0.02 0.05 0.05 0.05 0.05 0.05
12 603 12 603 23 773 67 882 24 471 24 471 24 471 68 463 17 976 17 976 17 976 67 287 35 227 35 227 35 227 68 015
0.01 0.01 0.02 0.05 0.02 0.02 0.02 0.05 0.01 0.01 0.01 0.05 0.03 0.03 0.03 0.05
6762 6762 6762 13 012 12 741 12 741 12 741 18 886 7338 7338 7338 11 154 13 889 13 889 13 889 13 889
Includes both operating costs and annual fixed costs that include depreciation and interest on the investment.
8707 8707 13 914 29 044 16 631 16 631 16 631 30 574 9283 9283 9283 23 168 17 779 17 779 17 779 26 198
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Table 6 Investment and annual costs of converting growout ponds into settling basins for 64, 128, and 256-ha farm scenarios by the number of drainage directions, HRT, and proportions of pond volume to be treated
Foodfish pond size (ha)
64-ha farm 4 4 6 6
Draining layout
HRT (days)
Overflow caused Required sediby a 7.6 cm storm mentation area event (m3) (ha)
Basins constructed (number)
Area of indivi- Total investdual settling ba- ment costs sins (ha) ($)a
Annual costs
Operating costs ($)b
Fixed costs ($)c
Total costs
Single Double Single Double
1 1 1 1
41 634 41 634 41 634 41 634
3 3 3 3
1 2 1 2
3 1.5 3 1.5
96 356 100 314 96 356 100 314
1046 1046 1046 1046
11 086 11 740 11 086 11 740
12 132 12 786 12 132 12 786
128-ha farm 4 Single 4 Double 6 Single 6 Double
1 1 1 1
87 894 87 894 86 352 86 352
6.33 6.33 6.22 6.22
1 2 1 2
6.33 3.17 6.22 3.11
200 091 204 920 196 756 201 468
2185 2186 2148 2148
22 849 23 643 22 472 23 253
25 034 25 829 24 620 25 401
256-ha farm 4 Single 4 Double 6 Single 6 Double
1 1 1 1
172 704 172 704 175 788 175 788
12.44 12.44 12.67 12.67
2 2 2 2
6.22 6.22 6.33 6.33
390 856 397 402 397 779 404 072
4246 4247 4322 4321
44 495 45 576 45 278 46 329
48 741 49 823 49 600 50 650
a b c
Includes construction costs of basins and acquisition of pumps. Includes copper sulfate and pumping costs. Includes depreciation and interest on investment.
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Table 7 Total costs of construction of settling basins for overflow effluents from catfish farms by farm size
189
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existing farm ditches prior to final discharge, may be implemented to achieve reductions in solid and nutrient loads comparable to what could be accomplished with the construction of independent and expensive structures.
3.3. Costs of constructing settling basins for the treatment of overflow effluents Total investment costs associated with construction of settling basins to treat overflow effluents from catfish farms ranged from $96 356 to 100 314 for 64-ha farms, $196 756/204 920 for 128-ha farms, and $390 856/404 072 for 256-ha farms (Table 7) (see Appendix E for itemized costs). These costs represent increases of 16 / 17, 18, and 18% above investment costs for the 64, 128, and 256-ha farms, respectively, without the installation of settling basins.
4. Conclusions Costs associated with the implementation of settling basins on static levee-style pond catfish farms are dependent on the size and number of basins, and whether sufficient land is available for basin construction or if existing production ponds must be converted. The size of settling basins on catfish farms is controlled by factors such as the type of effluent to be treated, pond layout, size of the largest pond, number of drainage canals, and scope of regulations governing the release of aquacultural effluents. Moreover, the effectiveness of settling basins in practical operation on farms is unknown. There is potential for re-suspension of particles due to wind turbulence and erosion in the basins. Costs increase dramatically as the number of different farm drainage directions increases. Requirements to treat all effluents as compared with only the last 20% also increase costs sharply. Larger pond sizes result in greater treatment costs due to the necessity of constructing larger settling basins. Costs are higher on larger farms, but treatment costs on a per-ha basis are lower for larger farms than for smaller farms. The cost for small farms is particularly burdensome, increasing investment costs by 5/13%. Increasing HRT from 1 to 20 days did not increase costs by as much as the number of drainage directions or pond sizes. However, increasing HRT increases costs rapidly if all effluents are required to be treated. The costs estimated in this study do not cover all possible scenarios. Farms that drain in more than five directions and those that require pumping instead of gravity flow, will have much higher costs than those estimated in this analysis. Increased production costs of the magnitude of those estimated in this study cannot be passed on to processors. Many catfish farmers have long-term financing at maximum credit levels with little or no credit reserve and cannot borrow additional capital. The financial burden is particularly high for smaller-scale farms. The costs associated with the use of settling basins to treat effluents from catfish ponds are too high to be economically feasible.
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Acknowledgements The authors gratefully acknowledge the helpful review comments and suggestions of Nathan Stone, Steve Lochmann, and Bartholomew Green. The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 99-38500-7375 from the United States Department of Agriculture.
Appendix A Hourly pumping costs were calculated using the following equation (Yoo and Boyd, 1994): C
Q TDH 0:746 R 3960 Ep Ed Em
(3)
where C is the pumping cost per hour of operation ($ h 1); Q is the discharge rate (l s 1); TDH, total dynamic head (m); R is cost of electricity ($ kW 1 h 1); Ep is pump efficiency (decimal); Ed is drive train efficiency in decimal and Em is the motor efficiency (decimal). Total dynamic head (TDH ) is 3.66 m while assumed cost of electricity is $0.065 kW 1 h 1. Manufacturer specifications were consulted to obtain values for the remaining items in the equation. The annual number of pumping hours was estimated for each farm scenario, taking into account that foodfish ponds are drained only once every 6 years, while fingerling ponds are drained annually. It should be noted that although only a portion of effluents may be subject to treatment, the total volume of effluents needs to circulate through and be pumped out of the basin. Annual pumping costs were calculated based on the estimated costs of operation per hour and total number of hours.
Appendix B The following equation was used to estimate culvert flow rates (Yoo and Boyd, 1994): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2gH Q A (4) 1 km kc L where Q is the flow rate (cm s 1); A is the cross-sectional area of culvert flow (m2); g is the gravity acceleration (32.2 m s 2); H is the effective head causing flow in culvert (fm); km is the minor-loss coefficient caused by culvert entrance. Assumed to be 0.8. kc is the friction loss coefficient for culvert flowing full; and L is the culvert length (m).
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In turn, kc is calculated using the equation: kc
29:16n2 d 4=3
(5)
where n is the Manning’s roughness coefficient of culvert. Assumed to be 0.016 for plastic tubing and d is the culvert diameter (cm). Calculation of effective head is dependent on flow conditions at the culvert outlet. For a free-flow culvert, effective head is equivalent to water depth above culvert inlet in the head-pond minus 0.6 times the culvert diameter. When a sufficient volume of water has been transmitted to the receiving pond, both ends of the culvert are submerged and efficient head is expressed as the difference in water levels between the two ponds. The difficulty with using Eq. (4) is that it only provides an estimate of flow rate at a specific head. However, effective head is at a maximum at the initial stage of flow, but it constantly declines as the differences in level between headwater and tailwater drops. In order to estimate average flow rate during the entire period of time required to fill the receiving pond, one must obtain several estimates of Q at successive time intervals, which should be sufficiently short (e.g. 1 s). Effective head (H) is recalculated for each time interval because of the change in water level caused by the culvert flow. Successive estimates of Q are obtained until the tail-pond has been completely filled, then average flow rate is simply calculated as the arithmetic mean of the various Q estimates. Besides verifying water level in the tail-pond after each time interval, water level must also be monitored in the head-pond since water is constantly being pumped from the main drainage canal at an average rate and overflow needs to be avoided. The only variable in Eq. (4) that can be manipulated to regulate culvert flow is culvert diameter (A ).
Appendix C Table 8
Appendix D Table 9
Appendix E Table 10
Table 8 Itemized costs of newly-constructed settling basins for all scenarios evaluated Farm/settling Drainages basin size (ha) (number)
HRTa Volume treated (%)
Investment
Pump installa- Total investtion ($) ment ($)
Total operating costs
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
24 348 122 894 36 362 186 616 48 696 245 787 48 696 245 787 36 632 184 616 36 632 184 616 73 264 369 232 73 264 369 232
4730 4730 4730 4730 9460 9460 9460 9460 4730 4730 4730 4730 9460 9460 9460 9460
29 078 127 624 41 092 191 346 58 156 254 647 58 156 255 247 41 362 189 346 41 362 189 346 82 724 378 692 82 724 378 692
3442 14 569 4828 21 540 6884 29 138 6884 29 138 4828 21 540 4828 21 540 9657 43 081 9657 43 081
235 1173 352 1759 469 2345 469 2345 352 1759 352 1759 704 3517 704 3517
208 208 208 208 208 208 208 208 208 208 208 208 208 208 208 208
443 1381 560 1967 677 2553 677 2553 560 1967 560 1967 912 3725 912 3725
3885 15 950 5388 23 507 7561 31 691 7561 31 691 5388 23 507 5388 23 507 10 569 46 806 10 569 46 806
128-ha 4 4 4 4 4 4 4 4
1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100
24 348 122 894 61 245 307 498 48 696 245 788 73 264 369 232
5748 5748 5748 5748 11 495 11 495 11 495 11 495
30 096 128 642 66 993 313 246 60 191 257 283 84 759 380 727
3592 14 719 7757 35 559 7185 29 439 9958 43 381
234 1172 586 2931 469 2345 703 3517
381 381 381 381 381 381 381 381
615 1553 967 3312 850 2726 1084 3898
4208 16 273 8725 38 871 8035 32 164 11 042 47 280
193
64-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
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Excavation ($)
Annual fixed costs ($)
194
Table 8 (Continued ) Farm/settling Drainages basin size (ha) (number)
HRTa Volume treated (%)
Investment
Annual fixed costs ($) Pump installa- Total investtion ($) ment ($)
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
6 6 6 6 6 6 6 6
1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100
36 632 184 616 61 245 307 498 73 264 369 232 73 264 369 232
5748 5748 5748 5748 11 495 11 495 11 495 11 495
42 380 190 364 66 993 313 246 84 759 380 727 84 759 380 727
4979 21 691 7757 35 559 9958 43 381 9958 43 381
352 1759 587 2931 704 3518 704 3518
376 376 376 376 376 376 376 376
728 2135 963 3307 1080 3894 1080 3894
5707 23 826 8720 38 866 11 037 47 275 11 037 47 275
256-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
24 348 122 894 147 576 739 128 48 696 245 788 147 130 738 464 36 632 184 616 147 576 739 128 73 264 369 232 147 130 738 464
7887 7887 7887 7887 15 774 15 774 15 774 15 774 7887 7887 7887 7887 15 774 15 774 15 774 15 774
32 235 130 781 155 463 747 015 64 470 261 562 162 904 754 238 44 519 192 503 155 463 747 015 89 038 385 006 162 904 754 238
3909 15 035 17 823 84 619 7817 30 071 18 929 85 697 5295 22 007 17 823 84 619 10 590 44 014 18 929 85 697
234 1172 1407 7034 469 2345 1407 7034 351 1758 1406 7034 703 3517 1406 7034
739 739 739 739 739 739 739 739 748 748 748 748 748 748 748 748
973 1911 2146 7773 1208 3084 2146 7773 1099 2506 2154 7782 1451 4265 2154 7782
4882 16 947 19 969 92 393 9025 33 155 21 075 93 470 6394 24 513 19 978 92 401 12 041 48 279 21 084 93 479
a
Hydraulic residence time.
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Excavation ($)
Total operating costs
Table 9 Itemized costs of converting existing settling basins for all scenarios evaluated Farm/settling ba- Drainages sin size (ha) (number)
Total ($)
Annual fixed costsc ($)
Total operating costs
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
4300 4300 4300 4300 8600 8600 8600 8600 4300 4300 4300 4300 8600 8600 8600 8600
1537 1537 1537 4356 2291 2291 2291 2291 2113 2113 2113 2113 3439 3439 3439 3439
5837 5837 5837 8656 10 891 10 891 10 891 10 891 6413 6413 6413 6413 12 039 12 039 12 039 12 039
9893 9893 9893 19 214 19 707 19 707 19 707 19 707 14 470 14 470 14 470 14 470 28 862 28 862 28 862 28 862
879 879 879 1758 1758 1758 1758 1758 1319 1319 1319 1319 2638 2638 2638 2638
163 163 163 163 159 159 159 159 167 167 167 167 163 163 163 163
1042 1042 1042 1921 1918 1918 1918 1918 1485 1485 1485 1485 2801 2801 2801 2801
10 935 10 935 10 935 21 135 21 625 21 625 21 625 21 625 15 955 15 955 15 955 15 955 31 663 31 663 31 663 31 663
128-ha 4 4 4 4 4 4 4 4 6
1 1 1 1 2 2 2 2 1
1 1 20 20 1 1 20 20 1
20 100 20 100 20 100 20 100 20
5225 5225 5225 5225 10 450 10 450 10 450 10 450 5225
1537 1537 1537 7787 2291 2291 2291 8436 2113
6762 6762 6762 13 012 12 741 12 741 12 741 18 886 7338
9773 9773 9773 27 937 19 468 19 468 19 468 37 622 14 215
879 879 879 2638 1758 1758 1758 3517 1319
297 297 297 297 299 299 299 299 285
1176 1176 1176 2935 2058 2058 2058 3816 1604
10 949 10 949 10 949 30 872 21 526 21 526 21 526 41 438 15 819
195
64-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
C.R. Engle, D. Valderrama / Aquacultural Engineering 28 (2003) 171 /199
HRTa Volume trea- Investment ted (%) Pumpsb Drains/cul($) verts ($)
196
Table 9 (Continued ) Farm/settling ba- Drainages sin size (ha) (number)
Total ($)
Annual fixed costsc ($)
Total operating costs
Total annual costs ($)
CuSO4 ($)
Pumping ($)
Total ($)
6 6 6 6 6 6 6
1 1 1 2 2 2 2
1 20 20 1 1 20 20
100 20 100 20 100 20 100
5225 5225 5225 10 450 10 450 10 450 10 450
2113 2113 5929 3439 3439 3439 3439
7338 7338 11 154 13 889 13 889 13 889 13 889
14 215 14 215 27 752 28 353 28 353 28 353 28 353
1319 1319 2638 2638 2638 2638 2638
285 285 285 288 288 288 288
1604 1604 2923 2925 2925 2925 2925
15 819 15 819 30 675 31 278 31 278 31 278 31 278
256-ha 4 4 4 4 4 4 4 4 6 6 6 6 6 6 6 6
1 1 1 1 2 2 2 2 1 1 1 1 2 2 2 2
1 1 20 20 1 1 20 20 1 1 20 20 1 1 20 20
20 100 20 100 20 100 20 100 20 100 20 100 20 100 20 100
7170 7170 7170 7170 14 340 14 340 14 340 14 340 7170 7170 7170 7170 14 340 14 340 14 340 14 340
1537 1537 6744 21 874 2291 2291 2291 16 234 2113 2113 2113 15 998 3439 3439 3439 11 858
8707 8707 13 914 29 044 16 631 16 631 16 631 30 574 9283 9283 9283 23 168 17 779 17 779 17 779 26 198
11 089 11 089 21 379 61 972 22 100 22 100 22 100 62 575 16 031 16 031 16 031 61 385 31 985 31 985 31 985 62 136
879 879 1758 5275 1758 1758 1758 5275 1319 1319 1319 5275 2638 2638 2638 5275
635 635 635 635 613 613 613 613 627 627 627 627 604 604 604 604
1514 1514 2394 5910 2371 2371 2371 5888 1945 1945 1945 5902 3242 3242 3242 5879
12 603 12 603 23 773 67 882 24 471 24 471 24 471 68 463 17 976 17 976 17 976 67 287 35 227 35 227 35 227 68 015
a b c
Hydraulic residence time. Includes installation. Includes the revenue lost due to removing pond(s) from production, depreciation on pumps, pipes, and culverts, and interest on the investment.
C.R. Engle, D. Valderrama / Aquacultural Engineering 28 (2003) 171 /199
HRTa Volume trea- Investment ted (%) Pumpsb Drains/cul($) verts ($)
Farm/settling ba- Drainages sin size (ha) (number)
HRTa Investment Excavation ($)
Annual fixed costs ($) Pump installation ($)
Total investment ($)
Total operating costs CuSO4 ($)
Pumping ($)
Total annual costs ($) Total ($)
64-ha 4 4 6 6
1 2 1 2
1 1 1 1
81 432 82 199 81 432 82 199
4730 9460 4730 9460
92 056 45 857 92 056 45 857
11 086 11 740 11 086 11 740
223 223 223 223
879 440 879 440
1102 1102 1102 1102
12 189 12 842 12 189 12 842
128-ha 4 4 6 6
1 2 1 2
1 1 1 1
171 193 171 903 168 285 168 890
5748 11 496 5748 11 496
194 866 97 235 191 531 95 509
22 849 23 643 22 472 23 253
441 441 434 434
1856 929 1824 912
2297 2298 2258 2258
25 146 25 941 24 730 25 511
256-ha 4 4 6 6
1 2 1 2
1 1 1 1
168 285 336 570 171 193 342 385
7887 15 774 7887 15 774
383 686 191 531 390 609 194 866
44 495 45 576 45 278 46 329
802 802 816 816
3647 1824 3713 1856
4449 4451 4529 4528
48 945 50 027 49 807 50 858
a
Hydraulic residence time.
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Table 10 Itemized costs of newly-constructed settling basins for overflow effluents
197
198
C.R. Engle, D. Valderrama / Aquacultural Engineering 28 (2003) 171 /199
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