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Intensification of pond aquaculture and high rate photosynthetic systems
Aquacultural Engineering 28 (2003) 65 /86 www.elsevier.com/locate/aqua-online
Intensification of pond aquaculture and high rate photosynthetic systems D.E. Brune a,*, G. Schwartz a, A.G. Eversole b, J.A. Collier a, T.E. Schwedler b a
Department of Agricultural and Biological Engineering, Clemson University, Clemson, SC 29634, USA b Department of Aquaculture, Fisheries and Wildlife, Clemson University, Clemson, SC 29634, USA Received 15 August 2002; accepted 3 February 2003
Abstract Aquaculture production systems may range from tanks and raceways, in which water quality is controlled by water dilution and discharge to the environment to captive water systems, in which water quality is controlled by microbial reactions within the tank or pond. Attempts at intensification of pond aquaculture beyond the commonplace practice of supplemental aeration may be classified into categories of physical/chemical techniques and a broad range of microbial techniques. Most of these techniques are directed at raising the ‘ceiling’ of the system ammonia detoxification rate. Physical /chemical techniques for intensification of pond aquaculture have included use of in-pond cages and raceways, water blending and shading of the algal community, as well as, direct flocculation and removal of algal and bacteria biomass from ponds. A variety of microbial processes can be used to reduce ammonia levels in a conventional pond. These processes include nitrification/denitrification, photosynthesis, and heterotrophic bacterial re-growth. In this paper, simplified microbial growth fundamentals, and elemental mass balances are used to analyze and compare the various aquaculture intensification techniques and, in particular, to compare conventional and heterotrophic techniques to the use of high rate photosynthetic systems. Direct or indirect photosynthetic systems include enhanced algal systems (with water mixing), polyculture, hydroponics, wetlands, and terrestrial irrigation/fertilization. The development of Clemson University’s Partitioned Aquaculture System (PAS) constitutes an attempt to combine a number of the various physical, chemical, and microbial intensification techniques into a single integrated system. The PAS represents an adaptation of high rate microalgal culture to produce a sustainable, minimal discharge, high yield, and more controllable fish production
* Corresponding author. E-mail address: [email protected] (D.E. Brune). 0144-8609/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0144-8609(03)00025-6
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process. The PAS combines the advantages of process control of recirculating tank aquaculture with the lower costs of earthen pond aquaculture. Central to the economic success of the PAS is the use of low speed (1 /3 r.p.m.) paddlewheels as an energy efficient means of establishing a uniform water velocity field within an aquaculture pond. The PAS represents a redesign of the conventional aquaculture pond culture technology providing a spectrum of applications ranging from moderate yield (6700 /11 200 kg/ha) ‘engineered ecosystems’ to high yield (16 800 /33 600 kg/ha) controlled ‘production processes’. This high rate photosynthetic system offers the potential for a 90% reduction in total water usage per unit of fish produced. The modular nature of the PAS, the increased productivity per unit area, reduced water requirement, and reduced environmental impact offers the potential for fish culture systems to be installed at sites not currently suitable for conventional aquaculture. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Supplemental aeration; Intensification techniques; Partitioned aquaculture systems
1. Aquaculture production systems; overview Aquaculture production systems range from tanks and raceways, in which water quality is controlled by water dilution or discharge to the environment, to bacterial or algal-driven systems in which water quality is controlled by microbial reactions within captive water held in tanks or ponds. Trout production systems are typical of the flow-controlled aquaculture system. Fig. 1 illustrates the sequence of water quality constraints typically encountered in tank fish culture. If water flow to a typical culture system (operated at conventional carrying capacities) is interrupted, the dissolved oxygen level within the tank will fall to critical concentrations within 30 /60 min. Pure oxygen (O2) can be supplied to the tank, however, within 3/5 h respiratory carbon dioxide (CO2) production from the
Fig. 1. Typical water quality limitations in fish culture tanks and approximate time to impact (Gunther et al., 1981).
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fish will suppress the water pH to toxic levels. This outcome is predictable, and can be calculated, assuming a 1/1 molar ratio of CO2/O2 production/consumption and using pH predictions from carbonate equilibrium chemistry (see Fig. 1). Air aeration supplied to the tank will meet both the O2 supply and CO2 degassing needs, but within 24 h or less, free ammonia levels within the tank will reach toxic concentrations as can be predicted from a simple nitrogen mass balance (Fig. 1). Beyond this point in time, ammonia removal will be required and this need can be met through either water discharge/exchange or treatment of captive water with microbial processes or physical/chemical ammonia removal techniques. US pond aquaculture practices have changed significantly over the last 30 years. Pond catfish production has increased from around 1100 /1700 kg/ha in the 1960’s to a typical production of 4500/5600 kg/ha today. Supplemental pond aeration allows for the increase in fish production from 1700/5600 kg/ha. Algal primary production in a typical aquaculture pond can assimilate 1/3 gm C/m2-day corresponding to 0.5 mg/l-day of nitrogen addition or approximately 90 /112 kg feed/ha-day, ultimately limiting production at the 4500 /5600 kg/ha level. Attempts at intensification of pond aquaculture beyond the commonplace practice of supplemental aeration can be classified into categories of physical/chemical techniques and a broad range of microbial techniques. Most of these techniques are directed at raising the ‘ceiling’ of the pond ammonia detoxification rate.
2. Physical /chemical intensification techniques Physical /chemical techniques for intensification of pond aquaculture have included use of in-pond cages and raceways (Lorio, 1994; Lazur and Britt, 1997), water blending (Busch et al., 1978; Busch and Goodman, 1983), shading of the algal community and direct flocculation and removal of algal biomass from ponds. The cage culture or confinement technique offers the advantages of raceway fish culture (see below), however, without additional modifications in the management of bulk pond water, overall net production per unit water area is not improved. Water blending can offer advantages in improving oxygen distribution in ponds resulting in a reduction in the overall energy requirements for pond aeration however, standby equipment for emergency aeration of the pond is still required. Attempts to eliminate or shade the algal community within the pond can reduce the diurnal oxygen fluctuations induced by algal photosynthesis and occurrence of toxic algal blooms, however, this approach also does not address the required nitrogen management issue. Selective partial algal standing crop manipulation with copper sulfate addition to ponds has been shown to increase net profits from conventional ponds by approximately 10% (Tucker et al., 2000). Direct algal, bacterial or waste removal by flocculation or mechanical means has not yet been demonstrated to be economically viable (Chesness et al., 1976).
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3. Microbial intensification techniques A variety of microbial processes will react to remove or add ammonia from/to a conventional aquaculture pond. These include nitrification, denitrification, photosynthesis, mineralization or heterotrophic bacterial re-growth (illustrated in Fig. 2). In the following presentation a number of microbial intensification techniques that had been studied and presented by various investigators (for both shrimp and finfish) will be analyzed and compared using approximate pond organic and nitrogen mass balances (Fig. 2), and simplified microbial growth reactions (Fig. 3). It will be useful to represent organic and nitrogen input to an aquaculture water column as approximately 36% of the input feed (as BOD) and as 75% (90% for shrimp) of the input nitrogen (as ammonia/N, see Fig. 2) The BOD content of typical aquaculture feed is assumed to be 60% (as mg/l BOD5) of the dry weight of the feed. These values can vary significantly depending upon the handling of the fecal and bacterial solids within the system. If no solids are removed from the pond or tank these numbers represent the maximum expected BOD and nitrogen loading to the water column assuming no internal nitrogen recycle, which can occur, depending on the degree of interaction of the water column with the sediments. Heterotrophic microbial production occurs at a higher rate (10 h generation time)
Fig. 2. Typical aquaculture pond nitrogen cycle and fish oxygen demand mass balance.
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Fig. 3. The three important microbial processes dominating water quality in pond aquaculture systems.
that either autotrophic photosynthesis or nitrification (from 24 to 48 h generation time, see Fig. 3). The growth rates of algae and nitrifying bacteria are similar, however, the yield coefficient for photosynthesis is approximately 57 times larger than nitrification (Fig. 3). 3.1. Bacterial intensification techniques Bacterial or heterotrophic techniques for intensification of pond aquaculture have included the addition of nitrifying columns (Greene, 1971), increased pond aeration and mixing of sediments to improve in-pond suspended culture nitrification (Avnimelech et al., 1986, 1999), and addition of carbonaceous matter to the pond
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to stimulate nitrogen uptake by heterotrophic bacteria growth (Avnimelech et al., 1994). All of these techniques require high levels of water pumping, pond mixing, or aeration. Each technique has been demonstrated to increase production by decreasing the ammonia concentration in the pond however, the increased production associated with such techniques is usually insufficient to offset the increased energy costs and capital costs (in catfish culture in particular) associated with the increased water pumping and aeration. Pond sediment/sludge management or removal (not widely used in the aquaculture industry) has been demonstrated to decrease nitrogen recycle in ponds thereby increasing production (Hopkins et al., 1994). Most sludge removal techniques demonstrated to date have been limited to relatively small systems and have not been shown to be transferable to large-scale systems. The most widely applied technique for management of pond sediments has been drying of the pond sediments allowing for mineralization and denitrification processes to reduce the sediment organic carbon and nitrogen content, although this practice requires that pond water be discharged, or moved to another storage area and that the pond be removed from production for a significant length of time. Basic microbial growth fundamentals (Fig. 3) can be used to gain insight, in a general way, into the behavior of many aquaculture systems. As an example, a moderately fed shrimp pond (Hopkins et al., 1994) is typically seen to produce 5600 / 6700 kg shrimp/ha per season (Fig. 4). During the reported 130-day growing cycle, average water column concentrations of volatile solids (VS) are seen to range from 30 to 80 mg/l with an average of 40 mg/l. Oxidized levels of nitrogen (NO3 and NO2) are low (B/1 mg/l). Feed application rates to the pond are intentionally kept below an average of 67 kg/ha feed day to avoid excessive pond ammonia concentrations. Applying the microbial yield coefficient to the input nitrogen and input BOD to the pond (90% of feed nitrogen and 36% of feed BOD) suggests a mixed heterotrophic/ photosynthetic biomass within a conventional shrimp pond of 32/49 mg VS/l at a cell age controlled by the microbial sedimentation and degradation rate (typically 20
Fig. 4. Observed vs. predicted range of bacterial and algal standing crop in typical shrimp production pond at 53 kg feed/ha-day (Hopkins et al., 1994).
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days, see Fig. 4). Internal nitrogen recycle from the pond sediments is frequently observed to double conventional aquaculture pond algal production and standing crop, resulting in predicted pond total VS concentrations ranging from 36 to 98 mg / l, similar to the observed range in pond VS concentrations of 30 /80 mg/l. Differences in operator handling of pond sludge/biomass and short term release of ‘pulses’ of nitrogen from the sediments can alter the internal nitrogen/BOD recycle by as much as two to fourfold resulting in short bursts of increased algal production and standing crop. Alternatively, high rate shrimp production systems may be pushed to (reduced protein) feed application rates as high as 22/34 000 kg feed/ha season with shrimp production at 13 /16 000 kg/ha season (Fig. 5) by using a technique herein referred to herein as the Avnimelech/McIntosh method (McIntosh, 2001). The higher C/N ratio of the low percent protein feed drives heterotrophic bacterial production within the pond reducing ammonia concentrations in the water column below that which would be expected with 36/40% protein feeds typically used in shrimp aquaculture. Applying microbial yield coefficients to the observed input BOD and ammonia additions to this system suggest a bacterial/photosynthetic dominated microflora (Fig. 6). Although there is sufficient steady state water column ammonia concentration present in the pond to support algal growth, significant algal growth was not visually reported in this system, although direct measurements of photosynthesis were not made. At the increased observed microbial biomass ranging between 50 and 170 mg VS/l, rapid heterotrophic bacterial growth will likely insure that bacterial production within the system will overgrow algal biomass present at the surface interface of the (induced) flocculant cultures that were maintained in the ponds. Because of the presence of elevated levels of total ammonia within the water column (1 /2 mg/l) significant nitrification is observed to take place within the system, although the overall rate of nitrification is again likely limited by the faster growing heterotrophic microbial production. Overall, nitrification was seen to convert 15% of the input ammonia into NO3, with the greater fraction of the input ammonia (43%) being converted to heterotrophic microbial biomass. The availability of ammonia in excess of heterotrophic demand (in a culture where photosynthesis is depressed) is likely driving the nitrification reaction in this system. In very heavily loaded, aggressively mixed, and aerated tank tilapia aquaculture systems, (reported by Avnimelech et al. (1994)) nitrification can be ‘pushed’ to higher
Fig. 5. Annual feed loading and shrimp production in zero exchange shrimp culture (McIntosh, 2001).
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Fig. 6. Observed vs. predicted range of bacterial and algal production and standing crop in zero exchange shrimp culture (McIntosh, 2001).
levels with 29 /43% conversion of input ammonia (see Figs. 7 and 8). However, these systems are not ‘efficient’ nitrification reactors because of the high BOD/NH3-N ratio (typically greater than 7/1) reducing nitrifier populations to less than 5% of total microbial biomass, as is observed in conventional heterotrophic, activated sludge, wastewater treatment facilities (Fig. 9). As expected, overall nitrification rate in the tilapia tanks is low (0.013 mg N/mg VS per day) as compared to more efficient wastewater nitrification systems operating at BOD/NH3-N ratios of 1.2 /3 with oxidation rates of 0.1 /0.5 mg N/mg VS per day and nitrifier populations as high as 12/35% of reactor total bacterial dry weight (see Fig. 9).
Fig. 7. Predicted nitrifying bacterial production rate, and standing crop in tank tilapia culture (Avnimelech et al., 1994).
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Fig. 8. Overall nitrogen mass balance observed in 20 and 30% protein fed tilapia culture.
3.2. Photosynthetic intensification techniques Direct or indirect photosynthetic systems include enhanced algal systems, polyculture, hydroponics, wetlands and terrestrial irrigation/fertilization. Enhanced algal systems usually include transfer of water from intensive fish confinement areas to less intensive (extensive) algal culture ponds where the pond water is treated for reuse without discharge to public waters (Avnimelech, 1998). These systems increase algal production by improved mixing and perhaps, settling and removal of algal biomass which increases and stabilizes algal growth and algal populations. Field trials with such intensive /extensive systems have demonstrated sustained algal productivities and corresponding nitrogen removal rates equivalent to 6 g C/m2-day of treatment area (Diab et al., 1992). The confinement of fish into intensive tanks, ponds or cages solves a number of practical management issues. Confinement of fish eliminates the labor-intensive need for seining of ponds. Fish can be held in sorted cohorts allowing for more efficient and uniform feed applications, demonstrated to substantially increase conversion efficiency (Brune et al., 1999) and improved fish health management. Animal and bird predation of fish stocks is virtually eliminated as a result of placement of covers or nettings over concentrated fish populations. Crowding of fish into intensive production areas allows for more uniform and efficient management of water quality, particularly oxygen concentration, since it is supplied directly to the fish confinement volume and need not be applied to the entire pond volume. This allows for the water quality impacting the fish to be separated from the diurnal water quality variations of the pond (Fig. 10). The primary disadvantage of the enhanced photosynthetic technique in ponds has been
Fig. 9. Relationship between the fraction of nitrifying organisms and the BOD5/TKN ratio (from Metcalf and Eddy Inc., 1979).
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Fig. 10. Diurnal oxygen cycling in pond aquaculture system as a result of algal photosynthesis (Brune et al., 2001).
the use of energy intensive centrifugal pumping systems (20 /111 kW/ha) necessary to move the high volumes of water between the intensive/extensive ponds. Polyculture is, in effect, an enhanced algal technique. This culture method usually involves stocking the pond with populations of filter-feeding fish or detritivoirs. Net combined yields of primary and secondary fish of 9000 kg/ha has been demonstrated (Moav et al., 1977). This technique improves pond carrying capacity by ‘storing’ a portion of pond respiration (algal and sediment respiration) in the secondary fish biomass. In addition, the secondary organism recovers nitrogen from the algal standing crop and sediment ‘storing’ it in biomass thereby reducing the nitrogen recycle and loading of the water column. The disadvantage of the polyculture technique is the need for hand harvest and sorting of mixed fish populations, the need to market two or more species, and the typically, lower market value of the secondary fish. Additional photosynthetic techniques include the transfer of nitrogen-laden waters from ponds to aquatic macrophytes or terrestrial plants in hydroponic beds, created wetlands, or irrigation of conventional crops (McMurty et al., 1990; Olson, 1991; Jinescu and Brune, 1995). The hydroponic technique may offer value in limited niche markets but is not likely to be amenable to large-scale systems because of the large area requirement for plant biomass production and the disconnect between the relatively high nitrogen concentration requirement of plant culture in contrast to the low nitrogen tolerance limit of growing fish (Rakocy et al., 1993). The low nutrient value of aquaculture discharge water is typically of limited benefit to terrestrial crops
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(Ghate and Burtle, 1993) consequently, crop irrigation with aquaculture water is usually a technique of water disposal or groundwater recharge, particularly in the more humid eastern US. The usefulness of aerated and non-aerated wetlands as a technique for removal of nitrogen from aquaculture water has been demonstrated (Jinescu and Brune, 1995), however, net productivities of fish per total unit area are typically similar to conventional pond culture. Most wetland systems reduce water ammonia concentration as a result of the plant biomass acting as a substrate supporting nitrifying and denitrifying bacterial populations as opposed to direct nutrient uptake by the plant biomass.
4. High rate photosynthetic systems 4.1. Partitioned aquaculture systems Algal biosynthesis is the primary waste treatment process of the conventional aquaculture pond. The major disadvantage of algal production is the observed diurnal oxygen and ammonia cycle that is induced within the pond (Fig. 10). In addition to diurnal cycles, pond managers must also be prepared to deal with the consequences of algal population crashes. Many of the management problems of conventional pond culture are linked to the daily diurnal cycles of oxygen production and the longer term waxing and waning of algal populations as a result of uncontrolled algal photosynthesis in ponds. The continuous nutrient enrichment of the pond drives pond photosynthesis, however, unmanaged algal populations in conventional ponds typically yield only 2 /3 g C fixation/m2-day. As Oswald (1988) and others (Benemann et al., 1980; Benemann, 1990) have demonstrated, low energy paddlewheel mixed ponds (often referred to as ‘high rate ponds’) are capable of sustained algal yields of 10 /12 g C/m2-day. This three to fourfold increase in algal photosynthesis provides the potential for a similar increase in the rate of pond water detoxification (ammonia removal) while simultaneously providing a solar driven oxygen production system. By superimposing a water velocity field upon the pond it becomes possible to utilize velocity and hydraulic detention time as a controlling strategy in reconfiguring the aquatic ecosystems of pond aquaculture into a series of engineered aquatic processes. High rate photosynthetic systems offer the greatest potential for intensification of pond aquaculture. Nearly 20 years ago, Torrans (1984) suggested that pond production could be intensified through the use of paddlewheel enhanced water movement. Such as system takes advantage of the lower cost of open ponds while providing solar energy driven biological waste treatment capacity. In contrast to energy intensive systems (fossil fuel driven), solar energy based waste treatment offers the potential for long-term resource and energy sustainability. Oswald and Golueke (1960) proposed that algal photosynthesis could be used as a sustainable process for power generation and waste treatment. The Partitioned Aquaculture System (PAS) (patented, Brune et al., 2001) combines the advantages of process control of recirculating tank/raceway aquaculture with the lower costs of earthen pond aquaculture. The PAS is a re-design of conventional
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pond aquaculture using the principles of high rate microalgal production yielding a more sustainable, low impact, high yield, and more controllable fish production process. Beginning in 1989, scientists and engineers at Clemson University initiated development of the PAS as a system that offered the advantages of high density raceway culture of fish while coupling the necessary waste treatment of the fish raceway to high rate algal ponds previously developed for treatment of domestic wastewater (Drapcho and Brune, 1989; Brune and Wang, 1998). The development of the PAS constitutes an attempt to combine the various physical, chemical, and biological pond intensification techniques into a single integrated system. Central to the economic success of this technique is the use of an energy efficient means of moving large volumes of water at low velocities uniformly throughout the pond (Figs. 11 and 12). It has been previously demonstrated that uniform water velocities can best be achieved with the use of low speed (1 /3 r.p.m.) paddlewheels (Brune et al., 1995). Clemson University PAS units (1/3 acre) as operated in 1995 is represented schematically in Fig. 13. The low speed paddlewheel provides operator control of bulk water velocity through the algal basin and fish raceways allowing for operator control of oxygen and carbon dioxide surface exchange rates into or out of the bulk pond water. Additionally, the uniform water velocity field provides for mixing and nutrient dispersion into the entire water column and rapid turnover of algal biomass into the photic zone insuring maximal algal productivity and also control of water quality across the raceway confined fish. The raceway culture provides all the advantages previously described. A solids-settling basin at the discharge of the fish
Fig. 11. Overview of six 1/3 acre and single 2-acre PAS units.
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Fig. 12. Algal paddlewheels in 2-acre PAS unit.
raceway provides the opportunity for solid waste or flocculated algal solids removal. A relatively shallow (1.5 ft) mixed algal culture basin provides for an enhanced maximal rate of photosynthesis resulting in potential fish carrying capacities of potentially 11 /28 000 kg/ha with on-site waste treatment and consequently reduced environmental impact. The PAS technique (at the highest yields) is dependent upon co-culture of filter-feeding fish, shellfish or detritivoirs. This continuous harvest of the microalgal population provides control of the algal cell age and therefore, algal standing crop, rate of algal respiration and oxygen production rate (Smith, 1985). Filter feeder co-production promotes a more environmental friendly, sustainable practice of nutrient recovery in addition to valuable by-product fish biomass production (Edwards et al., 1981). The high rate PAS utilizes harvested algal populations (maintained at relatively short cell ages of 2 /3 days typically) at high nutrient and feed application rates resulting in demonstrated maximum fish production ranging from 17 to 19 100 kg/ha per season (Fig. 14). High rate algal culture systems typically do not accumulate significant NO3 concentrations since the suspended algal biomass tends to outcompete the lower yielding nitrifying populations for ammonia. The high nitrogen input to the water column and lowered relative rate of BOD application combined with the short cell age favors a higher fraction of photosynthetic biomass within the system over heterotrophic biomass. Application of microbial yield factors demonstrates the photosynthetic standing crop excess (relative to heterotrophic standing crop) and dramatic effect of reduced cell age (3 vs. 20 days in conventional systems) on resulting water column VS concentration (Figs. 15 and 16). The use of tilapia coculture and the (unwanted) high rate of nitrogen release from the sediments to the
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Fig. 13. Representation of the 1/3 acre PAS unit as operated in 1995.
water column within the PAS increases internal recycle of nitrogen further promoting algal growth and algal standing crop to 100/200% of predicted levels (24 /41 mg/l predicted VS as opposed to 50 mg/l VS typically observed).
4.2. PAS algal management with tilapia co-culture Nile tilapia (Oreochromis niloticus) was selected as the co-cultured fish in the Clemson PAS growth trials. These fish were selected because of previous reports of the ability of this species to feed on microalgal biomass (Edwards et al., 1981) and South Carolina regulations allow for its possession and culture. Co-culture of a filter feeding organisms plays an essential role in the PAS as previously reported (Drapcho, 1993). The photosynthesis equation (Fig. 3) suggests that production of algal biomass represents a ‘stored oxygen demand’ and ‘stored ammonia/nitrogen’ therefore, if this oxygen demand can be harvested from the system directly as algal biomass or indirectly as filter feeder biomass, then algal biosynthesis generates a net system oxygen production and a net ammonia /nitrogen sink, thereby reducing the need for external oxygenation and additional nitrogen assimilation capacity.
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Fig. 14. Six 1/3-acre PAS catfish production, maximum carrying capacities, and tilapia production from 1995 to 2001.
The impact of the tilapia on performance of the algal production and standing crop in the PAS was quantified over a period of 6 years. Tilapia treatments (as opposed to non-tilapia) in 1996 showed a reduction in average algal water column cell age from 6 /8 days to 4 /6 days with a corresponding reduction in algal standing crop from 100 /150 mg/l to 50 mg/l. By 1997, a successful tilapia stocking ratio of 25% of catfish biomass at the end of season stabilized algal biomass at approximately 50 mg/l and algal cell ages of 2 /4 days in spite of a catfish carrying capacity 60% greater than 1996. The important link between young algal cell age, stable algal populations and improved water quality was established. In general, if the PAS is operated with sufficient tilapia biomass (/25% of catfish biomass at the end of the season), algal densities typically range from 40 /60 mg/l producing secchi disk readings of 15 /18 cm. If tilapia populations are not maintained in PAS units
Fig. 15. Annual feed loading and catfish carrying capacity in high rate PAS (Brune et al., 2001).
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Fig. 16. Observed vs. predicted bacterial and algal production rate and standing crop in catfish PAS at average feed application of 143 kg/ha day (Brune et al., 2001).
exceeding 12 200 kg/ha of catfish carrying capacities, algal cell densities will typically exceed 100 /150 mg/l producing secchi disk readings of 5 /10 cm (Brune et al., 2001). Tilapia co-culture was observed to produce an additional advantage in management of the algal production by successfully reducing the dominance of cyanobacterial populations in the PAS (Lazur and Britt, 2000). Beginning in 1998, the PAS units were stocked with only tilapia ‘breeding pairs’ (100 male/female pairs per acre). As a result, the units were seen to shift from early cyanobacterial dominance to phytoplankton populations of more desirable green algae in both 1998 and 1999 as tilapia offspring from the breeding pairs expanded in number and weight as the season progressed (Brune et al., 2001). 4.3. Limitations of high rate algal systems Since the essential waste treatment function of the PAS is solar driven algal photosynthesis, there is concern that reductions in solar radiation, particularly during extended cloudy periods, could adversely impact the system performance. In fact, such an event did occur, and detailed observations of the impact of this event was obtained during an extended cloudy period from mid-July to early-August of growing season 2000. This unusually long period of cloudy weather lasted nearly 3 weeks (Fig. 17). During this time feed application to the 2 acre PAS was continued at fish acceptance levels. After 10 days of reduced sunlight and sustained feed applications, system total ammonia /nitrogen concentration was seen to peak at levels in excess of 16 mg/l (Fig. 18). As a consequence of the low light event combined with sustained application of feed, a period of fish mortality was observed
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Fig. 17. Percent of full sunlight during 2000 Clemson PAS season.
Fig. 18. PAS total ammonia /nitrogen concentration during extended cloudy period of 2000.
beginning on August 7 of 2000, ending 10 days later resulting in a net loss of 3.5% of total stocked fish. Water flushing was initiated after the ammonia spike was detected on August 1 reducing system total ammonia /nitrogen to below 10 mg/l. The observed reduction in solar radiation and the resulting water total ammonia / nitrogen was used to calibrate a finite element PAS computer model (Brune et al., 2001). In computer simulations a protocol was inserted into the model that provided for fish feeding to be suspended whenever two low light days (75% of full sunlight)
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Fig. 19. Predicted reduction in PAS ammonia levels without (top) and with (bottom) feed reduction protocol during period of extended low light levels.
were experienced sequentially. The model predicted water column total ammonia / nitrogen levels to be reduced by 50% to 10 mg/l as opposed to predicted levels of 20 mg/l in simulations without the feed protocol procedure (Fig. 19). Extended periods of cloudiness are relatively rare during the catfish production season for upstate South Carolina where the PAS trials have been conducted. However, the PAS is dependent on solar radiation and this fact must be considered when locating and planning such photosynthetically dependent culture systems. In the event of sustained low light, feed management protocols such as simulated should be added to the pond management procedures. 4.4. Significance of PAS high rate technology With cultured fish growing at about 20% per year, aquaculture will, within the next two decades, supplant natural fisheries and become a major protein source for
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mankind, and of increasing importance to the US economy. The PAS offers the potential to increase fish production per unit area by a factor of three to five over conventional aquaculture techniques. Future aquaculture development will be severely constrained by water availability and the potential for adverse environmental impact. The PAS represents a redesign of the conventional aquaculture pond culture technology providing a spectrum of applications ranging from moderate yield (6700 /11 200) kg/ha ‘engineered ecosystems’ to high yield (16 800/33 600 kg/ ha) controlled ‘production processes’. The system offers the potential for a 90% reduction in total water usage per unit of fish produced. The modular nature of the PAS, the increased productivity per unit area, and reduced water requirements offers the potential for fish culture systems to be installed at sites not currently suitable for conventional aquaculture production. Advanced PAS processes utilizing ‘controlled eutrophication’ on private land offers the potential for solar driven protein and energy self-sufficiency without release of waste nutrient discharges to the environment. Commercial realization of this potential will come through demonstration of economic competitiveness and long-term reliability of large-scale applications of the technology.
Appendix A: Supporting calculations to figures Figure 1 A) (25 kg fish biomass) /(0.01 kg O2 /kg day) /0.25 kg/day /173 mg O2/min demand; (900 l) /(10 mg/l O2 at saturation) /9000 mg O2 available per tank; 50% O2 reduction yields 4500 mg O2/(173 mg/min) /26 min, 100% O2 reduction yields 9000/173 /52 min. B) (250 gm feed/day) /(42% protein) /(16% nitrogen) /(0.60 excreted) /10 000 mg/day /11 mg/l day ammonia addition rate to tank; with ammonia ionization fraction of 0.008 free ammonia addition rate /0.09 mg/l-day or approximately 1 day to toxic levels. C) CO2 production/O2 uptake /1/1 molar; (173 mg O2/min)/(32 mg/mm) /5.4 mm CO2/min production; (5.4 CO2 mm/min)/(900 liters) /0.006 mm/l min /0.36 mm/l h; addition of 1.5 mmCO2/l drops pH from 7.5 to limiting pH of 6.6 requiring (1.5/0.36) /4.2 h. Figure 4 A) (53 kg/ha feed per day) /(0.36 kg BOD excreted/kg feed) /(19.1 kg/ha BOD added to water column per day) /(0.5 kg bacteria/kg BOD) /(9.5 kg bacterial VS/day produced) /(20 days average cell age) /190 kg VS/pond volume /13.1 mg/l at 1.45 m depth, requiring (9.5 kg VA/ha)/(8.6 kg N/kg VA) /1.1 kg N/haday. B) (53 kg/ha feed per day) /(0.30 protein) /(0.16 nitrogen) /(0.90 excreted) /(2.3 kg N/ha) /(11.4 kg VS/kg N) /(26.2 kg VS/ha per day) /(20 days average cell
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age) /524 kg VS/pond volume /36 mg/l VS maximum algal standing crop. (2.3 /1.1 kg N/ha) /(11.4 kg VS/kg N) /(13.6 kg VS/ha per day) /(20 days average cell age) /273 kg VS/pond volume /19 mg/l VS minimum algal standing crop. C) Maximum potential rate of photosynthesis /(2.3 kg N/ha-day) /(5.7 kg C/kg N)/area /1.3 gm C/m2-day at a typical seechi disk reading of 0.3 m. Figure 6 A) (233 kg/ha feed per day) /(0.36) /(84 kg/ha BOD added to water column per day) /(0.5 kg/ha yield) /42 kg bacterial VS/day produced) /(20 days average cell age) /840 kg VS/pond volume /73 mg/l at 1.15 m depth, requiring 4.9 kg N/ha. B) (233 kg/ha feed per day) /(0.22 protein) /(0.16 nitrogen) /(0.90 excreted) / (7.4 /4.9 kg N/day) /(2.5 kg N/day) /(11.4 kg VS/kg N) /(28.5 kg VS/day) / (20 days average cell age) /570 kg VS/volume /49 mg/l VS average algal standing crop. C) Observed alkalinity destruction /105 mg/l as CaCO3 representing 14.8 mg/l NO3 /N oxidation over 120 days /0.12 mg N /l-day; (0.12 mg N /day) /(0.2 mg VS /mg N) /(20 days average cell age) /0.48 mg VS/l; (0.48 mg VS/l)/(73 mg total VS/l) /0.006 nitrifying fraction; At BOD5/TKN /14/1 predicted nitrifying fraction B/0.01. Figure 7 A) (14 mg/l observed NO3 oxidation)/(5 day) /2.8 mg NO3 /N /l-day yielding (2.8 mg/l day) /(0.2 mg VS/mg N) /(0.56 mg/l nitrifying biomass production per day) /(20 days average cell age) /11 mg/l nitrifier biomass. At reported tank bacterial biomass of 210 mg/l, calculated nitrifier fraction /(11 mg VS)/ (210 mg VS) /5%, compared to predicted values of 3.7% at BOD/NH3 ratio of 7/1. B) Overall nitrification rate /(2.8 mg N/day)/(210 mg VS) /0.013 mg N /mg VSday. Figure 16 A) (143 kg/ha feed per day) /(0.36 BOD excreted/kg feed) /(51.4 kg/ha BOD added to water column per day) /(0.5 kg VS/ kg BOD) /(25.7 kg VS/ha per day) /(3 days average cell age) /77.2 kg VS/pond volume /16.9 mg/l VS requiring 2.98 kg N/ha-day. B) (143 kg/ha feed per day) /(0.36 protein) /(0.16 nitrogen) /(0.15 excreted) / 6.2 kg N ha day; (6.2 /2.98 kg N/ha-day) /(11.4 kg VS/ kg N) /36.7 kg VS/haday /(3 days) /110 kg VS/volume /24 mg/l VS. C) (25.7 kg bacterial VS/ha-day)/(36.7 kg algal VS/ha-day) /(62.4 kg VS/haday) /(200 days/season) /12 480 kg VS/ha-season. At a tilapia production of 5600 kg/season, conversion of VS to tilapia /12 900/5600 /2.2/l.
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References Avnimelech, Y., Weber, B., Hepher, B., Mitstein, A., Zorn, M., 1986. Studies in circulated fish ponds; organic matter recycling and nitrogen transformation. Aquaculture and Fisheries Management 17, 231 /242. Avnimelech, Y., Kochva, M., Diab, S., 1994. Development of controlled intensive aquaculture systems with a limited water exchange and adjusted carbon to nitrogen ratio. Bamidgeh 46 (3), 119 /131. Avnimelech, Y., 1998. Minimal discharge from intensive fish ponds. World Aquaculture 24 (2), 32 /37. Avnimelech, Y., Kochva, M., Hargreaves, J.A., 1999. Sedimentation and resuspension in earthen fish ponds. Journal of the World Aquaculture Society 30 (4), 401 /409. Benemann, J.R., Koopman, B.L., Weissman, J.C., Eisenberg, D.E., Goebel, R.P., 1980. Development of microalgae harvesting and high rate pond technology. In: Shelef, G., Soeder, C.J. (Eds.), Algal Biomass. Elsevier Science Publishers, Amsterdam, pp. 457 /499. Benemann, J.R. 1990. The future of microalgae biotechnology. In: Creswell, R.C., Rees, T.A.V., Shah, N. (Eds.), Algal and Cyanobacterial Biotechnology, London, pp. 317 /337. Brune, D.E., Collier, J.A., Schwedler, T.E., 1995. Nutrient recovery and reuse for water quality control in the partitioned aquaculture systems. In: Proceedings of the Conference on Sustainable Aquaculture, Pacific Congress on Marine Science and Technology. pp. 57 /67. Brune, D.E., Wang, J.-K., 1998. Recirculation in photosynthetic aquaculture systems. Aquaculture Magazine 24 (3), 63 /71. Brune, D.E., Collier, J.A., Schwedler, T.E., Eversole, A.G., 1999. Designed ecosystems for aquaculture; the clemson partitioned aquaculture system, American Society of Agricultural Engineers Paper No. 99 /5031. Brune, D.E., Reed, S., Schwartz, G., Collier, J.A., Eversole, A., Schwedler, T., 2001. High rate algal systems for aquaculture, Proceedings from the Aquacultural Engineering Society’s Second Issues Forum, Ithaca, NY, NRAES-157, pp. 117 /129. Busch, C.D., Goodman, R.K., 1983. Fish pond management without emergency aeration. Transactions of the American Society of Agricultural Engineers 26 (1), 290 /296. Busch, C.D., Flood, C.A., Allison, R., 1978. Multiple paddlewheels influence on fish pond temperature and aeration. Transactions of the American Society of Agricultural Engineers 21 (6), 1222 /1224. Chesness, J.L., Obenauf, K.S., Hill, T.K., 1976. Chemical flocculation of catfish raceway effluent. Transactions of the American Society of Agricultural Engineers 19 (1), 192 /194. Diab, S., Kochba, M., Mires, D., Avnimelech, Y., 1992. Combined intensive /extensive pond system. Aquaculture 101, 33 /34. Drapcho, C.M., 1993. Modeling of algal productivity and diel oxygen profiles in the partitioned aquaculture system. Ph.D. Dissertation, Department of Agricultural and Biological Engineering, Clemson University, Clemson, South Carolina, USA. Drapcho, C.M., D.E. Brune., 1989. Design of a partitioned aquaculture system. American Society of Agricultural Engineers Paper No. 89 /7527. Edwards, D., Sinchumpasak, O., Tabucanon, M., 1981. The harvest of microalgae from the effluent of a sewage fed high rate stabilization pond by tilapia Nilotica . Aquaculture 23, 107 /147. Ghate, S.R., Burtle, G.J., 1993. Water quality in channel catfish ponds intermittently drained for irrigation. In: Wang, J.-K. (Ed.), Techniques for Modern Aquaculture. American Society of Agricultural Engineers, St. Joseph, MI, pp. 177 /186. Gunther, D.C., Brune, D.E., Gall, G.A.E., 1981. Ammonia production and removal in a trout rearing facility. Transactions of the American Society of Agricultural Engineers 24 (5), 1376 /1380. Greene, G.N., 1971. Biological filters for increased fish production. Annual Conference of Southeastern Association Game and Fish Commission, 24, 483 /484. Hopkins, J.S., Sandifer, P.A., Browdy, C.L., 1994. Sludge management in intensive pond culture of shrimp. Aquacultural Engineering 13 (1), 11 /31. Jinescu, L., Brune, D.E., 1995. High rate nitrogen application to constructed wetlands. In: Proceedings of the Conference on Sustainable Aquaculture. Pacific Congress on Marine Science and Technology, pp. 205 /213.
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Lazur, A.M., Britt, D.C., 1997. Pond recirculating production systems. Southern Regional Aquaculture Center Publication No. 455. Lazur, A.M., Britt, D.C., 2000. Control of cyanobacteria algae in aquaculture ponds. Southern Regional Aquaculture Center, Thirteenth Annual Progress Report. Lorio, W.J., 1994. Production of channel catfish with water recirculation. The Progressive Fish-Culturist 56, 202 /206. McIntosh, R.P., 2001, High Rate Bacterial Systems for Culturing Shrimp. Proceedings from the Aquacultural Engineering Society’s Second Issues Forum, Ithaca, NY, NRAES-157, pp. 117 /129. McMurty, M.R., Nelson, P.V., Sanders, D.C., Hodges, L., 1990. Sand culture of vegetables using recirculated aquacultural effluent. Applied Agricultural Research 5 (4), 280 /285. Metcalf and Eddy, Inc. 1979. Wastewater Engineering: Treatment Disposal Reuse. Moav, R., Wohlfarth, G., Schroeder, G.L., Hulata, G., Barusch, H., 1977. Intensive polyculture of fish in freshwater ponds; substitution of expensive feeds by liquid cow manure. Aquaculture 10 (1), 25 /45. Olson, G.L., 1991. The use of trout manure as a fertilizer for Idaho crops. In: J. Blake, J. Donald, W. Magette, (Eds.), Proceedings of the National Livestock, Poultry and Aquaculture Waste Management Workshop. American Society of Agricultural Engineers, St. Joseph, Michigan, USA, pp. 198 /205. Oswald, W.J., 1988. Micro-algae and wastewater treatment. In: Borowitzka, M.A., Borowitzka, L.J. (Eds.), Micro-algal Biotechnology. Cambridge University Press. Oswald, W.J., Golueke, C.G., 1960. Biological transformation of solar energy. Applied Microbiology 2, 223 /262. Rakocy, J.E, Hargreaves, J.A., Bailey, D.S., 1993. Nutrient accumulation in a recirculating aquaculture system integrated with hydroponic vegetable production. In: Wang, J.-K. (Ed.), Techniques for Modern Aquaculture. American Society of Agricultural Engineers, St. Joseph, MI, pp. 148 /158. Smith, D.W., 1985. Biological control of excessive phytoplankton growth and the enhancement of aquacultural production. Canadian Journal of Fisheries and Aquatic Sciences 42, 1940 /1945. Torrans, E.L., 1984. Pond design for polyculture aquafarming. University of Arkansas Cooperative Extension Service, 2:2. Tucker, C., Hanson, T., Kingsbury, S., 2000. Routine application of low doses of copper sulfate to reduce the incidence of off-flavor in channel catfish Ictalurus punctatus . World Aquaculture Society Meeting, New Orleans, Louisiana, USA.
Intensification of pond aquaculture and high rate photosynthetic systems D.E. Brune a,*, G. Schwartz a, A.G. Eversole b, J.A. Collier a, T.E. Schwedler b a
Department of Agricultural and Biological Engineering, Clemson University, Clemson, SC 29634, USA b Department of Aquaculture, Fisheries and Wildlife, Clemson University, Clemson, SC 29634, USA Received 15 August 2002; accepted 3 February 2003
Abstract Aquaculture production systems may range from tanks and raceways, in which water quality is controlled by water dilution and discharge to the environment to captive water systems, in which water quality is controlled by microbial reactions within the tank or pond. Attempts at intensification of pond aquaculture beyond the commonplace practice of supplemental aeration may be classified into categories of physical/chemical techniques and a broad range of microbial techniques. Most of these techniques are directed at raising the ‘ceiling’ of the system ammonia detoxification rate. Physical /chemical techniques for intensification of pond aquaculture have included use of in-pond cages and raceways, water blending and shading of the algal community, as well as, direct flocculation and removal of algal and bacteria biomass from ponds. A variety of microbial processes can be used to reduce ammonia levels in a conventional pond. These processes include nitrification/denitrification, photosynthesis, and heterotrophic bacterial re-growth. In this paper, simplified microbial growth fundamentals, and elemental mass balances are used to analyze and compare the various aquaculture intensification techniques and, in particular, to compare conventional and heterotrophic techniques to the use of high rate photosynthetic systems. Direct or indirect photosynthetic systems include enhanced algal systems (with water mixing), polyculture, hydroponics, wetlands, and terrestrial irrigation/fertilization. The development of Clemson University’s Partitioned Aquaculture System (PAS) constitutes an attempt to combine a number of the various physical, chemical, and microbial intensification techniques into a single integrated system. The PAS represents an adaptation of high rate microalgal culture to produce a sustainable, minimal discharge, high yield, and more controllable fish production
* Corresponding author. E-mail address: [email protected] (D.E. Brune). 0144-8609/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0144-8609(03)00025-6
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process. The PAS combines the advantages of process control of recirculating tank aquaculture with the lower costs of earthen pond aquaculture. Central to the economic success of the PAS is the use of low speed (1 /3 r.p.m.) paddlewheels as an energy efficient means of establishing a uniform water velocity field within an aquaculture pond. The PAS represents a redesign of the conventional aquaculture pond culture technology providing a spectrum of applications ranging from moderate yield (6700 /11 200 kg/ha) ‘engineered ecosystems’ to high yield (16 800 /33 600 kg/ha) controlled ‘production processes’. This high rate photosynthetic system offers the potential for a 90% reduction in total water usage per unit of fish produced. The modular nature of the PAS, the increased productivity per unit area, reduced water requirement, and reduced environmental impact offers the potential for fish culture systems to be installed at sites not currently suitable for conventional aquaculture. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Supplemental aeration; Intensification techniques; Partitioned aquaculture systems
1. Aquaculture production systems; overview Aquaculture production systems range from tanks and raceways, in which water quality is controlled by water dilution or discharge to the environment, to bacterial or algal-driven systems in which water quality is controlled by microbial reactions within captive water held in tanks or ponds. Trout production systems are typical of the flow-controlled aquaculture system. Fig. 1 illustrates the sequence of water quality constraints typically encountered in tank fish culture. If water flow to a typical culture system (operated at conventional carrying capacities) is interrupted, the dissolved oxygen level within the tank will fall to critical concentrations within 30 /60 min. Pure oxygen (O2) can be supplied to the tank, however, within 3/5 h respiratory carbon dioxide (CO2) production from the
Fig. 1. Typical water quality limitations in fish culture tanks and approximate time to impact (Gunther et al., 1981).
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fish will suppress the water pH to toxic levels. This outcome is predictable, and can be calculated, assuming a 1/1 molar ratio of CO2/O2 production/consumption and using pH predictions from carbonate equilibrium chemistry (see Fig. 1). Air aeration supplied to the tank will meet both the O2 supply and CO2 degassing needs, but within 24 h or less, free ammonia levels within the tank will reach toxic concentrations as can be predicted from a simple nitrogen mass balance (Fig. 1). Beyond this point in time, ammonia removal will be required and this need can be met through either water discharge/exchange or treatment of captive water with microbial processes or physical/chemical ammonia removal techniques. US pond aquaculture practices have changed significantly over the last 30 years. Pond catfish production has increased from around 1100 /1700 kg/ha in the 1960’s to a typical production of 4500/5600 kg/ha today. Supplemental pond aeration allows for the increase in fish production from 1700/5600 kg/ha. Algal primary production in a typical aquaculture pond can assimilate 1/3 gm C/m2-day corresponding to 0.5 mg/l-day of nitrogen addition or approximately 90 /112 kg feed/ha-day, ultimately limiting production at the 4500 /5600 kg/ha level. Attempts at intensification of pond aquaculture beyond the commonplace practice of supplemental aeration can be classified into categories of physical/chemical techniques and a broad range of microbial techniques. Most of these techniques are directed at raising the ‘ceiling’ of the pond ammonia detoxification rate.
2. Physical /chemical intensification techniques Physical /chemical techniques for intensification of pond aquaculture have included use of in-pond cages and raceways (Lorio, 1994; Lazur and Britt, 1997), water blending (Busch et al., 1978; Busch and Goodman, 1983), shading of the algal community and direct flocculation and removal of algal biomass from ponds. The cage culture or confinement technique offers the advantages of raceway fish culture (see below), however, without additional modifications in the management of bulk pond water, overall net production per unit water area is not improved. Water blending can offer advantages in improving oxygen distribution in ponds resulting in a reduction in the overall energy requirements for pond aeration however, standby equipment for emergency aeration of the pond is still required. Attempts to eliminate or shade the algal community within the pond can reduce the diurnal oxygen fluctuations induced by algal photosynthesis and occurrence of toxic algal blooms, however, this approach also does not address the required nitrogen management issue. Selective partial algal standing crop manipulation with copper sulfate addition to ponds has been shown to increase net profits from conventional ponds by approximately 10% (Tucker et al., 2000). Direct algal, bacterial or waste removal by flocculation or mechanical means has not yet been demonstrated to be economically viable (Chesness et al., 1976).
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3. Microbial intensification techniques A variety of microbial processes will react to remove or add ammonia from/to a conventional aquaculture pond. These include nitrification, denitrification, photosynthesis, mineralization or heterotrophic bacterial re-growth (illustrated in Fig. 2). In the following presentation a number of microbial intensification techniques that had been studied and presented by various investigators (for both shrimp and finfish) will be analyzed and compared using approximate pond organic and nitrogen mass balances (Fig. 2), and simplified microbial growth reactions (Fig. 3). It will be useful to represent organic and nitrogen input to an aquaculture water column as approximately 36% of the input feed (as BOD) and as 75% (90% for shrimp) of the input nitrogen (as ammonia/N, see Fig. 2) The BOD content of typical aquaculture feed is assumed to be 60% (as mg/l BOD5) of the dry weight of the feed. These values can vary significantly depending upon the handling of the fecal and bacterial solids within the system. If no solids are removed from the pond or tank these numbers represent the maximum expected BOD and nitrogen loading to the water column assuming no internal nitrogen recycle, which can occur, depending on the degree of interaction of the water column with the sediments. Heterotrophic microbial production occurs at a higher rate (10 h generation time)
Fig. 2. Typical aquaculture pond nitrogen cycle and fish oxygen demand mass balance.
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Fig. 3. The three important microbial processes dominating water quality in pond aquaculture systems.
that either autotrophic photosynthesis or nitrification (from 24 to 48 h generation time, see Fig. 3). The growth rates of algae and nitrifying bacteria are similar, however, the yield coefficient for photosynthesis is approximately 57 times larger than nitrification (Fig. 3). 3.1. Bacterial intensification techniques Bacterial or heterotrophic techniques for intensification of pond aquaculture have included the addition of nitrifying columns (Greene, 1971), increased pond aeration and mixing of sediments to improve in-pond suspended culture nitrification (Avnimelech et al., 1986, 1999), and addition of carbonaceous matter to the pond
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to stimulate nitrogen uptake by heterotrophic bacteria growth (Avnimelech et al., 1994). All of these techniques require high levels of water pumping, pond mixing, or aeration. Each technique has been demonstrated to increase production by decreasing the ammonia concentration in the pond however, the increased production associated with such techniques is usually insufficient to offset the increased energy costs and capital costs (in catfish culture in particular) associated with the increased water pumping and aeration. Pond sediment/sludge management or removal (not widely used in the aquaculture industry) has been demonstrated to decrease nitrogen recycle in ponds thereby increasing production (Hopkins et al., 1994). Most sludge removal techniques demonstrated to date have been limited to relatively small systems and have not been shown to be transferable to large-scale systems. The most widely applied technique for management of pond sediments has been drying of the pond sediments allowing for mineralization and denitrification processes to reduce the sediment organic carbon and nitrogen content, although this practice requires that pond water be discharged, or moved to another storage area and that the pond be removed from production for a significant length of time. Basic microbial growth fundamentals (Fig. 3) can be used to gain insight, in a general way, into the behavior of many aquaculture systems. As an example, a moderately fed shrimp pond (Hopkins et al., 1994) is typically seen to produce 5600 / 6700 kg shrimp/ha per season (Fig. 4). During the reported 130-day growing cycle, average water column concentrations of volatile solids (VS) are seen to range from 30 to 80 mg/l with an average of 40 mg/l. Oxidized levels of nitrogen (NO3 and NO2) are low (B/1 mg/l). Feed application rates to the pond are intentionally kept below an average of 67 kg/ha feed day to avoid excessive pond ammonia concentrations. Applying the microbial yield coefficient to the input nitrogen and input BOD to the pond (90% of feed nitrogen and 36% of feed BOD) suggests a mixed heterotrophic/ photosynthetic biomass within a conventional shrimp pond of 32/49 mg VS/l at a cell age controlled by the microbial sedimentation and degradation rate (typically 20
Fig. 4. Observed vs. predicted range of bacterial and algal standing crop in typical shrimp production pond at 53 kg feed/ha-day (Hopkins et al., 1994).
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days, see Fig. 4). Internal nitrogen recycle from the pond sediments is frequently observed to double conventional aquaculture pond algal production and standing crop, resulting in predicted pond total VS concentrations ranging from 36 to 98 mg / l, similar to the observed range in pond VS concentrations of 30 /80 mg/l. Differences in operator handling of pond sludge/biomass and short term release of ‘pulses’ of nitrogen from the sediments can alter the internal nitrogen/BOD recycle by as much as two to fourfold resulting in short bursts of increased algal production and standing crop. Alternatively, high rate shrimp production systems may be pushed to (reduced protein) feed application rates as high as 22/34 000 kg feed/ha season with shrimp production at 13 /16 000 kg/ha season (Fig. 5) by using a technique herein referred to herein as the Avnimelech/McIntosh method (McIntosh, 2001). The higher C/N ratio of the low percent protein feed drives heterotrophic bacterial production within the pond reducing ammonia concentrations in the water column below that which would be expected with 36/40% protein feeds typically used in shrimp aquaculture. Applying microbial yield coefficients to the observed input BOD and ammonia additions to this system suggest a bacterial/photosynthetic dominated microflora (Fig. 6). Although there is sufficient steady state water column ammonia concentration present in the pond to support algal growth, significant algal growth was not visually reported in this system, although direct measurements of photosynthesis were not made. At the increased observed microbial biomass ranging between 50 and 170 mg VS/l, rapid heterotrophic bacterial growth will likely insure that bacterial production within the system will overgrow algal biomass present at the surface interface of the (induced) flocculant cultures that were maintained in the ponds. Because of the presence of elevated levels of total ammonia within the water column (1 /2 mg/l) significant nitrification is observed to take place within the system, although the overall rate of nitrification is again likely limited by the faster growing heterotrophic microbial production. Overall, nitrification was seen to convert 15% of the input ammonia into NO3, with the greater fraction of the input ammonia (43%) being converted to heterotrophic microbial biomass. The availability of ammonia in excess of heterotrophic demand (in a culture where photosynthesis is depressed) is likely driving the nitrification reaction in this system. In very heavily loaded, aggressively mixed, and aerated tank tilapia aquaculture systems, (reported by Avnimelech et al. (1994)) nitrification can be ‘pushed’ to higher
Fig. 5. Annual feed loading and shrimp production in zero exchange shrimp culture (McIntosh, 2001).
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Fig. 6. Observed vs. predicted range of bacterial and algal production and standing crop in zero exchange shrimp culture (McIntosh, 2001).
levels with 29 /43% conversion of input ammonia (see Figs. 7 and 8). However, these systems are not ‘efficient’ nitrification reactors because of the high BOD/NH3-N ratio (typically greater than 7/1) reducing nitrifier populations to less than 5% of total microbial biomass, as is observed in conventional heterotrophic, activated sludge, wastewater treatment facilities (Fig. 9). As expected, overall nitrification rate in the tilapia tanks is low (0.013 mg N/mg VS per day) as compared to more efficient wastewater nitrification systems operating at BOD/NH3-N ratios of 1.2 /3 with oxidation rates of 0.1 /0.5 mg N/mg VS per day and nitrifier populations as high as 12/35% of reactor total bacterial dry weight (see Fig. 9).
Fig. 7. Predicted nitrifying bacterial production rate, and standing crop in tank tilapia culture (Avnimelech et al., 1994).
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Fig. 8. Overall nitrogen mass balance observed in 20 and 30% protein fed tilapia culture.
3.2. Photosynthetic intensification techniques Direct or indirect photosynthetic systems include enhanced algal systems, polyculture, hydroponics, wetlands and terrestrial irrigation/fertilization. Enhanced algal systems usually include transfer of water from intensive fish confinement areas to less intensive (extensive) algal culture ponds where the pond water is treated for reuse without discharge to public waters (Avnimelech, 1998). These systems increase algal production by improved mixing and perhaps, settling and removal of algal biomass which increases and stabilizes algal growth and algal populations. Field trials with such intensive /extensive systems have demonstrated sustained algal productivities and corresponding nitrogen removal rates equivalent to 6 g C/m2-day of treatment area (Diab et al., 1992). The confinement of fish into intensive tanks, ponds or cages solves a number of practical management issues. Confinement of fish eliminates the labor-intensive need for seining of ponds. Fish can be held in sorted cohorts allowing for more efficient and uniform feed applications, demonstrated to substantially increase conversion efficiency (Brune et al., 1999) and improved fish health management. Animal and bird predation of fish stocks is virtually eliminated as a result of placement of covers or nettings over concentrated fish populations. Crowding of fish into intensive production areas allows for more uniform and efficient management of water quality, particularly oxygen concentration, since it is supplied directly to the fish confinement volume and need not be applied to the entire pond volume. This allows for the water quality impacting the fish to be separated from the diurnal water quality variations of the pond (Fig. 10). The primary disadvantage of the enhanced photosynthetic technique in ponds has been
Fig. 9. Relationship between the fraction of nitrifying organisms and the BOD5/TKN ratio (from Metcalf and Eddy Inc., 1979).
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Fig. 10. Diurnal oxygen cycling in pond aquaculture system as a result of algal photosynthesis (Brune et al., 2001).
the use of energy intensive centrifugal pumping systems (20 /111 kW/ha) necessary to move the high volumes of water between the intensive/extensive ponds. Polyculture is, in effect, an enhanced algal technique. This culture method usually involves stocking the pond with populations of filter-feeding fish or detritivoirs. Net combined yields of primary and secondary fish of 9000 kg/ha has been demonstrated (Moav et al., 1977). This technique improves pond carrying capacity by ‘storing’ a portion of pond respiration (algal and sediment respiration) in the secondary fish biomass. In addition, the secondary organism recovers nitrogen from the algal standing crop and sediment ‘storing’ it in biomass thereby reducing the nitrogen recycle and loading of the water column. The disadvantage of the polyculture technique is the need for hand harvest and sorting of mixed fish populations, the need to market two or more species, and the typically, lower market value of the secondary fish. Additional photosynthetic techniques include the transfer of nitrogen-laden waters from ponds to aquatic macrophytes or terrestrial plants in hydroponic beds, created wetlands, or irrigation of conventional crops (McMurty et al., 1990; Olson, 1991; Jinescu and Brune, 1995). The hydroponic technique may offer value in limited niche markets but is not likely to be amenable to large-scale systems because of the large area requirement for plant biomass production and the disconnect between the relatively high nitrogen concentration requirement of plant culture in contrast to the low nitrogen tolerance limit of growing fish (Rakocy et al., 1993). The low nutrient value of aquaculture discharge water is typically of limited benefit to terrestrial crops
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(Ghate and Burtle, 1993) consequently, crop irrigation with aquaculture water is usually a technique of water disposal or groundwater recharge, particularly in the more humid eastern US. The usefulness of aerated and non-aerated wetlands as a technique for removal of nitrogen from aquaculture water has been demonstrated (Jinescu and Brune, 1995), however, net productivities of fish per total unit area are typically similar to conventional pond culture. Most wetland systems reduce water ammonia concentration as a result of the plant biomass acting as a substrate supporting nitrifying and denitrifying bacterial populations as opposed to direct nutrient uptake by the plant biomass.
4. High rate photosynthetic systems 4.1. Partitioned aquaculture systems Algal biosynthesis is the primary waste treatment process of the conventional aquaculture pond. The major disadvantage of algal production is the observed diurnal oxygen and ammonia cycle that is induced within the pond (Fig. 10). In addition to diurnal cycles, pond managers must also be prepared to deal with the consequences of algal population crashes. Many of the management problems of conventional pond culture are linked to the daily diurnal cycles of oxygen production and the longer term waxing and waning of algal populations as a result of uncontrolled algal photosynthesis in ponds. The continuous nutrient enrichment of the pond drives pond photosynthesis, however, unmanaged algal populations in conventional ponds typically yield only 2 /3 g C fixation/m2-day. As Oswald (1988) and others (Benemann et al., 1980; Benemann, 1990) have demonstrated, low energy paddlewheel mixed ponds (often referred to as ‘high rate ponds’) are capable of sustained algal yields of 10 /12 g C/m2-day. This three to fourfold increase in algal photosynthesis provides the potential for a similar increase in the rate of pond water detoxification (ammonia removal) while simultaneously providing a solar driven oxygen production system. By superimposing a water velocity field upon the pond it becomes possible to utilize velocity and hydraulic detention time as a controlling strategy in reconfiguring the aquatic ecosystems of pond aquaculture into a series of engineered aquatic processes. High rate photosynthetic systems offer the greatest potential for intensification of pond aquaculture. Nearly 20 years ago, Torrans (1984) suggested that pond production could be intensified through the use of paddlewheel enhanced water movement. Such as system takes advantage of the lower cost of open ponds while providing solar energy driven biological waste treatment capacity. In contrast to energy intensive systems (fossil fuel driven), solar energy based waste treatment offers the potential for long-term resource and energy sustainability. Oswald and Golueke (1960) proposed that algal photosynthesis could be used as a sustainable process for power generation and waste treatment. The Partitioned Aquaculture System (PAS) (patented, Brune et al., 2001) combines the advantages of process control of recirculating tank/raceway aquaculture with the lower costs of earthen pond aquaculture. The PAS is a re-design of conventional
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pond aquaculture using the principles of high rate microalgal production yielding a more sustainable, low impact, high yield, and more controllable fish production process. Beginning in 1989, scientists and engineers at Clemson University initiated development of the PAS as a system that offered the advantages of high density raceway culture of fish while coupling the necessary waste treatment of the fish raceway to high rate algal ponds previously developed for treatment of domestic wastewater (Drapcho and Brune, 1989; Brune and Wang, 1998). The development of the PAS constitutes an attempt to combine the various physical, chemical, and biological pond intensification techniques into a single integrated system. Central to the economic success of this technique is the use of an energy efficient means of moving large volumes of water at low velocities uniformly throughout the pond (Figs. 11 and 12). It has been previously demonstrated that uniform water velocities can best be achieved with the use of low speed (1 /3 r.p.m.) paddlewheels (Brune et al., 1995). Clemson University PAS units (1/3 acre) as operated in 1995 is represented schematically in Fig. 13. The low speed paddlewheel provides operator control of bulk water velocity through the algal basin and fish raceways allowing for operator control of oxygen and carbon dioxide surface exchange rates into or out of the bulk pond water. Additionally, the uniform water velocity field provides for mixing and nutrient dispersion into the entire water column and rapid turnover of algal biomass into the photic zone insuring maximal algal productivity and also control of water quality across the raceway confined fish. The raceway culture provides all the advantages previously described. A solids-settling basin at the discharge of the fish
Fig. 11. Overview of six 1/3 acre and single 2-acre PAS units.
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Fig. 12. Algal paddlewheels in 2-acre PAS unit.
raceway provides the opportunity for solid waste or flocculated algal solids removal. A relatively shallow (1.5 ft) mixed algal culture basin provides for an enhanced maximal rate of photosynthesis resulting in potential fish carrying capacities of potentially 11 /28 000 kg/ha with on-site waste treatment and consequently reduced environmental impact. The PAS technique (at the highest yields) is dependent upon co-culture of filter-feeding fish, shellfish or detritivoirs. This continuous harvest of the microalgal population provides control of the algal cell age and therefore, algal standing crop, rate of algal respiration and oxygen production rate (Smith, 1985). Filter feeder co-production promotes a more environmental friendly, sustainable practice of nutrient recovery in addition to valuable by-product fish biomass production (Edwards et al., 1981). The high rate PAS utilizes harvested algal populations (maintained at relatively short cell ages of 2 /3 days typically) at high nutrient and feed application rates resulting in demonstrated maximum fish production ranging from 17 to 19 100 kg/ha per season (Fig. 14). High rate algal culture systems typically do not accumulate significant NO3 concentrations since the suspended algal biomass tends to outcompete the lower yielding nitrifying populations for ammonia. The high nitrogen input to the water column and lowered relative rate of BOD application combined with the short cell age favors a higher fraction of photosynthetic biomass within the system over heterotrophic biomass. Application of microbial yield factors demonstrates the photosynthetic standing crop excess (relative to heterotrophic standing crop) and dramatic effect of reduced cell age (3 vs. 20 days in conventional systems) on resulting water column VS concentration (Figs. 15 and 16). The use of tilapia coculture and the (unwanted) high rate of nitrogen release from the sediments to the
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Fig. 13. Representation of the 1/3 acre PAS unit as operated in 1995.
water column within the PAS increases internal recycle of nitrogen further promoting algal growth and algal standing crop to 100/200% of predicted levels (24 /41 mg/l predicted VS as opposed to 50 mg/l VS typically observed).
4.2. PAS algal management with tilapia co-culture Nile tilapia (Oreochromis niloticus) was selected as the co-cultured fish in the Clemson PAS growth trials. These fish were selected because of previous reports of the ability of this species to feed on microalgal biomass (Edwards et al., 1981) and South Carolina regulations allow for its possession and culture. Co-culture of a filter feeding organisms plays an essential role in the PAS as previously reported (Drapcho, 1993). The photosynthesis equation (Fig. 3) suggests that production of algal biomass represents a ‘stored oxygen demand’ and ‘stored ammonia/nitrogen’ therefore, if this oxygen demand can be harvested from the system directly as algal biomass or indirectly as filter feeder biomass, then algal biosynthesis generates a net system oxygen production and a net ammonia /nitrogen sink, thereby reducing the need for external oxygenation and additional nitrogen assimilation capacity.
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Fig. 14. Six 1/3-acre PAS catfish production, maximum carrying capacities, and tilapia production from 1995 to 2001.
The impact of the tilapia on performance of the algal production and standing crop in the PAS was quantified over a period of 6 years. Tilapia treatments (as opposed to non-tilapia) in 1996 showed a reduction in average algal water column cell age from 6 /8 days to 4 /6 days with a corresponding reduction in algal standing crop from 100 /150 mg/l to 50 mg/l. By 1997, a successful tilapia stocking ratio of 25% of catfish biomass at the end of season stabilized algal biomass at approximately 50 mg/l and algal cell ages of 2 /4 days in spite of a catfish carrying capacity 60% greater than 1996. The important link between young algal cell age, stable algal populations and improved water quality was established. In general, if the PAS is operated with sufficient tilapia biomass (/25% of catfish biomass at the end of the season), algal densities typically range from 40 /60 mg/l producing secchi disk readings of 15 /18 cm. If tilapia populations are not maintained in PAS units
Fig. 15. Annual feed loading and catfish carrying capacity in high rate PAS (Brune et al., 2001).
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Fig. 16. Observed vs. predicted bacterial and algal production rate and standing crop in catfish PAS at average feed application of 143 kg/ha day (Brune et al., 2001).
exceeding 12 200 kg/ha of catfish carrying capacities, algal cell densities will typically exceed 100 /150 mg/l producing secchi disk readings of 5 /10 cm (Brune et al., 2001). Tilapia co-culture was observed to produce an additional advantage in management of the algal production by successfully reducing the dominance of cyanobacterial populations in the PAS (Lazur and Britt, 2000). Beginning in 1998, the PAS units were stocked with only tilapia ‘breeding pairs’ (100 male/female pairs per acre). As a result, the units were seen to shift from early cyanobacterial dominance to phytoplankton populations of more desirable green algae in both 1998 and 1999 as tilapia offspring from the breeding pairs expanded in number and weight as the season progressed (Brune et al., 2001). 4.3. Limitations of high rate algal systems Since the essential waste treatment function of the PAS is solar driven algal photosynthesis, there is concern that reductions in solar radiation, particularly during extended cloudy periods, could adversely impact the system performance. In fact, such an event did occur, and detailed observations of the impact of this event was obtained during an extended cloudy period from mid-July to early-August of growing season 2000. This unusually long period of cloudy weather lasted nearly 3 weeks (Fig. 17). During this time feed application to the 2 acre PAS was continued at fish acceptance levels. After 10 days of reduced sunlight and sustained feed applications, system total ammonia /nitrogen concentration was seen to peak at levels in excess of 16 mg/l (Fig. 18). As a consequence of the low light event combined with sustained application of feed, a period of fish mortality was observed
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Fig. 17. Percent of full sunlight during 2000 Clemson PAS season.
Fig. 18. PAS total ammonia /nitrogen concentration during extended cloudy period of 2000.
beginning on August 7 of 2000, ending 10 days later resulting in a net loss of 3.5% of total stocked fish. Water flushing was initiated after the ammonia spike was detected on August 1 reducing system total ammonia /nitrogen to below 10 mg/l. The observed reduction in solar radiation and the resulting water total ammonia / nitrogen was used to calibrate a finite element PAS computer model (Brune et al., 2001). In computer simulations a protocol was inserted into the model that provided for fish feeding to be suspended whenever two low light days (75% of full sunlight)
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Fig. 19. Predicted reduction in PAS ammonia levels without (top) and with (bottom) feed reduction protocol during period of extended low light levels.
were experienced sequentially. The model predicted water column total ammonia / nitrogen levels to be reduced by 50% to 10 mg/l as opposed to predicted levels of 20 mg/l in simulations without the feed protocol procedure (Fig. 19). Extended periods of cloudiness are relatively rare during the catfish production season for upstate South Carolina where the PAS trials have been conducted. However, the PAS is dependent on solar radiation and this fact must be considered when locating and planning such photosynthetically dependent culture systems. In the event of sustained low light, feed management protocols such as simulated should be added to the pond management procedures. 4.4. Significance of PAS high rate technology With cultured fish growing at about 20% per year, aquaculture will, within the next two decades, supplant natural fisheries and become a major protein source for
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mankind, and of increasing importance to the US economy. The PAS offers the potential to increase fish production per unit area by a factor of three to five over conventional aquaculture techniques. Future aquaculture development will be severely constrained by water availability and the potential for adverse environmental impact. The PAS represents a redesign of the conventional aquaculture pond culture technology providing a spectrum of applications ranging from moderate yield (6700 /11 200) kg/ha ‘engineered ecosystems’ to high yield (16 800/33 600 kg/ ha) controlled ‘production processes’. The system offers the potential for a 90% reduction in total water usage per unit of fish produced. The modular nature of the PAS, the increased productivity per unit area, and reduced water requirements offers the potential for fish culture systems to be installed at sites not currently suitable for conventional aquaculture production. Advanced PAS processes utilizing ‘controlled eutrophication’ on private land offers the potential for solar driven protein and energy self-sufficiency without release of waste nutrient discharges to the environment. Commercial realization of this potential will come through demonstration of economic competitiveness and long-term reliability of large-scale applications of the technology.
Appendix A: Supporting calculations to figures Figure 1 A) (25 kg fish biomass) /(0.01 kg O2 /kg day) /0.25 kg/day /173 mg O2/min demand; (900 l) /(10 mg/l O2 at saturation) /9000 mg O2 available per tank; 50% O2 reduction yields 4500 mg O2/(173 mg/min) /26 min, 100% O2 reduction yields 9000/173 /52 min. B) (250 gm feed/day) /(42% protein) /(16% nitrogen) /(0.60 excreted) /10 000 mg/day /11 mg/l day ammonia addition rate to tank; with ammonia ionization fraction of 0.008 free ammonia addition rate /0.09 mg/l-day or approximately 1 day to toxic levels. C) CO2 production/O2 uptake /1/1 molar; (173 mg O2/min)/(32 mg/mm) /5.4 mm CO2/min production; (5.4 CO2 mm/min)/(900 liters) /0.006 mm/l min /0.36 mm/l h; addition of 1.5 mmCO2/l drops pH from 7.5 to limiting pH of 6.6 requiring (1.5/0.36) /4.2 h. Figure 4 A) (53 kg/ha feed per day) /(0.36 kg BOD excreted/kg feed) /(19.1 kg/ha BOD added to water column per day) /(0.5 kg bacteria/kg BOD) /(9.5 kg bacterial VS/day produced) /(20 days average cell age) /190 kg VS/pond volume /13.1 mg/l at 1.45 m depth, requiring (9.5 kg VA/ha)/(8.6 kg N/kg VA) /1.1 kg N/haday. B) (53 kg/ha feed per day) /(0.30 protein) /(0.16 nitrogen) /(0.90 excreted) /(2.3 kg N/ha) /(11.4 kg VS/kg N) /(26.2 kg VS/ha per day) /(20 days average cell
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age) /524 kg VS/pond volume /36 mg/l VS maximum algal standing crop. (2.3 /1.1 kg N/ha) /(11.4 kg VS/kg N) /(13.6 kg VS/ha per day) /(20 days average cell age) /273 kg VS/pond volume /19 mg/l VS minimum algal standing crop. C) Maximum potential rate of photosynthesis /(2.3 kg N/ha-day) /(5.7 kg C/kg N)/area /1.3 gm C/m2-day at a typical seechi disk reading of 0.3 m. Figure 6 A) (233 kg/ha feed per day) /(0.36) /(84 kg/ha BOD added to water column per day) /(0.5 kg/ha yield) /42 kg bacterial VS/day produced) /(20 days average cell age) /840 kg VS/pond volume /73 mg/l at 1.15 m depth, requiring 4.9 kg N/ha. B) (233 kg/ha feed per day) /(0.22 protein) /(0.16 nitrogen) /(0.90 excreted) / (7.4 /4.9 kg N/day) /(2.5 kg N/day) /(11.4 kg VS/kg N) /(28.5 kg VS/day) / (20 days average cell age) /570 kg VS/volume /49 mg/l VS average algal standing crop. C) Observed alkalinity destruction /105 mg/l as CaCO3 representing 14.8 mg/l NO3 /N oxidation over 120 days /0.12 mg N /l-day; (0.12 mg N /day) /(0.2 mg VS /mg N) /(20 days average cell age) /0.48 mg VS/l; (0.48 mg VS/l)/(73 mg total VS/l) /0.006 nitrifying fraction; At BOD5/TKN /14/1 predicted nitrifying fraction B/0.01. Figure 7 A) (14 mg/l observed NO3 oxidation)/(5 day) /2.8 mg NO3 /N /l-day yielding (2.8 mg/l day) /(0.2 mg VS/mg N) /(0.56 mg/l nitrifying biomass production per day) /(20 days average cell age) /11 mg/l nitrifier biomass. At reported tank bacterial biomass of 210 mg/l, calculated nitrifier fraction /(11 mg VS)/ (210 mg VS) /5%, compared to predicted values of 3.7% at BOD/NH3 ratio of 7/1. B) Overall nitrification rate /(2.8 mg N/day)/(210 mg VS) /0.013 mg N /mg VSday. Figure 16 A) (143 kg/ha feed per day) /(0.36 BOD excreted/kg feed) /(51.4 kg/ha BOD added to water column per day) /(0.5 kg VS/ kg BOD) /(25.7 kg VS/ha per day) /(3 days average cell age) /77.2 kg VS/pond volume /16.9 mg/l VS requiring 2.98 kg N/ha-day. B) (143 kg/ha feed per day) /(0.36 protein) /(0.16 nitrogen) /(0.15 excreted) / 6.2 kg N ha day; (6.2 /2.98 kg N/ha-day) /(11.4 kg VS/ kg N) /36.7 kg VS/haday /(3 days) /110 kg VS/volume /24 mg/l VS. C) (25.7 kg bacterial VS/ha-day)/(36.7 kg algal VS/ha-day) /(62.4 kg VS/haday) /(200 days/season) /12 480 kg VS/ha-season. At a tilapia production of 5600 kg/season, conversion of VS to tilapia /12 900/5600 /2.2/l.
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