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Suitability of a treatment wetland for dairy wastewaters
~
Wat. Sci Tech. Vol. 40, No.3, pp. 179-185, 1999
Pergamon
Publishedby ElsevierScience Ltd Printed in Great Britain. All nghts reserved 0273-1223199 $20.00 + 0.00
PH: S0273-1223(99)00464-3
SUITABILITY OF A TREATMENT WETLAND FOR DAIRY WASTEWATERS P. M. Geary* and J. A. Moore** • Department ofGeography & Environmental Science, The University ofNewcastle, Callaghan, 2308, NSW, Australia •• Department ofBioresource Engineering. Oregon State University, Corvallis, OR 97331-3906. USA
ABSTRACT A treatment wetland was constructed as part of a waste management system for dairy parlour waters and the performance of the wetland in reducing organic matter and nutrients monitored for a two-year period. The wetland was designed for a herd size of 110 with a detention time of approximately 10-14 days. Hydraulic loads to the wetland averaged 25 rnrnIday, although there were wide fluctuations due to rainfall and water use within the dairy. Average mass loadings to the wetland were 5.6 glm2/d for Biochemical Oxygen Demand, 2.6 glm2/d for Organic Nitrogen, 3.2 glm2/d for Ammonia and l.S glm 2/d for Total Phosphorus.
Monitoring results from the system indicated that significant BOD reductions were achieved, while Nitrogen and Phosphorus removals were variable but smaller. Calculated mean monthly pollutant reductions due to the treatment wetland were 61% for BOD, 43% for Organic Nitrogen, 26% for NH) and 28% for TP. The wetland received high hydraulic and pollutant loads and appeared to act as a sink for the nutnents which were removed. At this scale it did not appear to be suitable as a treatment option for significantly reducing nutrients in this type of agricultural waste. iC 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Agricultural waste; biochemical oxygen demand; macrophytes; nitrogen; phosphorus; pollutant removal.
INTRODUCTION Wastewaters from intensive agricultural activities (cattle feedlots, piggeries and dairies) typically have significantly higher concentrations of organic matter and nutrients than treated municipal effluent. The high pollutant loads which are generated pose particular problems and challenges for these industries as high concentrations of nutrients can contribute to water management problems if wastes are allowed to discharge directly to receiving waters. Agricultural wastes must be treated prior to disposal and constructed wetlands (in association with stabilisation ponds) have been suggested as a potential treatment option prior to land application. Within Australia intensive agricultural activities are being required to upgrade their waste management systems and the preparation and implementation of waste management plans is underway. Research which has examined the performance of constructed wetland systems for the treatment of dairy wastewaters overseas has shown that significant improvements in effluent quality may be achieved due to the physical, chemical and biological processes which occur in wetland ecosystems. A recent survey by CH2M Hill and 179
180
P. M. GEARY and J. A. MOORE
Payne Engineering (1997) reported that more than 65 pilot or full-scale wetlands have been built in North America since 1990 to treat livestock wastewater. While there are some difficulties in directly comparing pollutant removal efficiencies between studies, constructed wetlands do show potential for reducing BOD, suspended solids, faecal coliforms and, to a lesser extent, nutrients. Best management practice for the Australian dairy industry has been described by Wrigley (1994) and may include collection of milking shed wastes, pre-treatment, waste stabilisation pond(s) and land application by spray irrigation. While there has been research undertaken on a number of aspects of dairy waste treatment and disposal, little work has been undertaken in Australia on the use of constructed wetlands for reducing nutrient concentrations in dairy wastewaters. Masters (1993) described a low cost dairy waste treatment system using phosphorus sorbing materials for a farm in Western Australia, while in New Zealand. Tanner et al. (1995 a, b) have undertaken extensive work on the effect of different loading rates on the pollutant removal efficiencies of a constructed wetland system for dairy wastewaters. As a result, Tanner and Kloosterman (1997) have recently produced guidelines for the design of constructed wetlands for farm dairy wastewaters in that country.
In 1995 the Australian Dairy Research & Development Corporation and the Hunter Catchment Management Trust funded the construction of a wetland system at Tocal Agricultural College, near Maitland in the Hunter Valley (NSW). The objectives of the project were to examine the pollutant removal efficiencies which could be achieved by a constructed wetland receiving dairy wastewaters and to report on the suitability of such a natural treatment system for dairy waste management. METHODOLOGY Wetland description The dairy waste management system which existed at the College prior to 1995 consisted of a small coarse solids separator, a two-pond (anaerobic/facultative) storage system followed by spray irrigation to land. The modem dairy, which is managed by College staff and utilised in teaching agricultural students, is cleaned daily with water from two sources. Town water is used for the cleaning of milk storage vessels, while river water and a large diameter hose are used to clean the dairy stalls and concrete yards. All water used, apart from roof runoff, enters the dairy waste management system. Herd size typically varies but averages 110 head. Prior to commencing the study, water samples from the two-pond storage system were taken and analysed and each pond volume was surveyed. The results indicated that the existing treatment system did not result in significant improvements in effiuent quality and that inadequate on-farm storage existed for the wastewater volumes generated, particularly during wet weather periods. Existing pond volumes were calculated as 280 m3 (anaerobic) and 120 m 3 (facultative). Some initial remediation works such as redesign and enlarging of the coarse solids separator, and sludge removal from the anaerobic pond, were undertaken prior to the study commencing. There were some immediate improvements in wastewater quality as a result ofthis structural work, particularly in relation to reductions in BOD.
In late 1995 two 32m long surface flow wetland trenches were constructed as shown schematically in Figure 1. Wetlands I and 2 (WI and W2) had surface areas of 127 m2 (average depth 0.24 m) and 168 m2 (average depth 0.52 m) respectively. The total wetland volume was therefore 100 m 3• In their initial design, it was envisaged that the wetlands would be gravity fed by effiuent from the facultative pond and estimates of water use for wetland sizing were derived following an audit of dairy operations and consultation with dairy staff. After further discussions about the need to return effiuent to the facultative pond for effiuent irrigation and, as there was no electricity to the site, a decision was made to gravity feed effiuent from the anaerobic pond with return to the facultative lagoon. While the wetland system provided further detention time in an already overloaded system, it was clear that the wetland would receive higher organic and nutrient loads in this position.
Suitability of a treatment wetland for dairy wastewaters
coarse solids separator
-:
D
I
dairy
181
I diversion bank tipping bucket gauge
anaerobic pond
US class A panO & raingauge
tipping bucket gauge \
• facultative pond
suction Iysimeters
•I
o
•
to land application
SW1
•
•
SW2
SW3
10 I
metres Figure 1. Tocal Agricultural College dairy effluent management system
After the trenches were constructed, an impermeable, synthetic liner was positioned on the base and downslope walls of each wetland. A 0.2 m layer of bentonite clay was then placed on top of the liner and overlain by 0.3 m of topsoil which was obtained from the excavation. This soil was analysed and assessed as a suitable substrate for the wetland plants. Three species of native macrophytes (Baumea articulata. Phragmites australis and Schoenoplectus mucronatus) were hand planted in broad species bands (approximately 51m 2 ) in October 1995. The plants were then grown over the summer in shallow clean water obtained from the pasture irrigation system. Water was added to the trenches as the plants grew until April 1996, when dairy effiuent was diverted into the wetlands. Instrumentation and monitoring During 1996 monitoring instruments were installed at the dairy and at the wetland to measure a number of hydrological variables. Flow meters were fitted to both water sources used in dairy cleaning operations, and two large tipping bucket gauges (approximately 6 Lltip) were installed to measure inflows and outflows from the wetlands (Figure I). During 1997 a data logger was used to log flow rates and to verify the number of tips recorded from the mechanical counters on each bucket. A non-recording rain gauge and U.S. Class A evaporation pan were also placed on-site, although longer term daily records are also available from the College's meteorological station, approximately I km away. Other instruments which were installed included a series of tensiometers and suction Iysimeters, downslope from the second wetland, to enable an assessment to be made of the sub-surface loss for water balance calculations. Monitoring of a number of water quality parameters was undertaken at locations in the wetland: the inlet to Wetland I (WI-In) and the outlet from Wetland 2 (W2-0ut). A number of field analyses were conducted weekly between May 1996 and April 1998 using a Horiba Water Quality Checker (pH, electrical conductivity (EC), temperature and turbidity) and a YSI Dissolved Oxygen Meter (DO). Water quality sampling for BOD, ammonia (NH) total Kjeldahl nitrogen (TKN) and total phosphorus (TP) was undertaken monthly from May 1996 until October 1997 when a 2-weekly cycle was introduced, which then continued until April 1998. Both total oxidised nitrogen (TON) and dissolved reactive phosphorus (DRP) were also monitored periodically in the wetland
P. M. GEARY and J. A. MOORE
182
RESULTS AND DISCUSSION Flow monitoring The monitoring of inflows and outflows was undertaken using large tipfing bucket gauges which were regularly calibrated. Monthly flows to the wetland varied between 136.5 m (15.4 mm/d) and 295.7 m3(35.8 mm/d) depending on the volume of water used by the dai?, and variations in rainfall and evaporation. Monthly flows out of the wetland varied between 120.0 m and 281.7 m3, with the difference between inflows and outflows explained by evapotranspiration and some minor loss from the wetland by seepage. Hydraulic residence times based on the surveyed wetland volumes varied between 9 and 21 days using the monthly flow data. Analysis of the water balance data collected during the study has shown that the seepage component from the wetland is very small, approximately 0.2 to 0.3 m3/d (Geary, 1997), particularly in relation to the daily hydraulic loads to the wetland which typically varied between 6 and 10 m3(average 7.5 m3/d). The data also shows that evapotranspiration is the major factor responsible for the reductions in flow through the wetland and that the evapotranspiration rates from the wetland plants are variable thoughout the year. In wet periods and particularly following intense rainfall events, more water left the wetland than entered through the inflow pipe. This was due to rainfall falling directly on the wetland, while an upslope diversion bank prevented runoff entering the wetland from other sources. Water quality monitoring The monitoring data from the inlet to Wetland 1 and the outlet from Wetland 2 for the period June 1996 to April 1998 have been summarised and are presented in Table 1. The BOD test was for a 5 day period, while the Organic Nitrogen concentrations were calculated as the difference between the TKN and NH3 concentrations. The IDS concentrations were calculated by multiplying the electrical conductivity (uS/em) by 0.65. The effluent quality appeared to be typical of dairy runoff as it contained high concentrations of organic matter and nutrients, and its quality often varied. According to the mean concentration data in Table I, there was an improvement in effluent quality as a result of its passage through the wetland. The performance of the wetland in contributing to the removal ofthese pollutants is, however, affected by meteorological factors such as evaporation and rainfall, farm and dairy management practices and the growth/senescence of the wetland plants. Table 1. Wetland water quality (concentrations in mgll)
WI BOD-In W2BOD-Out WI TKN-In W2TKN-Out WI NH3-In W2 NH3-0ut WI Total P-In W2 Total P-Out WI IDS-In W2 TDS-Out
Mean 220 90 227 166 126 105 59.3 48.9 2307 2074
Number 32 31 32 31 30 29 32 31 94 94
Max 439 199 364 306 267 201 86.3 89.3 3243 2983
Min 64 16 108 88 36 35 40.2 24.2 1352 1131
Std Deviation 104 57 66 52 59 47 10.6 12.3 536 481
The variations in BOD entering and leaving the wetland are shown in Figure 2. The variability in inlet concentrations was moderated by the solids separator and desludging of the anaerobic pond, however, there were occasions when sludge entered the wetland, particularly following pipe blockages. While the concentrations at the outlet were also variable, they appeared to be less variable than the influent. The
Suitability of a treatment wetland for dairy wastewaten
183
detention offered by the wetland clearly resulted in significant reductions in BOD with concentrations several hundred to less than 50 mgIL. regularly being reduced
from
-'-Inlet BOD (mgIL) _ _ OutletBOD (mgIL)
Figure 2. Variations in BOD concentrations.
Animal waste is characterised by high concentrations of ammonia and organic nitrogen. The high TKN concentrations from the animal manure resulted in a high nitrogen loading to the wetland. Only minor TON concentrations are present in the wetland (typically <1 mgIL). The wetland system was clearly oxygen limited due to the high demand created by the organic matter, although some oxygen was usually present in the effluent from Wetland 2. Approximately 50-80% of the TKN was present as ammonia. Total Phosphorus concentrations were generally very high with the DRP usually about 50-60% of the Total Phosphorus. While phosphorus was present in the effluent from the animal manure, a number of cleaning agents used for milk storage vessels in the parlour contained phosphoric acid which contributed to the high concentrations. The total dissolved solids concentrations were primarily derived from the animal manure and the sourced river water.
100
-r-----------------,
90 80
~
70 60 50
-+- nlet P (rrg/\.) _ _ OUtlet P (mglL)
40 30 20 10 O-Jll,...,.........,.....,....,....,...,...,...,........-r......,..."T"T"~,...,...,r"'T""'T...,.....................l
OjIO OjIO OjIO OjIO ~ ~ ~ ~ OJ
tV'
tV'
Figure 3. Variations in Total Phosphorus concentrations
The variations in Total Phosphorus concentrations entering and leaving the wetland (Figure 3) clearly show that phosphorus removal was initially quite significant. After approximately 10 months the efficiency of the
P. M. GEARY and J. A. MOORE
184
wetland appeared to decrease and became quite variable. For the last 6 months of the study. the effiuent leaving the wetland had Total Phosphorus concentrations higher or very similar to those of the influent. While part of the phosphorus present in the effiuent would have been utilised by the macrophytes. the high phosphorus sorption of the topsoil used as substrate appeared to be responsible for a large proportion of the phosphorus removed. Once the capacity of the soil with respect to sorption was reached, the system commenced to leach phosphorus. The question of phosphorus removal in constructed wetland systems is problematic. The ability of sediments and wetlands to retain added phosphorus depends on the their P sorption capacity and physico-chemical properties (Reddy et aI., 1998). While some studies of aquatic plant systems, such as Brix (1997), have found the uptake capacity of emergent macrophytes to be quite high (30150 kg/ha/yr), other studies which have specifically looked at large changes in concentrations of nutrients around plant roots have found relatively small changes in tissue concentrations (Asher and Loneragan, 1967). It would appear that the wetland plants only uptake the phosphorus they need from the nutrient-rich effluent. Pollutant removal efficiencies To enable the pollutant removal efficiency of the wetland to be assessed, it is more appropriate to examine removals using a mass balance approach. This approach incorporates hydraulic loading and time variable transfers, such as precipitation (which may dilute) and evapotranspiration (which may concentrate). The detennination ofbudgets is important for understanding the nutrient removal functions of wetlands (Kadlec, 1986). From the hydraulic loadings to the wetland and the concentration data collected during the monitoring period, the percent mass removal of the wetland has been calculated. A summary of the monthly pollutant removal efficiencies achieved by the wetland is shown in Table 2. While the input-output budget for the dairy wetland serves to summarise the short-term net function of the system, it yields little information on the mechanisms by which these results are achieved. Nevertheless, this approach suggests that the constructed wetland performance was reasonable given the hydraulic and high constituent loads. Loading rates to the wetland averaged 5.6 g/m2/d for BOD (56 kg/ha/d), 2.6 g/m 2/d for Organic N (26 kg/ha/d), 3.2 g/m 2/d for NIl) (32 kg/ha/d), 1.5 g/m 2/d for Total P (15 kg/ha/d) and 46.9 g/m 2/d for IDS (469 kg/ha/d) during the study. The average monthly pollutant efficiencies from the flow weighted data in Table 2 are all higher than the removals calculated from the concentration data presented in Table 1. Table 2. Calculated monthly pollutant removal efficiencies (percentage)
BOD Organic N NH) Total P IDS
Mean 60.9 42.8 25.6 27.7 20.9
Number 21 21 21 21 21
Max 88.3 77.1 45.4 56.7 29.2
Min 16.5 12.65 3.2 - 8.6 8.9
Std Deviation 19.4 19.1 13.1 17.5 5.0
Constructed wetland studies have generally reported significant reductions in BOD due to the additional detention provided by further storage and the presence of plants assisting with sedimentation and filtration. These reductions are also due to various decomposition processes within the wetland ecosystem. Niswander (1997) in a similar dairy wetland study reported reductions in BOD of 52% with an average loading of 188 k/Yhald. while the 61% reported here is slightly lower than the 68% reported for dairy wetlands by Knight et al. (1996). In relation to the Tocal data, the high BOD loading of the wastewater quickly consumes any available oxygen and creates an anaerobic environment in Wetland 1. The removal efficiencies for nitrogen achieved by the wetland were quite variable. The reduction of Organic N (43%) is principally by decomposition and associated with some settlement of sludge carried over from the anaerobic pond. The anunonia concentrations in the effluent (while decreasing overall) generally increase as a proportion of the TKN, suggesting that ammonification is also an important process. The wetland is, however, oxygen-limited at these high loading rates, so there is little opportunity for the process of nitrification to occur. Ammonia removals through the wetland averaged only 26%.
Suitability of a treatment wetland for dairy wastewaters
185
Phosphorus removal rates were quite variable, but on average they were low (28%). Monthly figures suggest that the rate of removal decreased over time and on occasions the wetland released phosphorus. While phosphorus may be immobilised in constructed wetlands by plant uptake, adsorption, precipitation and incorporation into biological films, P removal is highly sensitive to loading rate and considered finite subject to the ability of the substrate to sorb phosphorus. In this wetland, a significant portion of the P removal is considered to occur in Wetland 1 in association with the deposition of sludge carried over from the anaerobic pond. Although the wetland acted as a sink for the storage of some of the phosphorus in the dairy effluent, the 28% reported here is less than the 42% reported by Niswander (1997) at a lower loading rate (I2 kglhald). In general, the long-term removal rate of phosphorus in wetlands is generally much lower than the removal rates of solids or BOD. CONCLUSIONS A treatment wetland constructed at Tocal Agricultural College received poor quality effluent from the dairy parlour and holding yards. The performance of the treatment wetland was monitored and the results suggested that varying degrees of pollutant removal are possible. The highest pollutant removals occurred for BOD, followed by Organic N, Total P, NH3 and IDS. At high loading rates the wetland appeared to act more as a sink for the pollutants which are removed from the dairy wastewater. This is particularly the case for phosphorus where the substrate of the wetland became saturated with this nutrient. On the basis of these monitoring results, a treatment wetland of this size would not appear to be suitable as a long-term option for the treatment of dairy waste. While harvesting the vegetation and replacing the substrate may increase the storage capacity of the system, neither of these practices is likely to find favour on a working dairy farm. A final report on the results of this study has been prepared for the Australian Dairy Research & Development Corporation (Geary, 1999). ACKNOWLEDGEMENTS Funding for this project was provided by the Dairy Research and Development Corporation and the Hunter Catchment Management Trust. Their support is acknowledged, in addition to the assistance provided by Peter Shimeld, Scott Burchell and College staff. REFERENCES Asher, C. J. and Loneragan, J. F. (1967). Response of plants to phosphate concentration in solution culture. I, Growth and phosphorus content. Soil Sci., 103,225-233. Brix, H. (1997). Do macrophytes playa role in constructed treatment wetlands? Wat. Sci. Tech., 35(5), 11-17. ClUM Hill and Payne Engineering (1997). Constructed Wetlands for Livestock Wastewater Management, EPA Gulf of Mexico Program, Nutrient Enrichment Committee, Stennis Space Center, USA. Geary, P. M. (1997). Performance of a Constructed Wetland Receiving High Hydraulic and Constituent Loads, In UNEP International Regional Conference Proceedings on Environmental Technologies for Wastewater Management, (G. Ho et al. eds), Murdoch University, perth, Western Australia. Geary, P. M. (1999). The Use of Artificial Wetlands for Nutrient Reduction in Dairy EfJ1uent, Report for Dairy Research and Development Corporation, Melbourne, Australia. Kadlec, J. A. (1986). Input-output budgets for small diked marshes, Con. J. Aquat. Sci., 43, 2009-2016. Knight, R. L., Payne, V., Borer, R. E., Clarke, R. A. and Pries, J. H. (1996). Final Draft, Livestock Wastewater Treatment Database, prepared for EPA Gulf of Mexico Program Masters, B. K. (1993). Management of dairy waste: A low cost treatment system using phosphorus-adsorbing materials. Wat. Sci. Tech., 27( I), 159-170. Niswander, S. F. (1997). Treatment of Dairy Wastewater in a Constructed Wetland System.Evapotranspiration, Hydrology. Hydraulics. Treatment Performance and Nitrogen Cycling Processes, PhD thesis, Department of Bioresource Engineering, Oregon State University, Corvallis. Reddy, K. R., O'Connor, G. A. and Gale, P. M. (1998). Phosphorus sorption capacities of wetland soils and stream sediments impacted by dairy effiuent. J. Environ. Qual. 27, 438-447. Tanner, C. C., Clayton, J. S. and Upsdell, M. P. (1995a, b). Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands. I: Removal of oxygen demand, suspended solids & faecal coliforms; II: Removal of nitrogen & phosphorus. Water Res., 29,17-34. Tanner. C. C. and Kloosterman, V. C. (1997). Guidelines for Constructed Wetland Treatment ofFarm Dairy Wastewaters in New Zealand. NIWA Science & Technology Series, Hamilton. Wrigley, R. (1994). Managing Dairy-Shed Wastes, Volume 2, Dairy Research and Development Corporation, Melbourne.
Wat. Sci Tech. Vol. 40, No.3, pp. 179-185, 1999
Pergamon
Publishedby ElsevierScience Ltd Printed in Great Britain. All nghts reserved 0273-1223199 $20.00 + 0.00
PH: S0273-1223(99)00464-3
SUITABILITY OF A TREATMENT WETLAND FOR DAIRY WASTEWATERS P. M. Geary* and J. A. Moore** • Department ofGeography & Environmental Science, The University ofNewcastle, Callaghan, 2308, NSW, Australia •• Department ofBioresource Engineering. Oregon State University, Corvallis, OR 97331-3906. USA
ABSTRACT A treatment wetland was constructed as part of a waste management system for dairy parlour waters and the performance of the wetland in reducing organic matter and nutrients monitored for a two-year period. The wetland was designed for a herd size of 110 with a detention time of approximately 10-14 days. Hydraulic loads to the wetland averaged 25 rnrnIday, although there were wide fluctuations due to rainfall and water use within the dairy. Average mass loadings to the wetland were 5.6 glm2/d for Biochemical Oxygen Demand, 2.6 glm2/d for Organic Nitrogen, 3.2 glm2/d for Ammonia and l.S glm 2/d for Total Phosphorus.
Monitoring results from the system indicated that significant BOD reductions were achieved, while Nitrogen and Phosphorus removals were variable but smaller. Calculated mean monthly pollutant reductions due to the treatment wetland were 61% for BOD, 43% for Organic Nitrogen, 26% for NH) and 28% for TP. The wetland received high hydraulic and pollutant loads and appeared to act as a sink for the nutnents which were removed. At this scale it did not appear to be suitable as a treatment option for significantly reducing nutrients in this type of agricultural waste. iC 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved
KEYWORDS Agricultural waste; biochemical oxygen demand; macrophytes; nitrogen; phosphorus; pollutant removal.
INTRODUCTION Wastewaters from intensive agricultural activities (cattle feedlots, piggeries and dairies) typically have significantly higher concentrations of organic matter and nutrients than treated municipal effluent. The high pollutant loads which are generated pose particular problems and challenges for these industries as high concentrations of nutrients can contribute to water management problems if wastes are allowed to discharge directly to receiving waters. Agricultural wastes must be treated prior to disposal and constructed wetlands (in association with stabilisation ponds) have been suggested as a potential treatment option prior to land application. Within Australia intensive agricultural activities are being required to upgrade their waste management systems and the preparation and implementation of waste management plans is underway. Research which has examined the performance of constructed wetland systems for the treatment of dairy wastewaters overseas has shown that significant improvements in effluent quality may be achieved due to the physical, chemical and biological processes which occur in wetland ecosystems. A recent survey by CH2M Hill and 179
180
P. M. GEARY and J. A. MOORE
Payne Engineering (1997) reported that more than 65 pilot or full-scale wetlands have been built in North America since 1990 to treat livestock wastewater. While there are some difficulties in directly comparing pollutant removal efficiencies between studies, constructed wetlands do show potential for reducing BOD, suspended solids, faecal coliforms and, to a lesser extent, nutrients. Best management practice for the Australian dairy industry has been described by Wrigley (1994) and may include collection of milking shed wastes, pre-treatment, waste stabilisation pond(s) and land application by spray irrigation. While there has been research undertaken on a number of aspects of dairy waste treatment and disposal, little work has been undertaken in Australia on the use of constructed wetlands for reducing nutrient concentrations in dairy wastewaters. Masters (1993) described a low cost dairy waste treatment system using phosphorus sorbing materials for a farm in Western Australia, while in New Zealand. Tanner et al. (1995 a, b) have undertaken extensive work on the effect of different loading rates on the pollutant removal efficiencies of a constructed wetland system for dairy wastewaters. As a result, Tanner and Kloosterman (1997) have recently produced guidelines for the design of constructed wetlands for farm dairy wastewaters in that country.
In 1995 the Australian Dairy Research & Development Corporation and the Hunter Catchment Management Trust funded the construction of a wetland system at Tocal Agricultural College, near Maitland in the Hunter Valley (NSW). The objectives of the project were to examine the pollutant removal efficiencies which could be achieved by a constructed wetland receiving dairy wastewaters and to report on the suitability of such a natural treatment system for dairy waste management. METHODOLOGY Wetland description The dairy waste management system which existed at the College prior to 1995 consisted of a small coarse solids separator, a two-pond (anaerobic/facultative) storage system followed by spray irrigation to land. The modem dairy, which is managed by College staff and utilised in teaching agricultural students, is cleaned daily with water from two sources. Town water is used for the cleaning of milk storage vessels, while river water and a large diameter hose are used to clean the dairy stalls and concrete yards. All water used, apart from roof runoff, enters the dairy waste management system. Herd size typically varies but averages 110 head. Prior to commencing the study, water samples from the two-pond storage system were taken and analysed and each pond volume was surveyed. The results indicated that the existing treatment system did not result in significant improvements in effiuent quality and that inadequate on-farm storage existed for the wastewater volumes generated, particularly during wet weather periods. Existing pond volumes were calculated as 280 m3 (anaerobic) and 120 m 3 (facultative). Some initial remediation works such as redesign and enlarging of the coarse solids separator, and sludge removal from the anaerobic pond, were undertaken prior to the study commencing. There were some immediate improvements in wastewater quality as a result ofthis structural work, particularly in relation to reductions in BOD.
In late 1995 two 32m long surface flow wetland trenches were constructed as shown schematically in Figure 1. Wetlands I and 2 (WI and W2) had surface areas of 127 m2 (average depth 0.24 m) and 168 m2 (average depth 0.52 m) respectively. The total wetland volume was therefore 100 m 3• In their initial design, it was envisaged that the wetlands would be gravity fed by effiuent from the facultative pond and estimates of water use for wetland sizing were derived following an audit of dairy operations and consultation with dairy staff. After further discussions about the need to return effiuent to the facultative pond for effiuent irrigation and, as there was no electricity to the site, a decision was made to gravity feed effiuent from the anaerobic pond with return to the facultative lagoon. While the wetland system provided further detention time in an already overloaded system, it was clear that the wetland would receive higher organic and nutrient loads in this position.
Suitability of a treatment wetland for dairy wastewaters
coarse solids separator
-:
D
I
dairy
181
I diversion bank tipping bucket gauge
anaerobic pond
US class A panO & raingauge
tipping bucket gauge \
• facultative pond
suction Iysimeters
•I
o
•
to land application
SW1
•
•
SW2
SW3
10 I
metres Figure 1. Tocal Agricultural College dairy effluent management system
After the trenches were constructed, an impermeable, synthetic liner was positioned on the base and downslope walls of each wetland. A 0.2 m layer of bentonite clay was then placed on top of the liner and overlain by 0.3 m of topsoil which was obtained from the excavation. This soil was analysed and assessed as a suitable substrate for the wetland plants. Three species of native macrophytes (Baumea articulata. Phragmites australis and Schoenoplectus mucronatus) were hand planted in broad species bands (approximately 51m 2 ) in October 1995. The plants were then grown over the summer in shallow clean water obtained from the pasture irrigation system. Water was added to the trenches as the plants grew until April 1996, when dairy effiuent was diverted into the wetlands. Instrumentation and monitoring During 1996 monitoring instruments were installed at the dairy and at the wetland to measure a number of hydrological variables. Flow meters were fitted to both water sources used in dairy cleaning operations, and two large tipping bucket gauges (approximately 6 Lltip) were installed to measure inflows and outflows from the wetlands (Figure I). During 1997 a data logger was used to log flow rates and to verify the number of tips recorded from the mechanical counters on each bucket. A non-recording rain gauge and U.S. Class A evaporation pan were also placed on-site, although longer term daily records are also available from the College's meteorological station, approximately I km away. Other instruments which were installed included a series of tensiometers and suction Iysimeters, downslope from the second wetland, to enable an assessment to be made of the sub-surface loss for water balance calculations. Monitoring of a number of water quality parameters was undertaken at locations in the wetland: the inlet to Wetland I (WI-In) and the outlet from Wetland 2 (W2-0ut). A number of field analyses were conducted weekly between May 1996 and April 1998 using a Horiba Water Quality Checker (pH, electrical conductivity (EC), temperature and turbidity) and a YSI Dissolved Oxygen Meter (DO). Water quality sampling for BOD, ammonia (NH) total Kjeldahl nitrogen (TKN) and total phosphorus (TP) was undertaken monthly from May 1996 until October 1997 when a 2-weekly cycle was introduced, which then continued until April 1998. Both total oxidised nitrogen (TON) and dissolved reactive phosphorus (DRP) were also monitored periodically in the wetland
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RESULTS AND DISCUSSION Flow monitoring The monitoring of inflows and outflows was undertaken using large tipfing bucket gauges which were regularly calibrated. Monthly flows to the wetland varied between 136.5 m (15.4 mm/d) and 295.7 m3(35.8 mm/d) depending on the volume of water used by the dai?, and variations in rainfall and evaporation. Monthly flows out of the wetland varied between 120.0 m and 281.7 m3, with the difference between inflows and outflows explained by evapotranspiration and some minor loss from the wetland by seepage. Hydraulic residence times based on the surveyed wetland volumes varied between 9 and 21 days using the monthly flow data. Analysis of the water balance data collected during the study has shown that the seepage component from the wetland is very small, approximately 0.2 to 0.3 m3/d (Geary, 1997), particularly in relation to the daily hydraulic loads to the wetland which typically varied between 6 and 10 m3(average 7.5 m3/d). The data also shows that evapotranspiration is the major factor responsible for the reductions in flow through the wetland and that the evapotranspiration rates from the wetland plants are variable thoughout the year. In wet periods and particularly following intense rainfall events, more water left the wetland than entered through the inflow pipe. This was due to rainfall falling directly on the wetland, while an upslope diversion bank prevented runoff entering the wetland from other sources. Water quality monitoring The monitoring data from the inlet to Wetland 1 and the outlet from Wetland 2 for the period June 1996 to April 1998 have been summarised and are presented in Table 1. The BOD test was for a 5 day period, while the Organic Nitrogen concentrations were calculated as the difference between the TKN and NH3 concentrations. The IDS concentrations were calculated by multiplying the electrical conductivity (uS/em) by 0.65. The effluent quality appeared to be typical of dairy runoff as it contained high concentrations of organic matter and nutrients, and its quality often varied. According to the mean concentration data in Table I, there was an improvement in effluent quality as a result of its passage through the wetland. The performance of the wetland in contributing to the removal ofthese pollutants is, however, affected by meteorological factors such as evaporation and rainfall, farm and dairy management practices and the growth/senescence of the wetland plants. Table 1. Wetland water quality (concentrations in mgll)
WI BOD-In W2BOD-Out WI TKN-In W2TKN-Out WI NH3-In W2 NH3-0ut WI Total P-In W2 Total P-Out WI IDS-In W2 TDS-Out
Mean 220 90 227 166 126 105 59.3 48.9 2307 2074
Number 32 31 32 31 30 29 32 31 94 94
Max 439 199 364 306 267 201 86.3 89.3 3243 2983
Min 64 16 108 88 36 35 40.2 24.2 1352 1131
Std Deviation 104 57 66 52 59 47 10.6 12.3 536 481
The variations in BOD entering and leaving the wetland are shown in Figure 2. The variability in inlet concentrations was moderated by the solids separator and desludging of the anaerobic pond, however, there were occasions when sludge entered the wetland, particularly following pipe blockages. While the concentrations at the outlet were also variable, they appeared to be less variable than the influent. The
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detention offered by the wetland clearly resulted in significant reductions in BOD with concentrations several hundred to less than 50 mgIL. regularly being reduced
from
-'-Inlet BOD (mgIL) _ _ OutletBOD (mgIL)
Figure 2. Variations in BOD concentrations.
Animal waste is characterised by high concentrations of ammonia and organic nitrogen. The high TKN concentrations from the animal manure resulted in a high nitrogen loading to the wetland. Only minor TON concentrations are present in the wetland (typically <1 mgIL). The wetland system was clearly oxygen limited due to the high demand created by the organic matter, although some oxygen was usually present in the effluent from Wetland 2. Approximately 50-80% of the TKN was present as ammonia. Total Phosphorus concentrations were generally very high with the DRP usually about 50-60% of the Total Phosphorus. While phosphorus was present in the effluent from the animal manure, a number of cleaning agents used for milk storage vessels in the parlour contained phosphoric acid which contributed to the high concentrations. The total dissolved solids concentrations were primarily derived from the animal manure and the sourced river water.
100
-r-----------------,
90 80
~
70 60 50
-+- nlet P (rrg/\.) _ _ OUtlet P (mglL)
40 30 20 10 O-Jll,...,.........,.....,....,....,...,...,...,........-r......,..."T"T"~,...,...,r"'T""'T...,.....................l
OjIO OjIO OjIO OjIO ~ ~ ~ ~ OJ
tV'
tV'
Figure 3. Variations in Total Phosphorus concentrations
The variations in Total Phosphorus concentrations entering and leaving the wetland (Figure 3) clearly show that phosphorus removal was initially quite significant. After approximately 10 months the efficiency of the
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wetland appeared to decrease and became quite variable. For the last 6 months of the study. the effiuent leaving the wetland had Total Phosphorus concentrations higher or very similar to those of the influent. While part of the phosphorus present in the effiuent would have been utilised by the macrophytes. the high phosphorus sorption of the topsoil used as substrate appeared to be responsible for a large proportion of the phosphorus removed. Once the capacity of the soil with respect to sorption was reached, the system commenced to leach phosphorus. The question of phosphorus removal in constructed wetland systems is problematic. The ability of sediments and wetlands to retain added phosphorus depends on the their P sorption capacity and physico-chemical properties (Reddy et aI., 1998). While some studies of aquatic plant systems, such as Brix (1997), have found the uptake capacity of emergent macrophytes to be quite high (30150 kg/ha/yr), other studies which have specifically looked at large changes in concentrations of nutrients around plant roots have found relatively small changes in tissue concentrations (Asher and Loneragan, 1967). It would appear that the wetland plants only uptake the phosphorus they need from the nutrient-rich effluent. Pollutant removal efficiencies To enable the pollutant removal efficiency of the wetland to be assessed, it is more appropriate to examine removals using a mass balance approach. This approach incorporates hydraulic loading and time variable transfers, such as precipitation (which may dilute) and evapotranspiration (which may concentrate). The detennination ofbudgets is important for understanding the nutrient removal functions of wetlands (Kadlec, 1986). From the hydraulic loadings to the wetland and the concentration data collected during the monitoring period, the percent mass removal of the wetland has been calculated. A summary of the monthly pollutant removal efficiencies achieved by the wetland is shown in Table 2. While the input-output budget for the dairy wetland serves to summarise the short-term net function of the system, it yields little information on the mechanisms by which these results are achieved. Nevertheless, this approach suggests that the constructed wetland performance was reasonable given the hydraulic and high constituent loads. Loading rates to the wetland averaged 5.6 g/m2/d for BOD (56 kg/ha/d), 2.6 g/m 2/d for Organic N (26 kg/ha/d), 3.2 g/m 2/d for NIl) (32 kg/ha/d), 1.5 g/m 2/d for Total P (15 kg/ha/d) and 46.9 g/m 2/d for IDS (469 kg/ha/d) during the study. The average monthly pollutant efficiencies from the flow weighted data in Table 2 are all higher than the removals calculated from the concentration data presented in Table 1. Table 2. Calculated monthly pollutant removal efficiencies (percentage)
BOD Organic N NH) Total P IDS
Mean 60.9 42.8 25.6 27.7 20.9
Number 21 21 21 21 21
Max 88.3 77.1 45.4 56.7 29.2
Min 16.5 12.65 3.2 - 8.6 8.9
Std Deviation 19.4 19.1 13.1 17.5 5.0
Constructed wetland studies have generally reported significant reductions in BOD due to the additional detention provided by further storage and the presence of plants assisting with sedimentation and filtration. These reductions are also due to various decomposition processes within the wetland ecosystem. Niswander (1997) in a similar dairy wetland study reported reductions in BOD of 52% with an average loading of 188 k/Yhald. while the 61% reported here is slightly lower than the 68% reported for dairy wetlands by Knight et al. (1996). In relation to the Tocal data, the high BOD loading of the wastewater quickly consumes any available oxygen and creates an anaerobic environment in Wetland 1. The removal efficiencies for nitrogen achieved by the wetland were quite variable. The reduction of Organic N (43%) is principally by decomposition and associated with some settlement of sludge carried over from the anaerobic pond. The anunonia concentrations in the effluent (while decreasing overall) generally increase as a proportion of the TKN, suggesting that ammonification is also an important process. The wetland is, however, oxygen-limited at these high loading rates, so there is little opportunity for the process of nitrification to occur. Ammonia removals through the wetland averaged only 26%.
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Phosphorus removal rates were quite variable, but on average they were low (28%). Monthly figures suggest that the rate of removal decreased over time and on occasions the wetland released phosphorus. While phosphorus may be immobilised in constructed wetlands by plant uptake, adsorption, precipitation and incorporation into biological films, P removal is highly sensitive to loading rate and considered finite subject to the ability of the substrate to sorb phosphorus. In this wetland, a significant portion of the P removal is considered to occur in Wetland 1 in association with the deposition of sludge carried over from the anaerobic pond. Although the wetland acted as a sink for the storage of some of the phosphorus in the dairy effluent, the 28% reported here is less than the 42% reported by Niswander (1997) at a lower loading rate (I2 kglhald). In general, the long-term removal rate of phosphorus in wetlands is generally much lower than the removal rates of solids or BOD. CONCLUSIONS A treatment wetland constructed at Tocal Agricultural College received poor quality effluent from the dairy parlour and holding yards. The performance of the treatment wetland was monitored and the results suggested that varying degrees of pollutant removal are possible. The highest pollutant removals occurred for BOD, followed by Organic N, Total P, NH3 and IDS. At high loading rates the wetland appeared to act more as a sink for the pollutants which are removed from the dairy wastewater. This is particularly the case for phosphorus where the substrate of the wetland became saturated with this nutrient. On the basis of these monitoring results, a treatment wetland of this size would not appear to be suitable as a long-term option for the treatment of dairy waste. While harvesting the vegetation and replacing the substrate may increase the storage capacity of the system, neither of these practices is likely to find favour on a working dairy farm. A final report on the results of this study has been prepared for the Australian Dairy Research & Development Corporation (Geary, 1999). ACKNOWLEDGEMENTS Funding for this project was provided by the Dairy Research and Development Corporation and the Hunter Catchment Management Trust. Their support is acknowledged, in addition to the assistance provided by Peter Shimeld, Scott Burchell and College staff. REFERENCES Asher, C. J. and Loneragan, J. F. (1967). Response of plants to phosphate concentration in solution culture. I, Growth and phosphorus content. Soil Sci., 103,225-233. Brix, H. (1997). Do macrophytes playa role in constructed treatment wetlands? Wat. Sci. Tech., 35(5), 11-17. ClUM Hill and Payne Engineering (1997). Constructed Wetlands for Livestock Wastewater Management, EPA Gulf of Mexico Program, Nutrient Enrichment Committee, Stennis Space Center, USA. Geary, P. M. (1997). Performance of a Constructed Wetland Receiving High Hydraulic and Constituent Loads, In UNEP International Regional Conference Proceedings on Environmental Technologies for Wastewater Management, (G. Ho et al. eds), Murdoch University, perth, Western Australia. Geary, P. M. (1999). The Use of Artificial Wetlands for Nutrient Reduction in Dairy EfJ1uent, Report for Dairy Research and Development Corporation, Melbourne, Australia. Kadlec, J. A. (1986). Input-output budgets for small diked marshes, Con. J. Aquat. Sci., 43, 2009-2016. Knight, R. L., Payne, V., Borer, R. E., Clarke, R. A. and Pries, J. H. (1996). Final Draft, Livestock Wastewater Treatment Database, prepared for EPA Gulf of Mexico Program Masters, B. K. (1993). Management of dairy waste: A low cost treatment system using phosphorus-adsorbing materials. Wat. Sci. Tech., 27( I), 159-170. Niswander, S. F. (1997). Treatment of Dairy Wastewater in a Constructed Wetland System.Evapotranspiration, Hydrology. Hydraulics. Treatment Performance and Nitrogen Cycling Processes, PhD thesis, Department of Bioresource Engineering, Oregon State University, Corvallis. Reddy, K. R., O'Connor, G. A. and Gale, P. M. (1998). Phosphorus sorption capacities of wetland soils and stream sediments impacted by dairy effiuent. J. Environ. Qual. 27, 438-447. Tanner, C. C., Clayton, J. S. and Upsdell, M. P. (1995a, b). Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands. I: Removal of oxygen demand, suspended solids & faecal coliforms; II: Removal of nitrogen & phosphorus. Water Res., 29,17-34. Tanner. C. C. and Kloosterman, V. C. (1997). Guidelines for Constructed Wetland Treatment ofFarm Dairy Wastewaters in New Zealand. NIWA Science & Technology Series, Hamilton. Wrigley, R. (1994). Managing Dairy-Shed Wastes, Volume 2, Dairy Research and Development Corporation, Melbourne.