Reciprocating Constructed Wetlands (ReCip) for Treating Anaerobic Lagoon Wastewater

Transcription

1 Reciprocating Constructed Wetlands (ReCip) for Treating Anaerobic Lagoon Wastewater Leslie L. Behrends and Bert R. Bock Tennessee Valley Authority Introduction and Technology Description This proposed on-farm demonstration to treat high-strength swine wastewater from an anaerobic lagoon relates to an innovative and economical wastewater treatment technology referred to as reciprocating constructed wetlands. Scientists, at the Tennessee Valley Authority's (TVA) Environmental Research Center, developed and refined the technology over a six-year period beginning in The reciprocating technology was patented in January of 1999, and subsequently two technology licenses have been issued by TVA to users and vendors of the technology. Subsurface-Flow Reciprocating Constructed Wetlands (ReCip) Reciprocation relates to patented improvements in the design and operation of paired subsurface-flow constructed wetlands, such that paired wetland cells are filled and drained on a recurrent basis (Behrends et al., 1993, 1996, 1999). The unique recurrent fill and drain technique turns the entire wetland system into a fixed-film biological reactor in which it is possible to alternate between aerobic and anaerobic environments on a controlled basis. Recurrent reciprocation enhances wastewater treatment because it promotes rapid development of a broad continuum of biologically mediated oxidation/reduction zones in which reactions, such as organic decomposition, nitrification/denitrification, biological phosphorus removal, iron reduction, sulfate reduction, and methanogenesis, occur. These reactions catalyze the degradation and transformation of organic and inorganic compounds (Zitomer and Speece, 1993). With the advent of reciprocating wetlands, it is now possible to manipulate the biological structure and function of wetland treatment systems by controlling such parameters as hydraulic retention time, frequency of reciprocation, reciprocation cycle time, depth of reciprocation, and size and composition of substrate. The robust nature of the treatment technology is based on a combination of design and operating factors, including: establishment of recurrent fill and drain cycles that are generally independent of influent flow rates,

2 ability to rapidly cycle between anaerobic, anoxic, and aerobic environments on a recurrent and controlled basis, thereby promoting high microbial diversity and a broad range of biotic and abiotic reactions, establishment of high specific surface area of graded substrate(s) for attachment of diverse microbial biofilms and rapid transfer of metabolic gases, selection and layering of specific substrates for enhancing hydraulic conductivity, biofiltration, adsorption, and alkalinity generation, and selection of specific aquatic/terrestrial plant assemblages to augment treatment (phytoremediation). Reciprocating wetlands have been used successfully in treating acid mine drainage, municipal wastewater, deicing compounds, food processing wastewater, and explosives-contaminated groundwater (see References). Since March of 1999, the technology has been under evaluation at TVA with respect to treatment of high-strength wastewater from an intensive tilapia aquaculture system. Biochemical oxygen demand (BOD5) concentrations and loading rates, during the study, have averaged 697 mg/l and 525 pounds/acre/day, respectively. This loading rate is 810 times higher than loadings to conventional wetlands. To date, the wastewater treatment system has consistently reduced BOD5, total nitrogen, total phosphorus, and odors (TVA, unpublished data; see Table 1). Table 1. Average influent and effluent parameter values for an experimental twostage reciprocating system for treating high-strength wastewater from an intensive tilapia aquaculture system. Parameter 1 n Influent Effluent ReCip 1 Effluent ReCip 2 Change(%) (Inf / ReCip 2) BOD5 (mg/l) COD (mg/l)

3 Total N (mg/l) 2 > Total P (mg/l) DO (mg/l) 16 <0.5 < ORP (millivolts) ph (unitless) Temp (oc) Alkalinity(mg/Las CaCO3) Specific cond. (umhos/cm) Mean values are based on 2-5 weekly observations or 16 daily observations. Farm Situation and Proposed Demonstration System Earlier this year, TVA scientists and engineers visited the farm site on two occasions to evaluate its suitability for an on-farm demonstration of treatment wetlands technology. During the second visit to the farm, TVA provided a technical presentation regarding the technology of constructed wetlands for treating high-strength wastewater. Technical documents, describing constructed wetlands and their applications, were also provided to the farm management team and USDA NRCS staff members. They subsequently provided TVA with detailed information on proposed waste management requirements, irrigation schedules, and cropping sequences. The farmer recently toured the TVA Wetlands R&D Facility in Muscle Shoals, Alabama, as well as the licensed commercial-scale reciprocating system operated in Tennessee. The farmer has expressed strong interest in the technology and the proposed demonstration of treating anaerobic lagoon water. Successful application of the technology will provide significant pretreatment of BOD5, nitrogen, and phosphorus in lagoon wastewater, thereby allowing higher levels of irrigation onto approximately 15 acres of coastal bermuda / small grains. Without wetlands pretreatment of nutrients, land requirements for wastewater irrigation increase to 43 acres, necessitating the costly harvesting of immature pine trees. Lagoon Sludge Management Currently, it is NRCS's recommendation that lagoon sludge be agitated and

4 pump-irrigated onto terrestrial crops every three to five years to maintain lagoon storage capacity. NRCS has estimated that up to 63.3 acres of hybrid bermuda / small grains would be required for proper disposal of agitated sludge wastewater through irrigation. These recommendations would require development of new forage land from immature and managed pine forests. Alternately, and as part of this wastewater treatment demonstration, it is proposed that approximately 10% of the wastewater to be treated in the constructed wetlands be pumped from the bottom of the lagoon to facilitate treatment of organic solids (sludge). This will reduce solids buildup in the lagoon and enhance the useful life of the lagoon. Treating solids daily, on a year-round basis will reduce, if not eliminate, the need for agitating and pumping sludge onto irrigation fields. Sludge depth in the lagoon and near the pump station will be measured quarterly to quantify the impact of continuous pumping of solids. Based on the expressed interest of the Farm's Management team, it is proposed that a demonstration-scale reciprocating wetland system be designed, constructed, and operated on the farm site. The on-farm demonstration will provide an excellent opportunity to document and validate the utility of reciprocating wetland systems for treating residual lagoon solids and highstrength animal wastewater. Figure 1 provides a conceptual view (not to scale) of the reciprocating wetland system in relation to an existing lagoon, swine barns (8), and spray-irrigation field. Figure 1. Conceptual diagram (not to scale) of proposed two-stage reciprocating system for treating high-strength wastewater from the lagoon of a confined swine facility.

5 In addition, the U.S. EPA has tentatively agreed to provide FY 2000 cost-share funds to help with construction and monitoring costs. Staff members of USDA's NRCS and the Alabama Department of Environmental Management (ADEM) will provide critical review of this proposal as well as technical input into the final draft of the on-farm waste management / demonstration plan. It is envisioned that two-stage, four-cell reciprocating systems can treat from 70,000 to 100,000 gallons/acre/day of concentrated lagoon wastewater, with significant reductions in odor, pathogens, BOD5, total nitrogen, and total phosphorus. Up to 10% of this flow will be composed of lagoon solids. Effluent from the treatment process will be stored in an earthen reservoir and used for irrigating hay and small grain crops during the growing season and for flushing under pit storage facilities during the winter months. Irrigation Plan The proposed reciprocating wetland design for the swine farm facility is based on two interrelated issues concerning irrigation and wastewater management: the mandated requirement to spray-irrigate up to 9.68 million gallons of lagoon wastewater (and solids), annually, in order to maintain the treatment lagoon water balance with the zero allowance for point discharge, and the need to sufficiently reduce total nitrogen in lagoon wastewater to allow reduction of irrigated acreage from 41.3 acres to approximately 15 acres. The irrigation plan recommended by NRCS for the existing anaerobic lagoon is presented in Table 2. This plan would require 41.3 acres of hybrid bermuda grass overseeded with a small grain in the fall. For comparison, the proposed irrigation plan for an anaerobic lagoon, coupled with a reciprocating wetland, is also presented in Table 2. The latter irrigation plan assumes that the reciprocating wetland would remove two-thirds of the lagoon N and reduce land requirements for land application by two-thirds, thus requiring only 13.8 acres. This provides a safety factor of 8.5%, since 15 acres of land is actually available for application of lagoon effluent. Generally, in the lagoon-wetlands system, the total volume of effluent applied per acre is three times higher than in the lagoon system, the number of applications is increased three-fold, and the volume per application remains the same. Increasing the number of applications should

6 prevent water logging, leaching, and runoff problems that could occur, if too much effluent were applied at one time. Table 2. Irrigation schedule for anaerobic lagoon and lagoon-reciprocating wetland systems. Anaerobic Lagoon Total application Month(s) Crop Acres Inches Acre-in. No. of applns. Average appln., in. February Small grain Mar Sep. 15 Hybrid bermuda October Small grain Nov. - Jan. Small grain Total Anaerobic Lagoon Coupled with Reciprocating Wetland to Remove 2/3 of the Lagoon N Total application Month(s) Crop Acres Inches Acre-in. No. of applns. Average appln., in. February Small grain Mar Sep. 15 Hybrid bermuda October Small grain Nov. - Jan. Small grain Total In February, the same volume of effluent is applied in both systems. February is usually sufficiently wet that applying more than 1.0 inch of effluent could cause water logging of the soil and/or potential for nitrate leaching. Untreated (before wetland treatment) lagoon water will be irrigated in February to achieve application of the recommended amount of available N. Since the amount of irrigation water applied per acre will be more nearly optimal for crop growth in the lagoon-wetland system, grass yields likely will be significantly higher than if one-third as much water were applied per acre in the lagoon system. This represents another safety factor regarding the amount of N

7 that the lagoon-wetlands system can handle. During the irrigation season (February to October), wetland effluent will be transferred to a holding pond and then used periodically for irrigation according to the irrigation plan in Table 2. If the wetland removes more than two-thirds of the N from the lagoon water for an extended period of time, untreated lagoon water will be used to supplement wetland effluent to provide to proper amount of N per acre. From November through January, no wetland effluent will be irrigated and the wetland effluent will be used to refill the hog house manure pits after they are flushed on a weekly basis. Because the wetland effluent is ultimately recycled to the lagoon during this period, the lagoon volume will not be reduced by the wetland; however, the amount of N and P in the lagoon will be reduced by cycling water from the lagoon to the wetland to the hog houses and back to the lagoon. Pine Tree Irrigation Experiments In addition to the irrigation scenario presented above, preliminary experiments will be planned and executed to evaluate the performance of pine trees under various irrigation options. These experiments will be designed to evaluate irrigation strategies for utilizing nutrients and water from either the anaerobic lagoon and/or the clean water reservoir. Since there is on the farm a substantial acreage of pine trees which are proximate to the swine facility, it was suggested by the farm owner that it would be prudent to evaluate this potential use of nutrients and water. Reciprocating System Design Assumptions and Design Calculations With respect to the farm, NRCS has estimated that acre-inches of nitrogen-rich lagoon effluent will be required, annually, to irrigate and meet the nitrogen requirements of 41.3 acres of coastal bermuda and small grains crops (Table 2). Based on the irrigation requirements for the reciprocating wetlands (Table 2), it is projected that there will be 16 irrigation events on 15 acres, ranging from 1 to 1.67 inches. During the period of March 15 to September 15, there will be 12 irrigation events totaling 301 acre inches (8,127,000 gallons). (This amount may be increased marginally to provide water for experimental irrigation of pine tree plots). Based on the above and for purposes of designing the two-stage (four-cell), reciprocating treatment wetland, it will be necessary to treat approximately 50,000 gallons per day (gpd), i.e., 40 gallons per minute (gpm), of lagoon wastewater to meet irrigation requirements. This treatment rate will also provide sufficient treated water for flushing one pit storage area daily, during the winter

8 months when there will be no need for irrigation water. Operational Guidelines for Reciprocating Constructed Wetlands After the reciprocating wetland system has been constructed, both the influent cells (Cells 1 & 3) and the effluent cells (Cells 2 & 4) should be filled with wastewater to within 4 inches of the top of the substrate and allowed to acclimate for approximately one week to establish anaerobic microbial biofilms. Within one to two weeks of full-scale operation, selected plants (wetland and/or terrestrial) can be planted at densities compatible with objectives (treatment vs. aesthetics), and the operating budget. However, for best survival and plant establishment, it is suggested that planting activities be delayed until late spring or early summer. Initial establishment of bacterial populations in the wetlands will provide adequate wastewater treatment during the first five to six months of operation. Influent and effluent flow rates and constituent concentrations of parameters (BOD5, COD, total suspended solids, total solids, ammonia, nitrate, total N, total P, ortho P, metals, and alkalinity) should be monitored to evaluate the health of the system, to monitor rates of evapotranspiration, and to facilitate mass balance calculations. These determinations should be done at least monthly, and according to the methods and materials outlined in Standard Methods (APHA, 1998). Monitoring of water quality should be continued for up to 24 months to validate the system's design and operational procedures and to quantify seasonal effects. References Behrends, L. L., H. S. Coonrod II, E. Bailey and M. J. Bulls Oxygen diffusion rates in reciprocating rock biofilters: potential applications for subsurface-flow constructed wetlands. 12 pp. Proceedings Subsurface-Flow Constructed Wetlands Conference, University of Texas at El Paso. Behrends, L. L., E. Baily, M. J. Bulls, H. S. Coonrod and F. J. Sikora. (1994). Seasonal trends in growth and biomass accumulation of selected nutrients and metals in six species of emergent aquatic macrophytes. Pp In: 4th International Conference on Wetland Systems for Water Pollution Control November Guangzhou, People's Republic of China. Behrends, L. L., F. J. Sikora, H. S. Coonrod, E. Baily and M. J. Bulls Coupled aerobic and anaerobic environments in constructed wetlands for removing ammonia, nitrate, and chemical oxygen demand: Potential for

9 remediating domestic, industrial, and agricultural wastewaters. In: Achieving High Performance at Low Cost in Environmental and Sanitation Control Systems. September 18-19, Salvador, Bahia, Brazil. Behrends, L. L. Patent Application Filed December 2, Reciprocating Subsurface-Flow Constructed Wetlands for Improving Wastewater Treatment. (Patent received January 1999). Behrends, L. L., F. J. Sikora, W. D. Phillips, E. Bailey, C. McDonald, and H. S. Coonrod Phytoremediation of explosives-contaminated groundwater in constructed wetlands: II. Flow through study. U.S. Army Environmental Center Report No. SFIM-AEC-ET-CR Behrends, L. L., F. J. Sikora, H. S. Coonrod, E. Bailey, C. McDonald, J. Clayton, and M. Higgins Reciprocating subsurface-flow constructed wetlands for treating domestic and municipal wastewater. Proceedings from Seventh Tennessee Water Resources Symposium, February 24-26, Nashville, Tennessee, Tennessee Section of the American Water Resources Association. Sikora, F. J., L. L. Behrends, and G. A. Brodie Manganese and trace metal removal in anaerobic and aerobic wetland environments. Proceedings of American Power Conference. Sikora, F. J., T. Zhu, L. L. Behrends, S. L. Steinberg, and H. S. Coonrod Ammonium removal in constructed wetlands with recirculating subsurface-flow: removal rates and mechanisms. Water Science and Technology 32: Sikora, F. J., L. L. Behrends, G. A. Brodie, and M. J. Bulls Manganese and trace metal removal in successive anaerobic and aerobic wetlands. Proceedings of American Society for Surface Mining and Reclamation 1996 Annual Meeting, May 1823, 1996, Knoxville, Tennessee. Sikora, F. J., L. L. Behrends, W. D. Phillips, H. S. Coonrod, E. J. Bailey, and D. F. Bader A microcosm study on remediation of explosives-contaminated groundwater using subsurface-flow constructed wetlands. Annals of New York Academy of Sciences 829: