Enwwra^Protocfcr Agmcy Constructed Wetlands for Wastewater Treatment and Wildlife Habitat Fort Deposit, AL: Constructed Wetland Treatment System Case History Fort Deposit, Alabama Canttrutfed W«U«nd Twrtnwm SyiMnt Background System Description Operations and Management Performance Ancillary Benefits Acknowledgements ------- Background The town of Fort Deposit, located south of Montgomery, Alabama, has a population of slightly more than 1,500. Until 1985, the town's wastewater was treated in a 10-acre waste stabilization pond and consistently met discharge limits. In 1985, a new discharge permit was issued by the Alabama Department of Environmental Management. This permit required the town to meet more stringent standards based on water quality limitations in the receiving water. Since the town's stabilization pond was unable to meet the new standards, an administrative order requiring the town to upgrade its system was issued. An engineering analysis of treatment alternatives was conducted by the environmental consulting firm CH2M HILL to compare a variety of conventional and innovative technologies. On the basis of an evaluation of environmental benefits, reliability, and cost, treatment by constructed wetlands was selected as the most cost-effective approach for compliance with the new permit limitations. The use of constructed wetlands to remove impurities in wastewater and to consistently achieve treatment levels that meet permit requirements was an emerging technology in 1985. To assist with funding their new system, the town applied for and was awarded a $610,000 U.S. Environmental Protection Agency (EPA) Innovative/Alternative Technology grant for its wetland project. This additional funding, coupled with low construction and maintenance costs associated with the wetland system, reduced the financial impact of the upgrade on the community and provided it with a system that would require only slightly more maintenance than the existing stabilization pond. Post-aeration is essential for compliance with the effluent standard for dissolved oxygen. ------- System Description As designed, the Fort Deposit wetland treatment system includes the following main components: • An 8.9-acre aerated pond • Two 7.5-acre constructed wetland cells • A 0.1-acre post-aeration pond The town's existing stabilization pond was modified to provide more effective pre-treatment. The modifications included relocating the influent and effluent points and adding floating mechanical aerators. Seven acres of the pond were aerated, leaving the remaining area to serve as a settling basin. These modifications improve 5-day biochemical oxygen demand (BODS) and ammonia nitrogen (NH3-N) removal efficiency, reduce organic and solids loading to the wetland cells, and provide additional flexibility in the overall treatment process. The wetland cells are configured side by side. Each cell covers 7.5 acres and has an aspect ratio (length to width) of 4.6:1. The cell floors are slightly sloped for easy draining during maintenance. Although most of the 15 acres of wetland cells are less than 2 feet deep, each cell has three "deep zones," which are 4 feet deep and about 20 feet wide. The deep zones remain free of rooted marsh vegetation, thus allowing effluent to be redistributed through the system and providing atmospheric aeration. The deeper water in these zones also furnishes year-round habitat for aquatic life, particularly mosquito fish and wetland birds. The Fort Deposit constructed wetland treatment system uses an aerated lagoon for pretreatment followed by two parallel wetland cells. The parallel operation of the two wetland cells gives the town the ability to direct all flow through a single cell during wetland resting and maintenance periods. Moreover, the rate of flow to each cell can be varied to allow flexibility in operations and to aid in testing or research. The treated effluent enters a post-aeration pond after passing through the wetland cells. This system component is used to meet the effluent dissolved oxygen limits specified in the permit. This 75,000- gallon earthen pond is equipped with a floating mechanical aerator. Final effluent flow rate from the post- aeration pond is continuously measured by a Parshall flume. ------- Operations and Management In the Fort Deposit wetland system, wastewater is treated by the naturally occurring bacteria and fungi that colonize the sediments on the bottom of the cells and the stems and leaves of the wetland vegetation below the water level. These microorganisms help transform and remove organic matter and nutrients that might otherwise degrade adjacent surface waters. The vegetation in the two wetland cells was selected to simulate a natural wetland and included an initial planting of 68,000 cattail and bulrush plants. Influent from the aerated pond is distributed to the cells by pipes with 1-inch holes drilled at 10-foot intervals. Outlet weir structures allow water level control for adjustment of hydraulic retention time. Influent distribution to the wetland cells is enhanced by perforated pipes on a rip-rap slope across the width of the wetland cells. This method of distributing influent starts the flow through the treatment system and reduces the buildup of solids at the head of the wetland cells. The system is designed so that the effluent takes up to 30 days to flow through the wetland cells. The actual retention time varies seasonally to account for changes in the reaction rate of microorganisms in the cells. Because the microorganisms react more quickly at higher temperatures, the retention time can be decreased during the summer and still provide the required contact time for effective removal of impurities. Conversely, during the winter's colder temperatures, the reaction rate of the microorganisms is lower and the retention time is increased by raising water levels. Aluminum stop logs, located in three outlet structures along the width of each wetland cell, control cell water depth and promote the flow of effluent through the treatment system. ------- Dense stands of submerged cattail stems and leaves serve as growth media for A n , . i ,1 *T j n f£\ t • ±u After treatment by the wetland cells, effluent is conveyed to the microorganisms matjeed on impurities in . . the influent. The natural transfer of post-aeration pond, where it receives supplemental aeration atomospheric oxygen to these microbes is from a floating aerator. essential in removing organic matter and ammonia from the wastewater. ------- Performance Deep zones in the wetlands provide open water for ducks and wading birds, enhance flow distribution in the wetland cells, serve as a sump for settling solids, and provide additional hydraulic residence time in the wetland cells. Construction of the cells began in June 1989, with planting starting during May 1990. By August 1990, the vegetation provided almost complete cover, and operation of the wetland cells began. Since then, with only one exception for NH3, the Fort Deposit constructed wetland treatment system has consistently achieved permit compliance and has caught the attention of others seeking a low cost, dependable natural treatment system. Because of its outstanding contribution to water resource conservation, the Fort Deposit system received several awards including the Alabama 1991 Governor's Conservation Achievement Award, the Alabama Engineering Excellence Award, and the Grand Award from the American Consulting Engineers Council. Month BOD5 TSS Nitrogen In Out In Out TKN In NHsOut 1990August 102 5 137 10 September 27 8 101 18 October 30 3 168 18 November 27 3 127 10 December 15 4 71 9 1991 January 20 5 52 10 February 13 4 18 4 March 26 7 40 8 April 22 10 97 15 May 21 9 52 20 June 29 10 72 25 July 33 7 69 10 August 56 7 183 7 September 24 4 87 12 October 30 8 125 18 November 32 4 106 7 December 33 12 64 16 1992January 39 4 83 19 February 22 4 32 4 March 34 4 58 5 20.0 11.0 19.0 14.0 10.0 8.0 11.0 19.0 10.0 80.0 5.0 21 20.0 10.0 6.0 11.0 11.5 10.0 6.7 10.0 0.57 0.66 0.78 0.93 2.60 1.10 0.74 0.89 0.70 0.35 0.94 6.43 0.90 0.99 0.75 0.21 0.87 0.38 0.15 0.22 ------- April 31 4 119 3 12.0 0.51 Wetland effluent BOD5 and total suspended solids (TSS) are consistenly in compliance with permit limits despite variable inflow quality to the wetland cells. Total Kjeldahl nitrogen (TKN) is mineralized in the wetland cells to NH3 and then nitrified to achieve the low discharge limits. ------- Ancillary Benefits In addition to improving the quality of the effluent discharged to the receiving stream, the creation of the Fort Deposit constructed wetland treatment system has significantly increased wildlife. This new habitat provides cover and food for various types of wetland-dependent vertebrate and invertebrate life including a variety of ducks and wading birds and their prey. As a result of the wetland1 s success and the desire of others to adopt similar technology, the town is receiving visitors from other areas of the state and the nation. Fort Deposit Wetland Design Criteria Average Daily Flow 0.24 mgd Influent Quality BODs TSS TN NH3-N Effluent Criteria BODs TSS NH3-N pH 40 mg/L lOOmg/L 20 mg/L 10 mg/L 10(18)amg/L 30 mg/L 2(5)a mg/L 6-9 units The Fort Deposit wetlands continue to diversify as new plant species colonize the cells. Areas Lagoon 10 acres Wetland Cells (2) 7.5 acres each ()a winter limits December-April ------- Acknowledgements The Waterworks and Sewer Board of the Town of Fort Deposit Henry Crenshaw, Chairman Leo Goldsmith, Board Member W.O. Ward, Board Member David Edwards, Manager Consulting Engineers Dennis A. Sandretto, CH2M HILL Project Manager Robert L. Knight, CH2M HILL Project Environmental Scientist Alabama Department of Environmental Management Truman Green, Chief, Municipal Branch, Water Division U.S. Environmental Protection Agency Robert Freeman, Municipal Grants Program, Region IV This brochure was prepared by CH2M HILL for the U.S. Environmental Protection Agency. ------- |