Engineered Ecosystems for on-Site Management of Water and Nutrients From Domestic Sewage
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N. C. State University,
Raleigh, NC 27695-8008,
The use and disposal of water and nutrients both waste these valuable resources and increases their potential for adverse environmental impacts upon ground and surface water. Water laden with nutrients may be managed by low-tech systems such as soil filters, constructed wetlands or the combinations. The systems mimic natural environments by utilizing energy directly from the sun to transform, store and recycle nutrients and water. Engineered ecosystems are management tools which enable us to reuse water and nutrients in a cost effective way.
The components for these engineered ecosystems and their arrangement as a system should be selected to match performance goals. Effectiveness of system performance is currently often defined by meeting regulatory compliance based on treatment. Compliance requirements vary and treatment as the primary goal minimizes environmental impacts but waste valuable resources such as water, nitrogen and phosphorus. The use of design based on increased knowledge rather than over design and safety factors should enable us to adjust our goals from disposal and treatment to reuse technologies. Engineered ecosystems, when properly designed, will recycle resources naturally and therefore represent one step toward the reuse goal.
Though simplicity is desirable, increased complexity generally increases the range and type of treatment process. As our understanding of engineered ecosystems increases and we become more capable with our designs, we may be able to peel away the layers of complex structure and maintain the complex process.
The use of engineered ecosystems to manage domestic sewage does not suggest that the natural model, for example wetlands, will somehow function the same and therefore can be used for the same purpose. Engineering a design permits the selection and arrangement of components which do not occur in nature for the purpose of managing domestic sewage. The engineered system, though based on the natural system, may not resemble its natural model and actually be a totally new arrangement of components.
The use of a natural model enhances design options for multiple functions. In addition to water and nutrient management, the systems may provide for recreational space, creation of aesthetic features and wildlife habitat and are excellent educational tools. The multiple function and emphasis on integration of the treatment components with each other within the system and with the surrounding landscape helps to insure operational effectiveness and efficiency.
Cell Depth and Treatment: The wastewater treatment system for the residence of Ms. Doris King in Vanceboro was installed during the fall of 1991 as a part of the Craven County I&A Program. The system utilizes a septic tank, two constructed wetland cells, UV disinfection and spray irrigation of the treated water.
Both wetland cells were planted along the influent end with wax myrtle, sweetbay magnolia and Virginia sweet spire on December 19, 1991. The emergent species; soft stem rush and hard stem rush were planted May 28, 1992 to take advantage of warmer weather to encourage their establishment. The emergent species were planted on two foot centers within the 15′ x 20′ cells.
Water samples to evaluate the performance of the cells were taken monthly from February 24, 1992-January 12, 1994.
Vegetation and Treatment: Another system which was a part of the Craven County I and A Program was constructed in 1990. The system treatment train consisted of a septic tank, 3 constructed wetland cells, an ozone disinfection unit inside an effluent pump tank, and a subsurface low pressure pipe network installed in fill for ground absorption. The wetland cells differed in their vegetation components. Cell A initially was planted with soft stem rush, Jancus effusus, but later bulrush, Scirpus validus was mixed in; cell B contained common reed, Phragmites australis; and Cell C initially was planted with cat-tail, Typha latifolia, but later soft stem rush was mixed in.
Operation and maintenance requirements were primarily related to the critical importance of water level control within the wetland cells and the lack of installation of the gravel substrate with a near level surface. The installation of the cells with a 1-3% bottom slope further contributed to vegetation mortality and maintenance by its contribution to decreased water level within the exit end of the wetland cells. Current information from other research indicates that a water level 2 inches below the gravel substrate provides for optimal emergent wetland plant growth in most cases. This level is near impossible to maintain without a water level control device on each cell and a near level gravel surface. Installation of the cells with a substantial bottom slope (1-3%) caused the exit end of the cells to be 3-8 inches deeper than the inlet end bottoms. During periods of low flow, all water could potentially move to the low exit end and further deprive plants near the inlet of water. Typically in other systems, plants near the inlet are more robust due to increased nutrients and water supply. This usual trend was reversed when the plants near the exit end of the cell planted with common reed grew taller and had broader leaves and those near the inlet were stunted. Water level control devices were added to each cell during February 1992. The discovery of the importance of water level control within this system evaluation paralleled similar discoveries elsewhere in the world. Design criteria used for this system were considered state of the art for constructed wetlands technology during its design and installation.
Filter Material and Treatment: The Merchants Millpond project in Gates County which was installed during November 1993, compares a constructed wetland filled with sand and one filled with gravel substrate. The systems are landscaped with native plants to show how constructed wetlands can be used in a residential area. The cell filled with sand substrate has generally performed better than the one filled with gravel. Early indications are that the sand filled cell is nitrifying the wastewater ammonium, a process that is unusual for horizontal flow constructed wetlands.
|Evaluation of Wastewater Treatment by a Sand Filled Low Marsh and Gravel Filled Low Marsh Installed at Merchants Millpond State Park (Avg. of 5 Samples-mg/L)|
|Sample Point||SS||NH4- NTKN||Total Nitrogen||PO4-P||Total PO4|
The use of combined engineered environments, such as constructed wetlands to reclaim water is based on extending treatment by combining multiple treatment environments. This system design consists of the combination of intermittently and constantly flooded wetlands to enhance nitrification and subsequent denitrification.
The concept was first used in another successful treatment system called a marsh/pond/meadow system (MPM) for a small town. The MPM design evolved from the initial research conducted at Brookhaven National Laboratory (Woodwell 1977;Small 1977). It was concluded that marshes and ponds in conjunction with terrestrial systems that can be spray irrigated can provide cost effective wastewater treatment (Woodwell 1977). Refinements of the design were used at a small retirement center in Pennsylvania and later in Iselin, Pennsylvania and Pembroke, Kentucky (SMC-Martin, Environmental Consultants 1980; Conway and Murtha 1989; Choate et al. 1990).
Another combination system which integrated an aerobic upland environment (mound) and a constructed wetland (horizontal subsurface flow) was used to successfully treat domestic sewage. The mound component provided near complete nitrification, phosphorus and organic removal. The wetlands further lowered nutrient concentrations (House et al. 1992). Hammer and Knight (1994) concluded that systems designed to treat nitrogen should have alternating aerobic (pond) and anaerobic (marsh) zonation within the wetland system.
Combination Mound-constructed Wetland System: An Upland-Wetland wastewater treatment system was installed in Pamlico County, N. C. in September 1989 to test its effectiveness in wastewater treatment for a single family home. Treatment effectiveness was evaluated from March 1990 to September 1991. The mound provided an aerobic environment that resulted in complete nitrification and reduction of phosphorus. Concentration of total nitrogen (TN) was lowered 64%, from 44.4 mg/L to 16 mg/L by the mound component. Nitrogen in the wastewater dosed into the mound was in the ammonium (NH4-N) and organic forms, while essentially all the nitrogen present in water that had passed through the mound was in the nitrate (NO3-N) form. The mound lowered total phosphorus (TP) concentration 86%, from 4.4 mg/L to 0.6 mg/L. The wetland cell planted with Phragmites australis was more effective than both the unplanted cell and the cell planted with Typha angustifolia. Concentrations of (TN), primarily NO3-N, were lowered from 16.0 mg/L to 11.1 mg/L or of that entering the cell. Total phosphorus was lowered 31%, from 0.6 mg/L to 0.3 mg/L. The Upland-Wetland Wastewater Treatment System has provided low cost, low maintenance and effective wastewater treatment.
Combination Vertical Flow-Horizontal Flow constructed Wetland System: A vertical flow-horizontal subsurface flow combined constructed wetland system was installed in Gates County, North Carolina during November 1992 to test its reclamation potential for sewage from an elementary school. Treatment effectiveness was evaluated from January 1993 to April 1996. The system effectively lowered total nitrogen (TN) concentration 75% and ammonium nitrogen (NH4-N) concentrations 98.5% during the four years of monitoring. The vertical flow cells provided and aerobic environment that resulted in near complete nitrification of influent NH4-N. The horizontal flow cells further lowered NH4-N concentrations. The system substantially lowered total phosphorus (TP) concentration during the first year of operation but was less effective in suspended solids (TSS) (55%) and biological oxygen demand (BOD) (97%). The TSS concentration removal increased from 16% the first year to 96% the fourth year. Hardwood mulch added to the wetland cells probably contributed to TSS within the discharge water.
Combination Constructed Wetlands, Aquatic and Soil Filters designed for Reclamation and Reuse of Water: Reclamation and reuse of water and nutrients at their source provide the opportunity to use simple, less costly technologies and lessens potentials for catastrophic effects due to centralized treatment system failures. The combination of multiple treatment environments within constructed wetlands can provide water quality suitable for reuse. A current project in rural Chatham County, North Carolina uses simple, aesthetically pleasing treatment components constructed in outdoor and indoor environments to reclaim domestic sewage for toilet flushing, landscape irrigation and water features. A courtyard containg constructed wetlands and a solarium with modular soil filter components and aquatic chambers are designed to treat sewage from within a small business facility and to provide recreational space for its 60 employees. The characteristics for the selected plant materials include: high rates of waster and nutrient utilization, disease resistance, temperature tolerance within a range of 10-40oC without photo period-induce dormancy and aesthetic value. The combination of vertical flow and horizontal flow constructed wetlands with fill and draw controls provides the necessary environments for nitrification-denitrification, removal of organic materials and phosphorus adsorption reactions. The system is designed to treat and reuse 4542 L/day (1200 gal/day) of domestic sewage from the business. After treatment within a septic tank, the partially treated water is pressure dosed into a vertical flow cell at a hydraulic loading rate of 40-120 Lm2/day (1-3 gal/ft2/day). Dosing is controlled by a time switch to insure 6-to-8 hour intervals between cycles to maintain an aerobic environment within the upper 1 ft of the substrate within the cell. After moving vertically to the cell bottom, the partially treated water flows by gravity into a horizontal flow cell that has a detention capacity of 7 days for denitrification. A solar panel operates a time switch and electro-mechanical valve set to open and close to the needed detention time. The water is then disinfected with ultraviolet light and pumped into 5 modular soil filter boxes each 1.2 m x 6.1 m x .5 m (4 ft. x 20 ft. x 1.5 ft.) within the solarium. The boxes contain different filter materials to test their effectiveness. The disinfected water is also pumped into aquatic plant components. Within these open, shallow containers designed to maximize the water surface area: volume ratio are grown free floating aquatic plants amenable to hydroponics culture such as duckwee, Lenna spp. And pennywort, Hydrocotyl spp. Regardless of the aquatic species used, the objective is to remove the low concentration of nutrients that remain. The plants used are selectively bred or genetically engineered to maximize their water reclamation potential. Utilization of simple treatment and reuse technology has permitted the business owner to renovate an abandoned and deteriorating school building into a home for two thriving and internationally based businesses and to protect the water quality of nearby Jordan Lake.
American Public health Association. 1992. Standard Methods for the Examination of Water and Wastewater: 18th Ed. American Public Health Association, Washington, D.C.
Choate, K. D., J. T. Watson and G. R. Steiner. 1990.Demonstration of constructed wetlands for treatment of municipal wastewater. Monitoring report for the period March 1988 to October 1989. TVA/WR/WZ-90/II. 107 pp. Water Resources River Basin Operations Resource development Tennessee Valley Authority
Conway, T. E. and L. M. Murtha. 1989. The Iselin marsh pond meadow. Pp.139-144. In D. A. Hammer (ed.) Constructed Wetlands for Wastewater Treatment. Proceeding from the First International Conference on Constructed Wetlands for Wastewater Treatment held in Chattanooga, Tenn. During June 13-17, 1988. Lewis Publisheres, Chelsea, Mich.
Hammer, D. A. and R. L. Knight. 1994. Designing consturcted wetlands for nitrogen removal. At. Sci. Tech., 29(4), 15-27.
Hatano, K. C. C Trettin, C. H. House, and A. G. Wollum, II. 1993. Microbial populations and decomposition activity in three sub-surface flow constructed wetland. In: G. A. Moshiri (ed.). Constructed Wetlands for Water Quality Improvement. Proceedings constructed Wetlands for Water Quality Improvement: an International Symposium. University of West Florida, Institute for Coastal and Estuarine Research. Lewis Pub.
House, C. H., S. W. Broome and M. T. Hoover. 1994. Treatment of nitrogen and phosphorus by a constructed upland-wetland wastewater treatment system. Wat. Sci. Tech., 29(4), 177-184.
House, C. H., S. W. Broome and M. T. Hoover. 1992. Treatment of nitrogen and phosphorus by a constructed Upland-Wetland wastewater treatment system. In Proceeding, “Wetland systems in Water Pollution Control.”International Specialist conference. Held Nov. 30-December 3, 1992 in Sydney, Australia.
House, C. H., S. W. Broome and M. T. Hoover. 1991 Constructed Upland-Wetland wastewater treatment system efficacy. Pp. 346-354. In: Proceedings of the Sixth National symposium on Individual and Small community Sewage System. Held in Chicago, Illinois on December 16-17, 1991. American Society of Agricultural Engineers, St. Josephs, Michigan.
House, C. H., S. W. Broome. 1990. Constructed Upland-Wetland wastewater treatment system. IN: P.F. Cooper and B. C. Findlater (eds.) constructed Wetlands in Water Pollution Control. Proceedings of the International conference on the Use of Constructed Wetlands in Water Pollution control, Cambridge, U. K. Pergamon Press, London.
Small, M. M. 1977. Natural sewage recycling sytems, brookhaven National Laboratory, United States Energy Research and development Administration Report EY-76-C-02-0016.
SMC-Martin, Environmental Consultants. (1980). “final report: marsh-pond-meadow sewage treatment facility,” Experimental Permit No. 4677452.
Woodwell, G. M . (1977). Recycling sewage through plant communities. American Scientist, 65, 556-562.