Wetland wastewater assimilation is a cost effective method of managing secondarily treated effluent.

     Mandeville started with a concept that's a great example of a first-generation sewage treatment program. The facility consists of three (200 X 600 feet) aerated lagoon cells, a three celled rock reed filter, and an ultraviolet disinfection system. Between 1964 and 1989 Mandeville operated two oxidation lagoons: one at the south end of Heavens Drive, discharging into the northern part of the East Tchefuncte River Marsh, and the other discharging into Bayou Chinchuba upstream of its current discharge location. Since 1989, upon construction of the current facility, the City of Mandeville has discharged all of its treated effluent into Bayou Chinchuba wetland and discontinued the use of the oxidation lagoons. The facility is operating with a current NPDES EPA permit and an administratively continued DEQ permit. The City is expected to meet standards by July, 2000.

From treating sewage to enhancing wetlands

     Comite Resources, Inc. is helping the City of Mandeville evolve into one of the best of the next generation of Cities by extending Mandeville's wastewater management to include more then just aerated lagoons, rock reed filters, and a disinfection system. Mandeville is going to use the free energies of the natural system to drive the cycle of production, use, degradation, and reuse.

     The basic principle underlying wetland assimilation of treated effluent is that the rate of application must balance the rate of decay or immobilization. The primary mechanisms by which this balance is achieved are physical settling and filtration, chemical precipitation and adsorption, and biological metabolic processes resulting in eventual burial, storage in vegetation, and denitrification.

     Wetlands with long residence times are best suited for Biochemical Oxygen Demand (BOD) reduction and bacteria dieback. Many pathogenic microorganisms in sewage effluent cannot survive for long periods outside of their host organisms. Protozoa present in shallow waters actively feed on bacteria. The presence of vegetation can also improve the BOD purifying capacity by trapping particulate organic matter and providing sites of attachment for decomposing bacteria. Root excretions from some wetland plants can kill pathogenic bacteria (Hemond and Benoit 1988).

The right team with the right approach

     Comite Resource's ecological approach helped address all facets of Mandeville's treatment system. Effluent discharge generally introduces nutrients as NO3, NH4, or organic forms. The nitrogen and phosphorus supplied in the wastewater can be removed in the short -term by plant uptake, in the long-term by peat and sediment accumulation, and permanently by the process of denitrification.

     The project began with the City's proximity to wetlands, designing a wastewater strategy to use the free energies of the natural system to assimilate treated domestic effluent. We will use the scientific research developed in the past several decades to design a comprehensive management approach. The ability of wetlands to perform certain water purification functions is well established for natural watersheds (Conner et al. 1989) Studies in the southeastern United States show that wetlands chemically, physically, and biologically remove pollutants, sediments and nutrients from water flowing through them (Wharton 1970; Shih and Hallett 1974; Kitchens et al. 1975; Boyt 1976; Nessel 1978; Yarbro 1979; Nessel and Bayley 1984; Yarbro et al. 1982; Tuschall et al. 1981; Kuenzler 1987). Recent studies show that cypress trees that received wastewater effluent for 50 years at Breaux Bridge, Louisiana, had higher growth rate than trees not receiving effluent (Hesse et al. 1998)

     The purpose of the Louisiana Water Control Law and Federal Clean Water Act is to protect or enhance the quality of public water, including wetlands. Three components of the water quality standards adopted by Louisiana and approved by the EPA are; 1) beneficial water uses such as propagation of fish and wildlife, 2) criteria to protect these beneficial uses and 3) an antidegradation policy that limits the lowering of water quality. In Louisiana, discharging treated effluent into wetlands can allow for the potential enhancement and restoration of the functional attributes associated with wetlands (e.g. groundwater re-charge, flood control, biological productivity) (Kadlec and Knight 1996; Rybczyk et al. 1996a). Specifically, most coastal wetlands have been hydrologically altered, and are isolated from the alluvial systems responsible for their creation (Boesch 1994). This makes these wetlands especially vulnerable to the high rates of relative sea level rise (RSLR: subsidence plus eustatic sea level rise) associated with deltaic systems (Penland 1988) and to predicted increases in eustatic sea level rise (Gornitz 1995).

     Wetlands have been shown to persist in the face of RSLR when vertical accretion equals or exceeds the rate of subsidence (Baumann et al. 1984, Delaune et al. 1983, Stevenson et al. 1986). In the past, seasonal overbank flooding of the Mississippi River deposited large amounts of sediments into the interdistributary wetlands of the delta plain. Not only did these floods provide an allochthonous source of mineral sediments, which contributed directly to vertical accretion, but the nutrients associated with these sediments promoted vertical accretion through increased autochthonous organic matter production and deposition, and the formation of soil through increased root growth. This sediment and nutrient source has been eliminated since the 1930's with the completion of levees along the entire course of the lower Mississippi, resulting in vertical accretion deficits (RSLR > accretion) throughout the coastal region.

     Contributing further to the problem of vertical accretion deficits, many wetlands in the deltaic region have been hydrologically isolated from surrounding marshes, swamps and bayous due to an exponential increase in the construction of canals and spoil banks during the past century (Turner and Cordes 1987). In addition to impeding drainage and, in many cases, physically impounding wetlands, these spoil banks also prevent the overland flow of sediments and nutrients into coastal wetland forests, creating essentially ombrotrophic systems.

     The total acreage of swamp forest in the Louisiana coastal zone has decreased by 50% from 1956 to 1990 (Barras et al. 1994). Furthermore it has predicted that increased rates of eustatic sea level rise could eliminate most of the remaining forested wetlands (Delaune et al. 1987). In the wetland forests of southeastern Louisiana, Conner and Day (1988) estimated vertical accretion deficits ranging form 2.5 to 10.8 mm yr-1, which leads directly to increased flooding duration, frequency and intensity. Productivity decreases that have been observed in these wetlands may be attributed to either the direct physio-chemical effects of flooding (i.e. anoxia or toxicity due to the reduced species of S and Fe), flood related nutrient limitations (i.e. denitrification or the inhibition of mineralization), nutrient limitations due to a reduction in allocthonous nutrient supplies, lack of regeneration, or most likely, some combination of these factors (Mitsch and Gosselink 1986).

     Recent efforts to restore and enhance wetlands in the subsiding delta region have focused on attempts to decrease vertical accretion deficits by either physically adding sediments to wetlands or by installing sediment trapping mechanisms (i.e. sediment fences), thus increasing elevation and relieving the physio-chemical flooding stress (Boesch et al 1994). Day et al. (1992) proposed an alternate restoration strategy by hypothesizing that adding nutrient rich secondarily-treated effluent to hydrologically isolated and subsiding wetlands could promote vertical accretion through increased organic matter production and deposition. Their work, and other studies, have shown that treated effluent does stimulate productivity and accretion in wetlands (Odum et al. 1975, Mudroch and Copobianco 1979, Bayley et al. 1985, Turner et al. 1976; Knight 1992; Craft and Richardson 1993; Hesse et al. 1998; Rybczyk 1997).

Creating the right wetland wastewater assimilation design

     For City managers, Comite Resources, Inc. created a specific plan for this ecosystem and added scientific information to help them manage their treated effluent cost effectively. And for Mandeivlle's citizens -- ranging from homeowners, students, and businesses -- long-term solutions have been built into wastewater management. Mandeville's officials and residents get relevant, timely information on the progress of the project.

Thinking strategically about what's next

     Comite Resources, Inc. works closely with Mandeville's Director of Public Works to keep the project on schedule, helping to identify new state and federal water quality requirements, and seek out the best opportunities for the City. Comite Resources, Inc. constantly monitors state and federal regulatory agencies as they develop rules and regulations -- from attending agency meetings, to reviewing proposed rule changes.