تجزیه و تحلیل اقتصادی استفاده از تالابها برای درمان تصفیه خانه پساب فرآوری میگو - یک مطالعه موردی در Dulac، LA

Two crucial environmental problems in Louisiana are high rates of wetland loss and surface water pollution. Using wetlands for wastewater treatment can address both of these concerns by reducing the amount of pollutant discharge into surface water bodies while simultaneously serving to restore and replenish deteriorating marshes by enhancing productivity and accretion. Using wetlands for wastewater assimilation can also result in considerable cost savings when compared with conventional, non-wetlands wastewater treatment options. In order to determine these cost savings in a specific case, an avoided cost economic analysis was performed for two potential wastewater treatment options for a shrimp processor in Dulac, LA: (1) conventional, on-site treatment with dissolved air flotation (DAF); and (2) wetland treatment. Annualized costs for DAF implementation are ≈$208 000/year for 25 years. Wetland treatment costs around 25% of DAF with an annual cost of ≈$63 000. Yearly savings would be almost $150 000. This is a capitalized cost savings of over $1.5 million.

Louisiana is one of the nation’s leaders in seafood production with an annual production rate of over half a billion kilograms, valued in excess of $250 million dockside (US DOC, 1997). Zachritz and Malone (1991) identified over 690 seafood processors in Louisiana, most of which are located in the coastal zone. Dulac, LA, consistently ranks in the top ten US ports for value of fishery landings, and four other Louisiana ports rank in the top ten (US DOC, 1997). Currently, seafood processors in Dulac, LA, are permitted by the State of Louisiana to discharge all the untreated wastewater used during the production process into Bayou Grand Caillou. The State of Louisiana, through delegated authority from the US EPA, has issued Louisiana Pollution Discharge Elimination System (LPDES) permits that require only removal of shell material. However, the remaining untreated effluents cause low dissolved oxygen (DO) concentrations in the bayou that violate the water quality standards for DO criterion (Waldon, 1991). More importantly, the untreated waste input high amounts of nutrients, such as nitrogen and phosphorus, which contribute to the eutrophication of coastal waters. This increase in primary productivity is one probable cause for the persistent hypoxic zone affecting much of the Gulf of Mexico during the summer months (Rabalais et al., 1994 and Rabalais et al., 1996). Additionally, Louisiana suffers high rates of wetland loss (Britsch and Dunbar, 1993). Using wetlands for wastewater treatment can reduce the amount of pollutants discharged into surface water bodies while simultaneously serving to help restore and replenish deteriorating marshes by enhancing productivity and accretion. We conducted this study to determine the ecological and economic feasibility of using wetlands for treatment of shrimp processing effluent in Dulac, LA (Day et al., 1998). This report presents the findings of the economic portion of that feasibility analysis. 1.2. Treatment options The wastewater treatment options for seafood processors in Dulac, LA, are the same as they have been for the past 20 years (Breaux et al., 1995): (1) continue disposal into Bayou Grand Caillou; (2) discharge into an expanded treatment plant in Houma (≈32 km (20 miles) away); (3) treat wastes at the individual plants before discharging into Bayou Grand Caillou; or (4) discharge into the wetlands. Many consider options (2) and (3) to be too expensive to be feasible solutions to this problem. Pumping wastewater for extended distances or on-site treatment is often prohibitively expensive for operations such as the shrimp processors in Dulac, LA. This paper considers options (3) and (4) as they pertain to shrimp processors. Breaux et al. (1995) and Breaux (1992) performed a generalized preliminary avoided cost estimate of options (3) and (4) for seafood processors in Dulac, LA. This study builds from those previous efforts by focusing specifically on one processor, Sea Tang Fisheries, Inc, 6320 Grand Caillou Road, Dulac, LA 70353 (hereafter Sea Tang), and the costs associated with conventional on-site treatment or wetland treatment. 1.3. Wetlands treatment Previous studies indicate that both natural and constructed wetlands can purify wastewater effluents (Richardson and Davis, 1987, Reed, 1991 and Kadlec and Knight, 1996). Wetlands are efficient at removing excess nutrients and pollutants by physical settling and filtration, chemical precipitation and adsorption, and biological metabolic processes that result in burial, storage in vegetation, and denitrification (Kadlec and Alvord, 1989, Patrick, 1990 and Breaux and Day, 1994). US EPA (1986) conducted a study in Alabama that successfully used wetlands to treat shrimp wastewater. Breaux and Day (1994) identified four primary benefits derived from wetlands wastewater treatment in Louisiana: (1) improved surface water quality; (2) increased accretion rates to balance subsidence; (3) increased productivity of vegetation; and (4) the financial savings of capital not invested in conventional advanced secondary and tertiary treatment systems. Subsidence in deltas results naturally from compaction, consolidation and dewatering of sediments. Localized sinking can also be increased due to withdrawals of water, oil, and gas. If wetlands in deltas do not accrete vertically at a rate equal to the rate of RSLR, they will become stressed due to waterlogging and salt stress, and ultimately disappear (Mendelssohn and McKee, 1988 and Day et al., 1997). The high rate of burial due to subsidence and higher than national average rates of denitrification due to warm temperatures are additional reasons for the use of wetland treatment in Louisiana. Projects conducted elsewhere in Louisiana, such as Amelia, Breaux Bridge, Thibodaux, and St. Bernard, demonstrate its usefulness to purify municipal and industrial effluent as well as enhance productivity (Day et al., 1999). Increasing vegetative productivity is especially crucial in many parts of Louisiana where coastal subsidence in the Mississippi Delta results in a relative sea level rise about ten times greater than eustatic sea level rise (Conner and Day, 1988 and Penland et al., 1988). Increasing productivity can enhance the accretion necessary to offset the subsidence that is contributing to wetland loss. Much wetland treatment has focused on constructed wetlands due to a variety of factors. For example, in some regions, natural wetlands are not available to use for treatment, regulations forbid the use of natural wetlands for wastewater treatment and/or constructed wetlands provide a high degree of control for treatment. In Louisiana, the multitude of canals and levees have left many wetlands hydrologically isolated and confer the same degree of control as constructed wetlands. With natural wetlands plentiful, it is often unnecessary to build artificial wetlands in Louisiana. These isolated wetlands provide a practical economic solution for the small communities that are widely dispersed in the coastal zone. Conventional treatments are often too expensive for the loads generated from these small producers (Breaux et al., 1995). Non-toxic, industrial processors, such as shrimp processors, can benefit from using wetlands for their highly seasonal loads. 1.4. Ecological feasibility Results from the ecological feasibility study are reported in Day et al. (1998) and indicate favorable conditions for wetland treatment of shrimp processing effluent. To briefly summarize, the selected site is a 183 ha forced drainage wetland that is hydrologically isolated by a system of levees, a system common to much of southern Louisiana. The wetlands have been under forced drainage since the 1970s and an automatic pump operates to keep the land and roads from flooding during heavy rains. Shrimp processing in this region is highly seasonal. Seafood processing effluent is rich in total nitrogen (N) and total phosphorus (P). In coastal Louisiana, the peak operating season is the early summer, resulting in a strong seasonal pulse of nutrients into Bayou Grand Caillou peaking in the month of June. Consequently, nutrient loading rates follow the same patterns and coincide with the long growing season in Louisiana. For Sea Tang, the loading rates are 12.5 g N m−2 year−1 and 1.6 g P m−2 year−1. A study conducted by the US EPA (1986) on marsh uptake of shrimp processing effluent recommends a maximum nitrogen loading level of 0.73 g m−2 day−1. Assuming a 300 day work year, this loading rate would be well over 200 g N m−2 year−1, 17 times the Dulac loading rate. The EPA study further calculated that a seafood facility producing 1 million gallons/day of wastewater would require 133 ha of salt marsh to accommodate recommended loading rates. Sea Tang effluent levels are less than 20% of that and have 183 ha available for treatment. A review of wastewater treatment loading rates by Richardson and Nichols (1985) places Dulac loading rates well within the low end of the spectrum and indicates the potential high nitrogen and phosphorus removal capacity of at least 70 and 68%, respectively. These results do not take into account the burial of nutrients caused by the subsidence and accretion that characterizes Louisiana wetlands. Furthermore, a positive impact is expected on productivity and accretion. Sampling in the proposed treatment area indicates a high aboveground primary productivity of 2578 g m−2year−1 as estimated by peak standing crop (Day et al., 1998). Phragmites australis (Roseau cane) is the dominant species comprising 66% of the vegetation harvested in the clip plots. Meo et al. (1975) conducted an overland flow pilot study for the treatment of a fishmeal processing plant in Dulac and the receiving wetland was composed primarily of Phragmites. Vegetation sampling results indicated that live standing crop increased by 55% after application of wastewater. With such similar vegetative characteristics, it is likely that current productivity will increase given the similar nature of the effluent. This added vegetative productivity will help offset the accretion deficit balance of ≈0.22 cm year−1 that characterizes this region ( Day et al., 1999) by contributing to organic soil production.

An ecological analysis conducted at the site (Day et al., 1998) indicates that the area is suitable for wastewater treatment and would benefit from it. Thus coupled with the substantial economic savings, a pilot wetland treatment study in Dulac is recommended to treat shrimp processing waste. When the regulatory authorities change current discharge permit requirements to mandate more stringent handling of the wastes, this project presents shrimp processors with a cost-effective option. Such a project can serve as a model for many of the other seafood processors in Louisiana, as well as in other Gulf of Mexico states. Many small communities are facing similar waste disposal problems. With the abundance of impounded or isolated wetlands in the coastal zone, wetland treatment can be a viable, cost effective option for these small communities and industrial processors. The Coastal Ecology Institute developed a wastewater treatment manual (Day and Sutula, 1998) that provides interested planners and processors with guidance on how to determine the feasibility of wastewater treatment. It can be accessed on the Internet by visiting the Coastal Ecology Institute's web site at: http://www.lsu.edu/guests/wwwcei/wastewaterguide/Wetman.html