Preliminary investigation of an integrated aquaculture–wetland ecosystem using tertiary-treated municipal wastewater in Los Angeles County, California

Abstract

The objective of this study was to determine the feasibility of using a designed integrated aquaculture–wetland ecosystem (AWE) for experimental food production and inorganic nitrogen removal from tertiary-treated wastewater. The AWE connected polyculture aquaculture ponds with in-pond aquatic plant systems (water hyacinths, Eichhornia crassipes, and Chinese water spinach, Ipomea aquatica), a solar energy aeration system, and an artificial wetland. Ponds were stocked with hybrid tilapia ( Oreochromis mossambicus× O. urolepis hornorum), common carp ( Cyprinus carpio), mosquitofish ( Gambusia affinis), and red swamp crayfish ( Procambarus clarkii), and were flushed weekly with new wastewater at 20%. Fish were fed a 32% protein floating ration at 1% fish body weight per day, and wheat bran was added at 1 mg l −1 when water conductivities exceeded 900 μmhos cm −1. Plants were allowed to grow until they reached approximately 50% of the pond surface area, then maintained at this area by manual harvesting. Pond water quality (temperature, conductivity, pH, oxygen) was monitored twice daily, and weekly water samples were taken for analyses of inorganic nitrogen (ammonia and nitrate-N) in the ponds, wetland, and wetland discharge waters ( n=30). Tilapia harvest from three ponds was 1134.5 kg. Fish standing crop biomass increased from 0.16 to 0.21 at stocking to 1.50–2.00 kg m −3 at harvest. Tilapia grew from an average stocking weight of 21 to 362–404 g at harvest but had poor survival (48–64%) due to heavy bird predation. Total food conversion ratios ranged 0.9–1.2. Approximately 70% of the tilapia were marketed live at $2.20 kg −1. An estimated standing crop of 1.4 tons wet weight of Ipomea aquatica grew luxuriantly in one 200-m 2 polyculture pond which could be harvested sustainably at 20 kg week −1. Water hyacinths removed approximately 90% of the ammonia and nitrate-N in wastewater, and the wetland removed an additional 7% (total removal was 97% of wastewater input concentrations). Overflow water exiting the wetland had less than 0.4 mg ammonia–nitrogen l −1 and no detectable nitrate–nitrogen. The experimental AWE accomplished aquatic food production and almost complete removal of inorganic nitrogen from wastewater, functioning as a `quartenary' wastewater treatment/food production ecosystem. However, more rigorous experimentation is required to optimize fish- and plant-carrying capacities, nutrient cycles, and testing for bioaccumulation of metals in order for the AWE to be socially and economically relevant. The concept of using tertiary-treated wastewater for aquatic food production may be attractive in the peri-urban areas of many meagcities like Los Angeles, both for fish markets and to stem the growing discharges of wastewaters that are causing coastal pollution.