Fate of Nitrogen and Phosphorus in a Waste-water Retention Reservoir Containing Aquatic Macrophytes

Potential use of retention/detention reservoirs stocked with vascular aquatic macrophytes was evaluated, using a microcosm reservoir for reducing the N and P levels of agricultural drainage effluents (waste water). The treatments evaluated were reservoirs stocked with (i) pennywort (Hydrocotyle umbellata L.), (ii) water hyacinth (Eichhornia crassipes [Mart] Solms), (iii) cattails (Typha latifolia L.) and elodea (Egeria densa P), and (iv) control (no macrophytes). Labeled "N was used to differentiate preferential uptake of "1NH4* and "NOj-, and to follow the fate of added "NH.* and "NO,-. Results showed that 34 to 40% of the added inorganic "N ("NH/ + NOr) was removed through plant uptake, while 45 to 52% of the added "N was unaccounted for, presumably lost through NH, volatilization and nitrification-denitrification processes. In the control reservoir, algal biomass removed 4.4% of added "N, while 41% of the added "N was not accounted. Pennywort and cattail-elodea systems were found to be most effective, with about 50% inorganic N removal in a 4-day detention period. All aquatic macrophytes preferred "NH4* over "NO3-, but the difference in uptake was not significant, except for pennywort and cattails, which removed 84 and 92% of the added "NH4* as compared to 16 and 8% of the added "NO3-, respectively. About 25 to 29 d were required by the systems with macrophytes to remove 50% of the wastewater P. Plant removal of P was in the range of 3 to 65% of added P, while 7 to 87% of the added P was lost through precipitation and adsorption reactions. Additional Index Words: water hyacinth, pennywort, cattails, elodea, aquatic system. Reddy, K. R. 1983. Fate of nitrogen and phosphorus in a waste water retention reservoir containing aquatic macrophytes. J. Environ, qual. 12:137-141. Vascular aquatic macrophytes such as water hyacinths (Eichhornia crassipes [Mart] Solms), duckweed (Lemna minor), and cattails (Typha sp.), cultured in ponds and reservoirs, offer potential alternatives for treating sewage and industrial effluents (Boyd, 1969; Wooten and Dodd, 1979; Wolverton and McDonald, 1979), and agricultural effluents (Reddy et al., 1982). The capacity of vascular plants to assimilate nutrients from polluted waters has been recognized for several years (Rogers and Davis, 1972; Stewart, 1970; Boyd, 1976). Nutrient removal efficiency of a system containing plants will depend on the type of aquatic plant, growth rates of plants, nutrient composition of the water, and physicochemical environment in the water. Studies reported by several researchers (Clock, 1968; Scarsbrook and Davis, 1971; Cornwell et al., 1977) calculate the nutrient removal rates, based on the changes in concentrations at the inflow and outflow of a pond or reservoir. Although these calculations provide information on the nutrient removal efficiency from waste water, they provide very little understanding on the rate of N and P removal in these systems. Presence of aquatic macrophytes in a pond alters the physicochemical environment of the water, and the role of these changes are often ignored in evaluating the efficiency of a biological treatment system. The dense cover of floating water hyacinths depletes dissolved O2 of the underlying water, thus creating anaerobic conditions (Reddy, 1981). These conditions favor the denitrification process, thus maximizing NO3" removal. Presence of submersed plants, such as elodea or algae, can deplete dissolved CO2 in the water during the periods of high photosynthetic activity (mid-afternoon) and increase the dissolved O2 of the water, thus resulting in increased water pH (Reddy, 1981). This condition can maximize NH/ removal through volatilization and soluble ortho-P removal by chemical precipitation. The role of underlying sediments of a pond or reservoir as a nutrient source or sink to the overlying waters is often ignored in calculating the nutrient removal efficiencies. In central Florida, organic soils (Histosols) planted with vegetable crops are artificially drained during wet seasons, and the drainage effluent discharged from these fields is being pumped into retention/detention reservoirs and subsequently into Lake Apopka. These retention reservoirs are needed to reduce the nutrient ' Florida Agricultural Experiment Stations Journal Series no. 3766. Received 6 Mar. 1982. 2 Associate Professor, University of Florida, Inst. of Food & Agricultural Sciences, Agricultural Research & Education Center, P.O. Box 909, Sanford, FL 32771. J. Environ. Qual., Vol. 12, no. 1,1983 137