WATER-QUALITY IMPROVEMENT BY 4 EXPERIMENTAL WETLANDS

Four constructed, experimental wetlands at the Des Plaines River Wetlands Demonstration Project near Chicago, Illinois, USA, were studied during the 1990 and 1991 growing seasons, to evaluate certain water quality functions. The wetlands, ranging in size from 2 to 3.5 hectares with a maximum depth around 1.5 m, were supplied with water from the nearby Des Plaines River. Concentrations of total suspended solids, volatile suspended solids, nitrate-nitrogen, and total phosphorus, measured at the inlet and outlet of each wetland, were consistently lower at the outlets. Based on constituent mass balances for each wetland, trap efficiencies (percent removal) ranged from 76 to 99% for total suspended solids, from 39 to 99% for nitrate-nitrogen, and from 52 to 99% for total phosphorus. Although the wetlands were subjected to different hydraulic loading rates, their effluent water quality was similar. Also, although constituent concentrations of the influent were often quite high and varied significantly with respect to time, the outlet concentrations remained low and steady. High removal rates and low variability in effluent with respect to both inlet concentration and hydraulic loading rate suggest that the wetlands were not stressed, but rather, are capable of treating substantially greater quantities of water than were applied during 1990 and 1991.