AWWA ACE93074

To stem deterioration of water quality in the Occoquan Reservoir, caused by discharge of effluent from several wastewater treatment plants, the Virginia Water Control Board mandated water reclamation of all wastewater generated in the reservoir's watershed. The reclamation plant employs conventional primary and secondary treatment followed by five advanced treatment processes. In a comparison of the raw and reclaimed water, the reclaimed water showed higher levels of alkalinity, total hardness, and nitrate, but lower levels of chemical oxygen demand, turbidity, and total suspended solids. Most significant was the reduction in phosphorus, which had caused uncontrollable algae blooms. Before the reclamation plant was functioning, the average phosphorus concentration during the summer ranged from 0.14 mg/L (in 1973) to 0.36 mg/L (in 1977). Since the reclamation plant began to function, the average phosphorus concentration has been consistently below .10 mg/L. The reservoir is used to supplement the Fairfax County Water Authority's primary source on the Potomac River. Not only does the Occoquan Reservoir provide some storage for drought but it can also be used after rains, when the Potomac becomes quite turbid. Thus the reclamation plant not only reduces water quality problems in the reservoir, the discharge also helps to supplement water supplies for the utility. An expansion to 54 mgd is in progress.