THE IMPROVED WATER QUALITY OF LONG LAKE FOLLOWING ADVANCED WASTEWATER TREATMENT BY THE CITY OF SPOKANE, WASHINGTON

Abstract

ABSTRACT Long Lake, Wash., an impoundment of the Spokane River, has experienced high algal standing crop; low water clarity and extensive hypolimnetic anoxia during summer stratification. The City of Spokane's primary sewage treatment plant was shown to be the primary contributor of phosphorus to the reservoir and the major cause of its eutrophic state. To reduce influent phosphorus (P) loading and improve Long Lake's water quality, the city provided advanced wastewater treatment (AWT) with chemical (alum) phosphorus removal in 1977. Monthly mean P load from the AWT plant has decreased approximately 90 percent and the overall load to the reservoir during the growing season (June-October) has declined about 74 percent. Mean post-AWT reservoir algal biovolumes and chlorophyll a concentrations are approximately 60 and 45 percent, respectively, less than pre-AWT values. A phosphorus load–chlorophyll a relationship, based on 5 years each of pre- and post-AWT data, was developed and provided excellent predictions of mean reservoir chlorophyll a concentrations for the growing season. As a result of AWT, Long Lake has changed from a eutrophic to a mesotrophic body of water. In studying the effects of reduced P loading, it was determined that seasonal chemical phosphorus removal (April through October) could be as effective in reducing algal growth in Long Lake as year-around removal. This conclusion was based on the premise that temperature was the primary limiting variable outside the growing season. The city was granted a change in their AWT plant discharge permit and has implemented seasonal chemical P removal with no detrimental effects on the improved water quality of Long Lake.