Converting an extended aeration plant to BNR

There are many extended aeration sewage treatment package plants installed in Arizona which have reliably treated sewage to secondary effluent requirements and because of their long detention times and sludge age, converted ammonia nitrogen to nitrate. The Arizona Statutes require that discharges to groundwater do not "cause or contribute to a violation of aquifer quality standards" (ARS 49.243.B.2). One of the quality requirements is nitrate. Ln order to meet the requirements the authors converted a 756 m(3)/d (200,000 gpd) extended aeration plant to denitrification at a cost of less than $25,000. Using a hypalon baffle, one of the aeration tanks was divided into anoxic and aerobic zones. The influent sewage was redirected to the anoxic zone which already received return sludge. The aerated mixed liquor was recycled into the anoxic zone using air lift pumps. The sludge age was controlled to a maximum of 20 days to provide an "aggressive mixed liquor bacteria population". Mixing was provided in the anoxic zone. A flux analysis was carried out on the clarifier to understand the impact of increased sludge inventories in the system and sludge storage variations between the reactor and the clarifier. Within two weeks of start up, effluent total nitrogen values reduced to below 10 mg/L. Operation of this plant and other plants designed by the authors revealed insights into the importance of flow equalization, mixed liquor recycle rate, sludge age, and raw sewage strength in the control of denitrification. The approach has been used to modify two 76 m(3)/d (20,000 gpd) extended aeration plants and a new 2,340 m(3)/d (620,000 gpd) plant with good success.