Conversion of Existing Primary Clarifiers According to the EASC Process for Biological Phosphorus Removal

At the end of 1988 a 22,000 p.e. municipal wastewater treatment plant in Northern Germany was converted to the EASC-biological phosphorus removal process. By simple modifications of the flow scheme of the plant, one of two existing primary clarifiers was converted to an anaerobic basin, into which both sewage and recycle sludge are fed. The supernatant as well as the sludge withdrawn from the bottom are discharged into the aeration basin. This operation mode achieves very good phosphorus uptake in the aeration basin. Since start up in November '88, the uptake-capacity increased continually, since April '89 phosphorus is removed down to concentrations of less than 1 mg/l PO₄-P in the aeration basin. Due to an inadequate design and size of the existing final clarifier, phosphorus bleedback occurs and reduces removal efficiency. This bleedback could be minimized by either intensifying denitrification or reducing sludge detention time in the final clarifier.