PHOSPHORUS RELEASE IN THE SECONDARY CLARIFIER OF A FULL-SCALE BIOLOGICAL PHOSPHORUS REMOVAL SYSTEM

This study investigated the release of phosphorus in secondary clarifiers of a 90 MGD (million gallons per day) wastewater treatment plant that uses EBPR for phosphorus removal. On site and laboratory measurements of EBPR operating parameters provided evidence that denitrification occurring in the sludge blankets of the clarifiers may be responsible for sporadic high TP concentrations in the effluent. The nitrate levels in the RAS were found to vary significantly from 0-12 mg/L. Lower levels of nitrate (0-4 mg/L) were found in the Eastern Cluster basins, which operate at longer retention times. This observation suggested that denitrification may be taken place within the clarifiers.Further investigations indicated that when denitrification is “weak"(high RAS nitrate) P release in the sludge blanket is minimal. “Moderate” levels of denitrification (lower RAS nitrates) may result in OP release, increasing the OP level in the clarifier effluent. When “Strong” nitrification occurs (very low RAS nitrate), the nitrogen bubbles produced may lift the polyP-containing sludge solids into the effluent. Indicators of this are very low nitrate concentrations in the RAS, a TP concentration that is noticeably higher than the OP concentration, and a TSS profile that correlates well with the TP profile.Plants considering retrofitting to implement EBPR and clarifier designers should pay special attention to clarifiers of EBPR systems. Retention times in clarifiers should be minimized to avoid excessive denitrification that may lift polyP-containing solids to the effluent, potentially affecting permit compliance. Accordingly, more attention must be given to identifying the parameters that control denitrification in the clarifier sludge blankets of EPBR systems in order to better design and operate clarifiers for biological phosphorus removal systems.