Increasing substrate for polyphosphate‐accumulating bacteria in municipal wastewater through hydrolysis and fermentation of sludge in primary clarifiers

The possibility of improving enhanced biological phosphorus removal (EBPR) by increasing the level of substrate for biological polyphosphate‐accumulating bacteria in influent wastewater, achieved through enhanced hydrolysis and fermentation of primary sludge in the primary clarifiers, was studied at a full‐scale University of Cape Town plant for more than 1 year. The sludge level in the clarifiers was increased to create anaerobic conditions and to increase the solids retention time in the clarifiers. Recirculation of sludge from the bottom to the top of the clarifiers was begun to wash out the fermentation products formed in the sludge phase into the water leaving the clarifiers. The performance of the EBPR plant improved considerably after enhanced hydrolysis and fermentation were established in the primary clarifier. Daily analyses at the treatment plant and laboratory‐scale fermentation experiments on sludge from the clarifiers showed that the soluble chemical oxygen demand (COD) in the wastewater increased by up to 10 mg CODIL. Measurements of volatile fatty acid potential (VFA‐potential) yielded an increase of 5 to 10 mg/L VFA‐COD. Long‐term fermentations in the laboratory showed that another 15 to 20 mg/L of VFA‐COD could be derived from the sludge, should the hydrolysis and fermentation be prolonged and optimized; if applied to a full‐scale wastewater treatment plant, this process would require the use of a separate reactor.