Increase of Soluble Phosphorus and Volatile Fatty Acids During Co-fermentation of Wastewater Sludge

Short-term fermentation of raw sludge from a high purity oxygen activated sludge systems was explored as a means of increasing the dissolved phosphorus and producing volatile fatty acids (VFA). Fermentation of primary sludge (PS), waste activated sludge (WAS), and co-thickened sludge (PS and WAS) showed significant phosphorus solubilization and VFA production at 2–4 days of fermentation in conditions of a completely mixed system of equal solids and liquid residence times. Factors found to have the most influence on phosphorus solubilization rates were sludge type, fermentation time, total phosphorus (TP) content in sludge, and process pH. WAS fermentation solubilized the most phosphorus per mass of volatile solids (VS) followed by co-thickened sludge. PS fermentation produced the most VFA per mass VS fermented while WAS produced the least. After 4 days of fermentation, co-fermented sludge converted 48 % of TP to dissolved phosphorus, and produced 1624 mg l−1 of VFA–COD which corresponded to a VFA–COD yield per mass of 0.139 mg per mg VS applied. In terms of total sludge management, co-fermentation resulted in greater overall VFA production and phosphorus solubilization than individual sludge fermentation.