Anaerobic Digestion of Sludges Containing Iron Phosphates

The phosphorus removals can be enhanced by precipitating bulk of the phosphorus with cation addition to the existing plants without costly alterations. Ferrous-iron was used to precipitate bulk of the phosphorus from raw waste water in the form of primary sludge and in aeration tanks in the form of waste activated sludge. The digestibility and the fate of phosphorus during the anaerobic digestion of such sludges was evaluated in bench scale studies and compared with those of corresponding control sludges containing very little or no iron precipitated phosphorus. The presence of iron in the sludge up to about 5% iron on the dry solids basis did not upset or affect the performance of the gas mixed digestion units. A significant release of phosphorus occurred on the anaerobic digestion of waste activated sludge containing high amounts of ferrous-iron precipitated phosphorus, while a significant uptake of phosphorus was observed on the digestion of primary sludge containing ferrous-iron precipitated phosphorus.