FATE OF NUTRIENTS DURING ANAEROBIC CODIGESTION OF SLUDGES FROM A BIOLOGICAL PHOSPHORUS REMOVAL PROCESS

Pilot scale (160 L) research on enhancement of anaerobic co-digestion of combined primary and high-phosphorus waste activated sludge (WAS) w as performed, using low-level alkaline solubilization of WAS. Solubili zation was performed by the addition of 15 meqL(-1) of calcium hydroxi de or sodium hydroxide, using a mechanical anoxic mixing time of 5 hou rs. The research examined the effects of decreasing solids retention t ime (SRT) on digester operational efficiency, as well as the fate of v arious nitrogen and phosphorus species in both the sludge and supernat ant. The overall performance of three pilot scale digesters, including a control unit was monitored. Nitrogen and phosphorus species demonst rated somewhat different response patterns for each chemical. Generall y, both chemicals were responsible for significant release of soluble TKN and ammonia during both the solubilization and actual anaerobic di gestion steps. Similarly, sodium hydroxide addition to WAS caused a si gnificant release of soluble phosphorus during both steps, resulting i n high supernatant levels of both soluble TP and PO4-P (similar to con trol unit, with no WAS pretreatment). Calcium hydroxide, however, rele ased considerably less soluble phosphorus into both the mixed liquor, during the solubilization step, and into the final supernatant during the digestion step.