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) was performed, using low-level alkaline solubilization of WAS. Solubilization was performed by the addition of 15 meqL(-1) of calcium hydroxide or sodium hydroxide, using a mechanical anoxic mixing time of 5 hours. The research examined the effects of decreasing solids retention time (SRT) on digester operational efficiency, as well as the fate of various nitrogen and phosphorus species in both the sludge and supernatant. The overall performance of three pilot scale digesters, including a control unit was monitored. Nitrogen and phosphorus species demonstrated somewhat different response patterns for each chemical. Generally, both chemicals were responsible for significant release of soluble TKN and ammonia during both the solubilization and actual anaerobic digestion steps. Similarly, sodium hydroxide addition to WAS caused a significant release of soluble phosphorus during both steps, resulting in high supernatant levels of both soluble TP and PO4-P (similar to control unit, with no WAS pretreatment). Calcium hydroxide, however, released considerably less soluble phosphorus into both the mixed liquor, during the solubilization step, and into the final supernatant during the digestion step.