LABORATORY STUDIES ON A SEQUENCING-BATCH TEMPERATURE-PHASED ANAEROBIC DIGESTION (TPAD) SYSTEM

With growing recognition of the difficulties in gaining US EPA's approval for PFRP equivalency, POTWs are undertaking pioneering projects aimed at developing creative operational schemes for thermophilic digestion to satisfy the definitional time-temperature standards under Part 503 Alternative 1. Temperature-Phased Anaerobic Digestion (TPAD) is one of the advanced digestion processes that could be adapted to meet the operational requirements for Class A. However, deviation from the intended continuous-flow mode of operation raised questions about the performance stability of the system. Laboratory studies at Iowa State University evaluated the performance and operational stability of a TPAD system modified to operate in the sequential-batch mode, retaining every “particle” at 55 °C for 24 hours. The sequential-TPAD system, fed with a 40:60 mixture (dry weight basis) of primary sludge and waste activated sludge at 5.5% total solids, showed stable performance at system retention times as short as 12 days. The system did not show any effects of shock loading at the retention times studied and outperformed a “conventional” mesophilic system operated at a longer retention time. However, while contemplating practical applications for the system, performance stability is not the sole issue. Critical applicability issues, such as digester heating, feed and discharge rates, and system configurations that have to be addressed while contemplating full-scale implementation of the process have also been discussed in this paper.