In-situ sludge degradation and kinetics of a full scale modified activated sludge system achieving near zero sludge production

This work reports on the performance of a full scale modified AS system that consists of anaerobic/anoxic/oxic and a settler with internal waste activated sludge (WAS) recycle that achieves sludge reduction through in situ cell lysis. This modification is termed as sludge process reduction activated sludge (SPRAS) process. The SPRAS plant achieved significant sludge reduction with sludge yield of 0.0823 kg TSS/kg COD which is much lower than the sludge yield of a conventional activated sludge process of 0.3-1.2 kg TSS/kg COD. The TSS concentration in the SPRAS tank was nearly constant at 4500 mg/L showing that there was no sludge build up, a sludge volume index of 80-120 mg/L implied adequate sludge settling ability in the settling chamber which facilitated sludge separation for subsequent degradation through anaerobic digestion. In addition, the process achieved ammonia and COD reductions above 85 %. Comparative FTIR analysis of the sludge in the anaerobic and oxic chambers indicated degradation of the proteinaceous biomass cell wall in the anaerobic chamber leading to release of carbohydrates that could easily be degraded in the oxic chamber. The maximum sludge yield and sludge decay coefficient were calculated as 0.103 kg TSS/kg COD and 0.0015 thus the technology is suitable for handling municipal wastewater at minimal sludge handling cost.