Enhanced Organic Matter Recovery in the High-Rate Contact Stabilization Process by Addition of Waste Activated Sludge: A Pilot-Scale Study

The high-rate contact stabilization (HiCS) process is a potential technology for recovering energy from organic matter in wastewater; however, further performance improvement is required. This study proposes a biologically enhanced HiCS (BE-HiCS) process that introduces waste-activated sludge (WAS) from a separate conventional activated sludge (CAS) train within a wastewater treatment plant (WWTP) into the HiCS stabilization tank. The performance of the proposed process was verified using a pilot-scale plant. In a scenario combining the CAS and BE-HiCS processes, which is considered feasible for practical WWTP implementation due to the ready availability of WAS, a recovery of 0.24 ± 0.03 g-COD/g-COD of the influent COD mass was projected. This value was statistically significantly higher than that achieved by either the CAS process alone or the HiCS process alone. In this scenario, WAS was added to the BE-HiCS process at a ratio of 0.15 ± 0.03 g-COD/g-COD, resulting in a net recovery rate of 0.33 ± 0.04 g-COD/g-COD, after subtracting the COD contribution of the added WAS. The superior organic matter recovery of the BE-HiCS process was attributed to its enhanced ability to convert non-particulate organic matter into sludge through absorption and adsorption, while maintaining adequate sludge settleability for effective solid–liquid separation.