Live Fast, Die Young: Optimizing Retention Times in High-Rate Contact Stabilization for Maximal Recovery of Organics from Wastewater

Wastewater is typically treated by the conventional activated sludge process, which suffers from an inefficient overall energy balance. The high-rate contact stabilization (HiCS) has been proposed as a promising primary treatment technology with which to maximize redirection of organics to sludge for subsequent energy recovery. It utilizes a feast–famine cycle to select for bioflocculation, intracellular storage, or both. We optimized the HiCS process for organics recovery and characterized different biological pathways of organics removal and recovery. A total of eight HiCS reactors were operated at 15 °C at short solids retention times (SRT; 0.24–2.8 days), hydraulic contact times (t_c; 8 and 15 min), and stabilization times (t_s; 15 and 40 min). At an optimal SRT between 0.5 and 1.3 days and t_c of 15 min and t_s of 40 min, the HiCS system oxidized only 10% of influent chemical oxygen demand (COD) and recovered up to 55% of incoming organic matter into sludge. Storage played a minor role in the overall COD removal, which was likely dominated by aerobic biomass growth, bioflocculation onto extracellular polymeric substances, and settling. The HiCS process recovers enough organics to potentially produce 28 kWh of electricity per population equivalent per year by anaerobic digestion and electricity generation. This inspires new possibilities for energy-neutral wastewater treatment.