High Solids Anaerobic Digestion with Recuperative Thickening: An effective method to increase digester capacity in the quest for energy positive municipal wastewater treatment plants

High solids anaerobic digestion with recuperative thickening is an effective strategy to increase the capacity and biogas generation of new and existing sewage sludge digesters. Anaergia partnered with Victor Valley Wastewater Reclamation Authority (VVWRA) to demonstrate at commercial-scale the Omnivore™ Digester: a digestion retrofit system that uses recuperative thickening and submersible high solids mixers. The system increases the solids retention time (SRT) and consequently the solids content in digesters. This is achieved by recovering and reintroducing digester solids, while discharging liquid in a digestate recirculation loop. This decouples solids residence time (SRT) from hydraulic residence time (HRT). Separate control of SRT and HRT improves digester performance and allows operation at higher organic loading rates. Biogas generation and volatile solids destruction increases with longer SRT while digester organic loading increases with shorter HRT. Increasing the solids content in the digester causes an increase in digestate viscosity to levels that are not suitable for conventional gas, jet, or mechanical mixers. The Omnivore™ system utilizes Anaergia low speed, high-torque submersible mechanical mixers. These mixers have been specifically designed to efficiently mix viscous digestates with high solids content. Conventional “pancake type” municipal sewage sludge anaerobic digesters can be retrofitted using the Omnivore™ system to increase organic loading and consequently biogas generation by 2 to 4 times. The increased loading capacity created in the digesters and the high solids mixing capability enable co-digestion of large amounts of imported organic waste streams. The resulting biogas generates electrical power that can meet or exceed a wastewater treatment plant’s energy demand.In this demonstration, the Omnivore™ Digester achieved an organic loading rate of 4.5 kg/m3/d (0.28 lb VS/cf-d) and 4.2% total solids (TS) content in the digester with sludge and external organic waste as feedstocks. This compared to a typical organic loading rate of 1.3 kg/m3/d (0.08 lb VS/cf-d) and 2% TS in two conventional digesters with primary sludge (PS) and thickened waste activated sludge (TWAS) sludge as feedstock. The Omnivore™ system tripled the capacity of the retrofitted conventional digester enabling production of the same quantity of biogas a conventional digester three times its size.Fecal coliform in the Omnivore™ digestate was found to be below the EPA 503 threshold for Class B Biosolids, indicating the ability for Omnivore™ digestate to meet Class B standards as a result of the extended solids retention time.Reductions in polymer demand for downstream dewatering of digestate where measured by Capillary Suction Time. Dewatering recuperatively thickened digestate from Omnivore™ was found to require 40% less polymer than conventional digestate.