Performance Evaluation of Anaerobic Digesters

Anaerobic digestion and food waste (FW) using Single-stage and two-stage reactors were studied to compare reactor startup, reactor stability and performance with respect to methane yield, volatile solids reduction and energy yield. The single-stage reactor (SSR) was a conventional reactor operated at a low loading rate with a maximum of 3% of total Solids, while the two-stage reactor consisted of an acidonegic reactor (TSAR) and a methanogenic reactor (TSMR). The TSAR was inoculated with methanogenic sludge similar to the SSR, but was operated with step-wise increase in the loading rate and with total recirculation of reactor solids to convert it into acidification sludge. Before each feeding, part of the sludge from TSAR was centrifuged, the centrifuge liquid (solubilized products) was fed to the TSMR and centrifuged solids were recycled back to the reactor. Single-stage digestion produced maximum methane yield of 1000 ml/d as CH4. The TSAR shifted to acidification mode at an (organic loading rate) OLR of 3% of total solid and then achieved stable performance at 4% of total solid and pH 6.53-6.68, a maximum methane yield of 3010 ml/d. The lower energy yield of the two-phase system was due to the loss of energy during hydrolysis in the TSAR and the deficit in methane production in the TSMR attributed to chemical oxygen demand (COD) loss due to biomass synthesis and adsorption of hard COD onto the flocs. These results including the complicated operational procedure of the two-stage process and the economic factors suggested that the two-stage process could be the preferred system for FW.