Use of a Hydraulic Flush Reactor in a Two-Stage Anaerobic Digestion Process for Biodegradable Municipal Waste

Performance of a hydrolysis reactor in which the solids and liquid retention times were uncoupled to give enhanced solids retention and removal of hydrolysis products was evaluated for a feedstock of biodegradable municipal waste (BMW). The reactor was operated both alone and as part of a two-stage process in series with an anaerobic filter. Operating with a hydraulic flush to give hydraulic retention times (HRT) between 1.6 and 5.3 days, and a solids retention time (SRT) of 20 days increased hydrolysis in comparison with a control reactor where HRT and SRT were equal. Overall biodegradability was low; however, with an effluent chemical oxygen demand (COD) equivalent to ∼30% of the substrate biochemical methane potential (BMP). When operated as part of a two-stage process using effluent from the anaerobic filter as the flush liquor the characteristics of the system changed substantially with an increase in performance to an overall methane production of 0.22–0.24 L g−1 volatile solids (VS) added. This was achieved even when the maximum loading of 7.5 gVS L−1day−1 was applied. During acclimatization the first-stage hydrolytic reactor became methanogenic despite the short HRT, and was responsible for as much as 70% of the overall methane production. The system could not revert to single-stage operation after acclimatization, showing the continuing importance of the anaerobic filter. The combined system was robust, and could recover from major disturbances and step increases in loading without reduction in continuous methane production, indicating some potential for larger scale application in BMW disposal.