BioWin modelling to reduce the Avonmouth digester commissioning programme from 6 months to 6 weeks

In this study a novel accelerated restart strategy has been developed for a two stage anaerobic digestion system, involving acid phase digestion of sewage biosolids, followed by mesophilic digestion. The strategy involves start-up of the acid phase process, and then restart of the secondary mesophilic digesters, following a feed starvation period, and without re-inoculation. Process simulation with BioWin 4.1 software was coupled with continuous lab-scale trials to inform a full sized trial, which in turn defined the full plant restart schedule. Biosolids feed shut down period, and feed loading and feed acceleration rates were explored via lab-scale trials and simulation. The lab and simulation trials lead to the proposal of a 21 day restart profile for start-up and recovery of the two-stage anaerobic digestion process. This profile involved an initial feed load of 39% of the normal steady-state flow, and a 5% daily feed flow ramp up rate, following a 3 week starvation period. This accelerated restart strategy was successfully deployed on a 24,500 m³ anaerobic digester system in the UK. During restart the concentration of VFAs remained less than 1,500 mg L^-1, whilst pH and alkalinity were stable around 7.8 and 5,000 mg L^-1, resulting in a stable recovery.