Proceedings

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

Forgacs, G.1, Smyth, M.1, Law, I.2 and Arnot, T.3, 1 Aqua Enviro, UK, 2 Wessex Water Enterprises trading as GENeco, UK, 3Water Innovation & Research Centre, University of Bath, UK

(free)

Abstract: 
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 at Avonmouth. 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.

Introduction:
Start-up is considered as a most sensitive, challenging and critical stage in the operation of anaerobic
digesters. During start-up the delicate balance between the different groups of microorganisms can be easily disrupted, because the microbial population varies in relation to pH optima, growth rates, inhibition levels etc. [1]. Moreover, a poorly executed start-up can lead to foaming, low treatment efficiency, or microbial death due to acid accumulation [2].

The main objective of this study was to develop a restart strategy for a two-stage anaerobic process, which
required some maintenance of the first stage. For this purpose, process simulation, continuous lab-scale
experiments, and full-scale trials were carried out. The result of these tests was used to determine an optimal restart profile for a full-sized AD plant at Avonmouth, UK.

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