Acid Phase Digestion at Derby – The Final Chapter

Wild, R., Severn Trent Water


During 2008, Severn Trent Water constructed an Acid Phase Digestion plant at Derby Wastewater Treatment Works, in the East Midlands of the UK.  The design of the plant was based upon laboratory scale work and learning from other, similar processes.  The plant was built with operational flexibility in order to determine the optimum process configuration and at the 14th European Biosolids Conference results were presented from the optimisation work, outlining the benefits realised from the plant to date.  Since this event, further work has been carried out and the performance of the plant increased further.  Now optimisation is complete, the purpose of this paper is to present the full set of results in context, together with a commentary on aspects of post-project appraisal such as standards of construction in order to identify the optimum Acid Phase Digestion plant specification for Severn Trent Water to construct elsewhere in AMP5 and beyond.
Acid phase digestion (APD) is reported to increase volatile solids destruction and gas production from anaerobic digestion.  Following research into a range of configurations, an APD plant of Severn Trent Water’s own design was constructed upstream of existing mesophilic anaerobic digestion (MAD) at Derby STW.  The design of this plant was outlined by Edgington & Thompson at the 12th European Biosolids & Organic Resources Conference in 2007 and an report of the first year’s performance was given by Wild & Bjorn at the 14th European Biosolids & Organic Resources.  This paper updates the latter to give a full set of operational results.
The plant was designed with the flexibility to operate the APDs in either parallel or series modes and variable hydraulic retention times.  The plant was commissioned in February 2009, and a programme of optimisation has been progressing since.  This paper relays the operational experience from the first stage of the this programme, operating the plant in parallel mode, and performance data set against the benchmark of previous performance.
Keywords: Acid Phase Digestion, vivianite, methane, volatile solids destruction, optimisation.
Anaerobic digestion has been the primary technology used to stabilise sewage sludge in the UK for over 30 years.  More recently technological innovations have been developed to optimise the anaerobic digestion of sludge.  One such advanced anaerobic digestion technology is Acid Phase Digestion (APD).  Anaerobic digestion takes place in three stages, hydrolysis, acetogenesis and methanogenesis.  The bacteria responsible for the first two stages prefer an environment of pH 5 and 1-3 days retention time, while methanogenic bacterial prefer pH 7.5 and 7+ days retention time.  APD works on the principle of physically separating the two stages, making it possible to provide optimal conditions for the two sets of bacteria consecutively, maximising volatile solids destruction (VSD) and therefore gas production.
Following research undertaken by Severn Trent Water in 2004 (Quinn, 2004) and visits to other sites in the UK Severn Trent were keen to take advantage of the technology’s benefits.  The company decided to undertake their own detailed design for APD at their Derby STW.
Renewable energy credits provide a significant income stream for the water companies in the UK.  Maximising this income stream is increasingly important with the drive for increased efficiencies through innovation in AMP 5, and the challenging wider economic landscape.  Severn Trent Water processes around 150,000 tonnes dry solids sewage sludge per annum, the majority of which is anaerobically digested, with currently over 30MW of installed CHP capacity to harness the energy in the resultant biogas.
Derby STW is a 475,000 PE works with conventional preliminary and primary treatment, followed by a biological nitrogen and phosphorous removal activated sludge plant.  The sludge stream consists of primary sludge thickening by centrifuge and surplus activated sludge thickened by belt thickeners, thickened sludge blending tank, APD, three MADs and dewatering by centrifuge.

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