CASE STUDY OF A FULL SCALE ADVANCED MESOPHILIC ANAEROBIC PLUG FLOW DIGESTER

Economic drivers have influenced the development of anaerobic digestion in the UK towards treating thicker sludges whilst reducing retention times. At the same time legislation from the European parliament and public perception has put the onus on water companies to provide a better quality product both low in pathogens and with greater stabilisation. The introduction of Treated and Enhanced Treated standards has led water companies to re examine their digester process trains in order to meet the new requirements. United Utilities response to these diverse pressures was to undertake fundamental studies in the anaerobic digestion process, which resulted in the development of the Enzymic Hydrolysis technology. There are now 4 full scale plug flow digestion plants either constructed or under construction incorporating the Enzymic Hydrolysis reactor.This paper discusses the benefits of converting the anaerobic digestion process to plug flow by looking at the first year of operational data from Bromborough STW. Bromborough STW treats sewage sludge from a 230,000 pe catchment and is the second full scale installation of Enzymic Hydrolysis within United Utilities. The design and build of the plant is detailed and experience gained during construction and commissioning of the plant discussed. The adoption of a standardised and modular plant design has provided a simple and reliable process, which can readily be incorporated into existing plants in a minimum of time. Commissioning of the process is also standardised and reliable startup is achieved in a matter of days.Data collected from the Enzymic Hydrolysis process and the anaerobic digesters during the first 18 months of operation is discussed and the ability of Enzymic Hydrolysis to produce both Class B and Class A biosolids reviewed. Furthermore the effectiveness of the hydrolysis achieved under plug flow conditions is examined and the implications for volatile solids reduction and increased biogas production assessed. An energy balance for the plug flow plant is presented, and the advantages of a sustainable biological process, operating at mesophilic temperatures and not requiring supplemental heating, chemical addition, heat recovery or inter stage cooling is considered.Finally the paper briefly reviews upcoming projects and proposes that plug flow Enzymic Hydrolysis Digestion will allow greater digester loading and a better biosolids product than currently possible with completely mixed anaerobic digesters.