Multi phase digestion – A low cost solution to enhance sludge assets

Two stage digestion processes have generated huge interest for the potential to enhance digestion performance, pathogen reduction and process stability. Laboratory research carried out by United Utilities (UU) proved that log 6 pathogen reduction could be achieved in 3 to 5 days at mesophilic temperatures compared to conventional MAD followed by secondary storage which achieves between 1.5 log reduction and 2.5 log reduction.Based on this bench scale work UU has constructed two full scale, two-stage digestion process incorporating a novel plug-flow reactor in order to meet pathogen consent and improve process stability at these facilities. A third plant is currently in detailed design.This paper discusses the novel plant design successfully employed by UU. The design concept of the plug-flow reactor is reviewed and the benefits accrued from optimising the preliminary digestion phases are discussed. Although 2-stage digestion is not new, the reactor design employed and the resulting mode of process optimisation is unique and patents have been applied for these novel features.