Accelerated hydrolysis and acidification of municipal solid waste (MSW) in a flushing anaerobic bio-reactor using treated leachate recirculation

Abstract

The efficiency of anaerobic digestion for reducing the solid content of the organic fraction of municipal solid waste (OFMSW) was compared using a conventional single pass reactor (SPR) and a two stage anaerobic–aerobic process. The first stage of the two stage system used an anaerobic hydraulic flush reactor (HFR) in which the solids and liquid retention times were decoupled by retention of solids within the system. The purpose of the hydraulic flush was to wash out rapidly the fermentation intermediates from the anaerobic reactor and thus prevent their accumulation; these were subsequently removed in the second stage aerobic activated sludge system, the effluent from which was then used as the flush liquor to the first stage. The hydraulic flush reactor was operated at hydraulic (liquid) retention times of 2, 3 and 4 days with solids retention times of 30, 25, 20, 15, 10 and 5 days. In all cases the total solid content in the feed was 10% giving loading rates to the reactor in the range 3 to 21 kg total solids per m3 day; this same loading range was used in the single pass reactor. At high loadings the SPR system showed an accumulation of volatile fatty acids, peaking at 13000 mg l–1, which resulted in process failure and cessation of solids destruction. At the same process loadings the HFR system showed only a slight decline in the rate of solids destruction when using a high hydraulic flush rate.