Development of a coarse membrane bioreactor for two-stage anaerobic digestion of biodegradable municipal solid waste

A laboratory-scale coarse membrane bioreactor was developed to test its potential for the enhanced degradation of biodegradable municipal (solid) waste. The purpose of the mesh was to retain solid substrate and biomass in the reactor, promoting optimal degradation while also allowing intermediate soluble compounds to be removed and degraded in a second reactor. Three reactors with nylon woven mesh membranes of pore sizes 30, 100 and 140 μm were operated at a solid and liquid retention time of 20 and 1.5 days respectively and fed at an organic loading rate of 3.75 gVS l⁻¹ d⁻¹. Liquid effluent was fed to an anaerobic filter reactor. The total methane production for the two stage systems was very similar at 0.21–0.22 l g⁻¹ VS added (c.f. 0.26 l g⁻¹ VS added for BMP). The effect of increasing the pore size of the mesh was to reduce the methane production in the first stage and transfer more of this to the filter reactor, with the proportion of the total methane produced in the first stage changing from 72% to 49% between 30 and 140 μm meshes. The VS content of the first stage effluent also increased with pore size so it is likely that the mechanism for the differences in methane production is one of solids/biomass retention. The 30 μm pore size is recommended for further work since it transmitted a lower solid and soluble loading to the anaerobic filter. Solids accumulation in the second stage, although causing no problems in the 85-day operational period of this trial, could lead to blocking of the anaerobic filter, which is undesirable on a large scale.