Design implications for anaerobic membrane bioreactors and the metabolic influence of cycle time for the treatment of liquid dairy manure

This research developed a design approach for an anaerobic membrane bioreactor (AnMBR) treating liquid dairy manure with consideration of the cycle time impact on microbial activity. The research builds from initial comparison experiments with an AnMBR and a complete mix digester (CMD) and concludes with testing of various cycle time conditions necessary to develop a qualitative understanding of the associated microbiology. The results from this research and those of previous researchers were integrated into specific design considerations for an AnMBR treating liquid dairy manure.;A pilot-scale AnMBR and an identically sized CMD were designed and constructed to treat a sand-separated dairy manure. The CMD produced 54% more methane than the AnMBR operating at a cycle time of 84. Despite the apparent negative impact on microbial activity, the AnMBR produced an effluent permeate devoid of suspended solids with a COD reduction of 89%. There was also a strong correlation between membrane flux rate and the total solids (TS) concentration of the digester system that indicated declining flux rate with increasing digester TS concentration.;Based on the initial results, a combined CMD/AnMBR digester configuration was studied where the CMD effluent was used as the AnMBR influent. Metabolic evaluation of the biomass from the CMD and the AnMBR using a respirometer setup indicated a reduction in the interaction between fatty acid oxidizing bacteria and hydrogen consuming methanogens (syntrophic relationship); however, some activity remained.;A final set of experiments evaluated the impact of cycle time, digester volatile solid concentration and cross-flow velocity on the rate of methane production for two AnMBR systems and a control CMD. All digesters received the same sand and solid-liquid separated manure feedstock.;Cycle times as high as 27/day and cross flow velocities up to 4.5 m/s did not produce a negative effect on methane production compared to a CMD control while total VFA concentration for the AnMBR digesters was lower than that of the CMD. Metabolic evaluation illustrated a reduction in syntrophic activity compared to the CMD; however, even at a cycle time of 27/day, the AnMBR biomass retained approximately 25% of the syntrophic activity of the CMD biomass.;Operation at the higher VS concentration of the AnMBR did not confer a methane production advantage compared to the CMD for the operating conditions tested. Considering low VFA concentrations in all of the systems, it was theorized that once steady-state operation was attained, hydrolysis mass transfer limitations controlled available substrate for anaerobic degradation.;Based on the findings of this research, the AnMBR process, when operated at cycle times of 27/day or less, provided equal gas production to a CMD while reducing the COD, phosphorus and pathogen/virus loading by approximately 90%, 95% and 99.96% respectively.