Effect of High Shear on Anaerobic Digestion in an Anaerobic Membrane Bioreactor (Anmbr) Treating High Strength Wastewater

156 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.Anaerobic membrane bioreactors (AnMBRs) provide an alternative for advanced wastewater treatment since they facilitate retention of slow growing anaerobic microorganisms resulting in efficient treatment in small reactor volumes and excellent effluent quality. Hence, these reactors are ideal for densely populated areas with a demand for water reuse. In order to reduce membrane fouling, high shear rates are applied at the surface of the membrane in AnMBRs. However, this high shear may lead to (i) breakup of spatial interactions between hydrogen-producing syntrophic bacteria and their methanogenic partners, (ii) increased hydrolysis rates, or (iii) prevention of growth of methanogenic zones required for methane conversion. The objective of this study was to evaluate if and to what extent high shear in AnMBRs can decrease reactor performance. A 6-liter AnMBR with an ultrafiltration membrane was fed swine waste. Methanogenic population dynamics monitored using terminal restriction fragment length polymorphism indicated that poor reactor performance under increased shear conditions could be explained by increased hydrolysis rates, but that the syntrophic interactions remained intact. To confirm this hypothesis, the reactor was fed a synthetic waste stream, which had faster hydrolysis rates compared to swine waste, and hence reduced the possibility of increased hydrolysis rates as a result of changes in shear. Reactor performance and microbial floc size distributions were not affected under increased shear conditions. Microbial floc structure analysis with fluorescence in situ hybridization indicated close proximity of archaeal and bacterial cells (within 10 mum of each other) before, during, and after a high shear period, indicating that syntrophic interactions were intact. To test the shear resisting ability of the AnMBR biomass, specific methanogenic activity batch experiments were performed to compare the effect of short-term exposure to high shear in AnMBR biomass to full-scale anaerobic digester biomass. Only the digester biomass was affected as a result of high shear, most likely due to the prevention of formation of methanogenic zones, but the sheared flocs were able to re-flocculate in the batch reactors and produced methane at a rate similar to that of the non-sheared biomass. These data suggest that anaerobic treatment is possible under high shear conditions that are present under normal operation of AnMBRs as long as care is taken to avoid an increase in shear in the presence of particulate matter.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD