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Exploration of fouling propensity in an anaerobic membrane bioreactor treating municipal wastewater and comparison to that of an aerobic membrane bioreactor Pattanayak, Soubhagya Kumar

Abstract

Anaerobic biodegradation is a successful technology that has been used in industrial, food processing, and agricultural wastewater treatment for many decades. The operational costs associated with anaerobic systems are typically lower than with aerobic systems and anaerobic systems also generate less waste sludge. However, the application of anaerobic treatment systems is limited for low strength wastewaters in colder climates. In colder climates, the biomass growth yield and the growth rate are relatively low, resulting in a low net biomass production. To maintain adequate biomass concentration in an anaerobic bioreactor, membrane modules can be coupled to the reactor to effectively treat low strength wastewater in colder climates. One of the important advantages of the membrane bioreactors (MBRs) is that the membrane component of the system can retain virtually all of the biomass within the bioreactor. Membrane units in an anaerobic MBR can operate either as external units or as submerged units, depending on the requirements of the process. Currently, the application of submerged AnMBRs is limited as compared to external AnMBRs. However, the vacuum-driven submerged membrane process shows a lot of promise as compared to external membrane processes. This is because high energy consumption is one of the biggest limiting factors in external membranes. Also, the use of head space gas for reducing fouling in submerged anaerobic membranes can be a very successful technology in limiting energy consumption in bioreactors. However, the widespread use of membrane technology has been limited due to the fouling of the membrane fibers. Membrane fouling is an inherent problem with membrane processes, which not only affects the long term operational stability, but also leads to significant operational costs due to increased membrane replacement frequency and added energy consumption. Therefore, a considerable amount of research and engineering effort has been devoted to understanding the mechanisms of membrane fouling and to work out fouling prevention and control strategies. The broad objective of the present study was (1) to assess the treatment performance of a submerged membrane AnMBR treating low strength municipal wastewater at an ambient temperature and (2) to identify and characterize the fouling mechanism in the AnMBR. The anaerobic process was effective in removing chemical oxygen demand (COD) and volatile fatty acid (VFA) from the effluent VFA removal was essentially complete and 80% COD removal was achieved under acetate-supplemented conditions. Nonetheless, high concentration of effluent COD (i.e. 72 mg/L) indicated that aerobic post treatment is needed to achieve secondary quality effluent. On-line filtration studies were conducted simultaneously on both the anaerobic membrane and the aerobic membrane of membrane enhanced biological phosphorus removal (MEBPR) process. The on-line tests were conducted to compare the fouling mechanism in the anaerobic membrane process and the aerobic membrane process. The results from energy dispersive X-ray (EDX) analysis suggest that inorganic materials were not the prominent foulants in the AnMBR. Also, scanning electron microscopy (SEM) analysis indicated that very little microbial colonization was found in the anaerobic membranes. Mixed liquor characterization tests were conducted to verify the role of extracellular polymeric substances (EPS) on membrane fouling. The tests concluded that the total bound EPS concentration was higher in the aerobic mixed liquor and the total soluble EPS concentration was higher in the anaerobic mixed liquor. The high soluble EPS concentration was probably the reason for rapid fouling of anaerobic membranes. In addition, the higher SRT in the case of the anaerobic membrane bioreactor might be responsible for the high soluble EPS production observed in some studies.

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