Control of biomass in anaerobic reactors using ultrafiltration membranes

Abstract

Anaerobic processing has become recognized as a simple and energy-efficient means of treating and stabilizing many high strength organic industrial wastes and is also not subjected to the operating limitations of aerobic processes. The literature review presented in this thesis outlines the advances made in the understanding of the microbiology and biochemistry of the process and the considerable advances in reactor configurations in achieving high SRTIHRT ratios. The efficacy of the anaerobic process is dependent on maintaining a high biomass concentration in the reactor, which in tum is dependent on the performance of the solids separator. The anaerobic contact process employs gravity settling for solid-liquid separation but the poor settleability of the anaerobic sludge may result in a poor performance of the contact process. By employing a much more efficient separation process such as ultrafiltration (UF), the performance of the anaerobic system can be significantly improved. In this study, therefore, a new method of operating a completely mixed anaerobic digester using a crossflow UF membrane technique was investigated in order to control the concentration of active biomass in the reactor and to determine the extent of any other advantages that can be gained over other reactor configurations. The study was carried out in four stages. In the first stage the performance of an anaerobic contact digester using a crossflow UF membrane technique was investigated for brewery wastewater treatment. The results obtained from this stage showed that under steady-state conditions, at an influent substrate concentration of approximately 120 g COD/l (100 g BOD/I) with a hydraulic retention time of 4.2 days giving an organic loading rate of 28.5 kg COO/m3.d , overall COO and BOD removal efficiencies of 99% and almost 100% respectively were achieved and the system had not reached its maximum loading capacity. Throughout the operation, HRT was maintained in the range of 2.5-4.2 days, largely determined by the flux rate. Microbiological analyses including Microscopic Count, Plate Count, Most Probable Number and Specific Methanogenic Activity test confirmed that there was almost no biomass loss through the membrane which, in tum, resulted in the maintenance of a high stability of the system under load changes. The UF membrane showed a remarkable consistency throughout the study. retaining a high concentration of active biomass in the digester and demonstrating that fouling by anaerobic biomass will reach a limiting level. In the second stage of the study the effect of Mixed Liquor Suspended Solids (MLSS) on the kinetics of the membrane reactor was investigated. The results showed that the kinetic coefficients estimated from the four steady-state runs had slight variations from each other but which could be mainly due to the changes in the numbers and the dominant species throughout the operation of the system. The increase in the MLSS concentrations did not significantly affect the kinetics of the system, In the third stage of the study the Specific Methanogenic Activity (SMA) technique was used to determine the methane production capacity of the membrane reactor, thus allowing suitable OLRs to be applied and to assess the effects of MLSS concentration on the activity of acetoclastic methanogenic bacteria in the digester. The results showed that any deterioration in acetoclastic methanogenic capacity of the system can be improved by increasing the sludge wastage rate. Ratios of the actual methane production rate to the potential methane production rate of less than 0.7 were found to be satisfactory in order to run the system efficiently in terms of COD removal and methane yield. In the final stage of the study the possible effects that membrane systems may have on the microbial population in the reactor was investigated. Therefore, microscopic examinations have frequently been carried out in order to determine the effects of the new configuration on any variation in the morphology or on the properties of methanogens as well as any change in the number of non-methanogens throughout the operation of the membrane reactor. This investigation showed that the membrane system configuration did have an apparent effect on the dominant methanogenic species throughout operation of the membrane reactor. For example Methanococcus species were the most dominant group at the beginning of the start-up period, becoming the third most dominant group at the end of the study. As a result, studying the changes in the number of viable methanogens and the dominant species may help to determine a reason for the deterioration in performance of a digester.