In recent years, anaerobic membrane bioreactor (AnMBR) technology has been increasingly researched for municipal wastewater treatment as a means to produce nutrient-rich, solids free effluents with low levels of pathogens, while occupying a small footprint. An AnMBR can be used n
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In recent years, anaerobic membrane bioreactor (AnMBR) technology has been increasingly researched for municipal wastewater treatment as a means to produce nutrient-rich, solids free effluents with low levels of pathogens, while occupying a small footprint. An AnMBR can be used not only for on-site wastewater treatment, but also for the generation of nutrient-rich irrigation water leading to reuse and recycling possibility for agricultural applications as well. Furthermore, biogas produced in the anaerobic process could potentially be used for minimising the energy requirements of AnMBR operation. Despite the mentioned advantages, the current state-of-the-art AnMBR technology for potential full-scale application raises some concerns related to energy requirements and investment costs for membrane fouling control, as well as the impact of high shear stress on biomass activity. Municipal wastewater in many countries can be characterised as low strength and is generated at high flows rates, related to the population served. However, ongoing efforts in water conservation and the implementation of source separated sewer systems may possibly change municipal wastewater characteristics in the near future. Several factors involved in design and operation of AnMBR systems may influence membrane fouling, such as treatment plant configuration and membrane characteristics, feeding and biomass properties and operational conditions. Among them, reactor design is proposed as an important factor that can change the AnMBR sludge fouling propensity. To date, especially in the last decade, many studies have been conducted in which various types of anaerobic reactors, including completely stirred tank reactor (CSTR), upflow anaerobic sludge blanket (UASB) reactor, expanded granular sludge bed reactor (EGSB), were used in combination with various types of membranes. Among them, a membrane coupled UASB reactor may be a promising approach to overcome problems related with fouling since the membrane is only subjected to supernatant filtration and not by bulk sludge. The purpose of this thesis was to investigate the applicability of an innovative AnMBR configuration in order to produce a pathogen-free but nutrient-rich effluent for use in agricultural irrigation, concomitantly enabling energy recovery. With the realization of this study, flux enhancement by controlling the total solids (TS) load to the membrane was the major starting point, bringing AnMBR technology for full-scale sewage treatment one step closer to realisation. The AnMBR-Digester system as developed in this study may offer a solution for the mentioned challenges. The system consisted of a membrane integrated UASB reactor coupled to a parallel operating digester that can be operated at any required process temperature between 0-40 °C. The external cross-flow tubular membrane module contained 28 membrane fibers with a very small internal tube diameter of 1.5 mm in order to increase the filtration area for accommodating large sewage flows. Blocking of the membrane module lumens is minimised by the preceding UASB reactor, which scavenges the large part of suspended solids (SS), having only the supernatant subjected to membrane filtration. Besides that, the combination of a UASB reactor and a sludge digester, the so-called UASB-Digester system, has been shown to be successful for mutual sewage treatment at low temperature and sludge stabilization. Therefore, membrane integration to the UASB-Digester system can be attractive for producing high quality and nutrient-rich effluents for reuse purposes under moderate climate conditions. Based on this general aim, the optimum upflow velocity that will result in an effluent with good filterability is determined in a laboratory-scale UASB reactor for the case of membrane coupled UASB reactor systems. Filterability tests were carried out in order to assess the effect of upflow velocity on subsequent membrane performance. Results indicated a significant impact of upflow velocity on both biological performance and physicochemical effluent characteristics. Operation at a higher upflow velocity caused the washout of colloidal particles. Effluent characterization results coincided with filterability tests. Results showed that filterability of the effluent during the operation at 0.6 m/h was better than that during the operation at 1.2 m/h. The observed differences in protein/carbohydrate ratio (P/C) and particle size distribution (PSD), which play important roles in membrane fouling, lead to the hypothesis that upflow velocity is a critical parameter for effluent filterability in membrane coupled UASB reactors. In subsequent experiments, the membrane was integrated into the UASB reactor and the system was operated as an AnMBR system with an upflow velocity of 0.6 m/h in order to understand the impact of membrane addition on both the biological performance and sludge characteristics. Membrane incorporation induced an accumulation of fine particles and a decrease in extracellular polymeric substances (EPS), resulting in a decrease in PSD and thus, a drop in sludge settleability. Deterioration of sludge settleability led to an increase in sludge washout, with a resultant increase in chemical oxygen demand (COD) and total suspended solids (TSS) concentrations in the UASB effluent. However, SS-free permeate with an average COD of 42 mg/L was obtained and despite the sludge bed deterioration, the average transmembrane pressure (TMP) value was 85 mbar during reactor operation, indicating that no severe membrane fouling occurred in the AnMBR. Following that investigation, the AnMBR was operated at 15 °C in order to investigate whether the membrane coupled UASB is a technically feasible alternative in the treatment of municipal wastewater at lower temperatures. The results showed that membrane fouling at 15 °C was more severe than at 25 °C. Increased COD and soluble microbial products (SMP) concentrations, reduced particle’s diameter, and higher turbidity in the UASB reactor effluent at lower temperature aggravated membrane fouling. However, treatment performance was not considerably affected by temperature possibly due to the retention capacity of membrane. Cake resistance was found responsible for over 40% of the total fouling at both temperatures. However, an increase was observed in the contribution of irreversible fouling resistance to the total filtration resistance (RT) at lower temperature, related to the larger amount of fine particles in the UASB reactor effluent. During the operation at 15 °C, differences were observed in sludge characteristics at different heights along the UASB reactor. The best location in the sludge bed for conveying the sludge from the UASB reactor to the digester needed to be determined. Analysis over the height of the reactor with time showed that TS, volatile solids (VS), TSS and volatile suspended solids (VSS) concentrations in the reactor decreased with height, and highest COD concentration of 46 g/L was observed at the bottom of the reactor. The active biomass remained near the inlet of the reactor; whereas, non-active biomass consisted of loose, suspended particles and flocculents moved towards the top. This was confirmed by the high specific COD consumption rate near the inlet and the poor specific COD biodegradation in the remaining portions of the bioreactor. Apparently, the assumption of a completely mixed sludge bed behavior for the UASB reactor, being part of an AnMBR system, does not hold for this type of reactor systems even at low temperatures, which makes the location in the sludge bed from where the sludge is to be conveyed to the digester of operational importance. Considering the observed sludge bed stratification with regard to sludge stability and solids concentration, the sludge to be recirculated from the UASB reactor to the digester is recommended to be extracted from the sampling point where the sludge has the lowest stability. A low value of stability coincides with a high amount of anaerobic biodegradable organic compounds present in the sludge. Finally, the impact of digester coupling on removal efficiency and filtration performance of the AnMBR was investigated. Digester incorporation remarkably influenced the characteristics of the UASB reactor effluent, resulting in a decrease in turbidity, soluble and total COD and an increase in median particle size (D50), which led to a substantial decrease in the RT. Improved stability and specific methanogenic activity (SMA) of the sludge were achieved in the UASB reactor with an increase in total biogas production of the AnMBR-Digester system. The sludge recirculation from the sampling point having the lowest stability in the AnMBR-Digester system improved both the solids physical removal and the conversion, which confirms the importance of sludge transfer point selection, especially for the filtration performance of AnMBR-Digester systems. Overall, it can be concluded that the UASB reactor is a suitable alternative for coupling membranes in AnMBR systems at 25 °C. Despite the sludge bed deterioration, SS-free permeate with an average COD of 42 mg/L was obtained and the average TMP of 85 mbar was maintained during the operation period, indicating no severe membrane fouling. However, this configuration is not found technically feasible at 15 °C, considering the deterioration of the filtration performance, which would be a bottleneck to the practical engineering application of AnMBRs. Thus, the filtration performance of the investigated single stage AnMBR was limited by the low temperature. In that situation, the results revealed the high potential of the new AnMBR-Digester configuration for treating municipal wastewater at 15 °C, as it couples wastewater treatment and sludge stabilization. Under optimized sludge recirculation conditions, the integrated AnMBR-Digester system represents an efficient technology to mitigate rapid membrane fouling for the low temperature applications of membrane coupled UASB reactors.@en
