Optimization of anaerobic membrane bioreactors for sludge treatment

Abstract

Wastewater treatment is an energy intensive process. The energy balance is positively affected by anaerobic sludge digestion, especially primary sludge. The inclusion of a primary clarifier before the biological reactors results in a higher sludge total production compared to the direct treatment of raw wastewater. Conventional anaerobic digesters for sludge treatment are designed as completely mixed reactors operated at long solid retention times (SRTs) for enhanced solids conversion and to maintain the methanogenic activity in the reactor. Consequently, anaerobic digesters are commonly built with large volumes to ensure sufficient reduction of volatile solids (Xu et al., 2011). Anaerobic membrane bioreactor (AnMBR) is a promising alternative to conventional anaerobic digesters for sludge digestion. AnMBRs are operated at long SRTs independent from hydraulic retention time (HRT) by means of physical separation of the membrane. Thus, slow growing methanogenic biomass can be kept longer in the reactor, resulting in enhanced methane production. Moreover, a smaller footprint of the anaerobic reactor can be achieved since the HRT can be controlled by manipulating the flux. To understand the rationale behind the thesis, Chapter 1 presents a brief description about the energy consumption for wastewater treatment and its distribution in the wastewater treatment plant (WWTP). The organic matter removal mechanism in the conventional WWTP is explained. Novel process configurations for organics capture are presented. Anaerobic digestion process and the design parameters of the anaerobic digester are explained as well. The advantages of using AnMBR for sludge treatment are defined. The chapter ends with research gap and an outline of the thesis. The current status and perspectives of the AnMBR technology for sludge treatment are critically reviewed in Chapter 2. It discusses the historical development of the AnMBR for sludge treatment, and factors influencing the AnMBR performance reported in the literature. The operational conditions such as SRT, HRT and temperature have a noticeable effect on the methane production and permeate quality. Volatile fatty acids (VFAs) can be recovered simultaneously during sludge treatment, which can improve the economics of the WWTP. However, there are still problems, such as membrane fouling, which hinder the adoption of AnMBR technology for sludge management, as well as a lack of studies demonstrating the economic benefits of using AnMBRs for sludge treatment. Suggestions for research perspective are given, aiming for overcoming the challenges and for optimization of the AnMBR for sludge treatment. The aim of this thesis was to investigate the applicability of the AnMBR for sludge treatment in the view of energy-positive WWTPs. The objectives of this thesis were met through four different studies. Chapter 3 explains the material used and methods followed during these studies. The results of these studies are explained and disscussed in Chapter 4. In order to maximize organic capture and thus energy recovery from wastewater, novel configurations including an A-stage and CEPT have been proposed as alternatives to primary settling. However, it remains to be investigated to which extent these configurations affect the sludge characteristics, which may affect the economic feasibility of the integrated systems. Therefore, the first study focuses on the effect of these primary treatment methods on sludge characteristics and digestibility, and on plant-wide economics of wastewater treatment. A detailed characterization of sludge obtained from primary settling (primary sludge), A-stage treatment (A-sludge) and CEPT showed significantly different sludge characteristics. The organic compounds in primary sludge consisted mainly of 40% carbohydrates, 23% lipids, and 21% proteins. A-sludge was characterized by a high amount of proteins (40%) and a moderate amount of carbohydrates (23%), and lipids (16%), while in CEPT sludge, organic compounds were mainly 26% proteins, 18% carbohydrates, 18% lignin, and 12% lipids. The biomethane potential test showed that primary sludge and A-sludge had the highest methane yield (347 and 333 mL CH4/g VS, respectively), while methane yield of CEPT sludge was lower(245 mL CH4/g VS). A plant-wide economic evaluation for the three systems, indicated that energy surplus was the highest with CEPT. The inclusion of an A-stage had the lowest positive net energy due to the relatively high energy consumption in aeration. Considering the effluent quality of the three systems, CEPT had the highest benefits, followed by A-stage. Overall, integration of CEPT or A-stage, instead of primary clarification in existing wastewater treatment plants, has the potential to improve the effluent quality and energy recovery. AnMBRs have been applied as compact alternatives for anaerobic digesters for sludge treatment in conventional WWTPs. However, there is no information about the impact of integrating an A-stage, instead of primary clarifier, on sludge digestion in an AnMBR. The second study examines the performance of lab-scale AnMBRs, in terms of treatment and filtration performances, for both digestion of primary sludge and A-sludge. The results showed that anaerobic digestion of A-sludge yielded more methane and improved methanogenic activity in the AnMBR compared to primary sludge. The permeate of the AnMBR fed with A-sludge contained higher nitrogen and phosphorous concentrations due to higher nitrogen and dissolved phosphorous concentrations of A-sludge. No coliforms were detected in the permeates, which showed that from the hygienic point of view, the permeate had the potential to be directly used for irrigation purposes. A higher EPS concentration was observed during the digestion of A-sludge compared to the primary sludge, which accumulated on the surface of the membrane and caused an increase in transmembrane pressure (TMP) and filtration resistance. On a plant-wide level, the integration of an A-stage increased the amount of organic matter (COD) recovered from wastewater in the form of methane gas by about 15% compared to a WWTP configuration with a primary clarifier. Anaerobic digesters are operated at either mesophilic (35°C) or thermophilic (55°C) conditions. In general, it is known that higher amounts of biogas are produced from digesters operated at thermophilic conditions because of higher biochemical reaction rates. However, the specific effect of temperature on AnMBR performance for A-sludge digestion has not yet been assessed. Therefore, the third study evaluates the treatment and filtration performances of lab-scale AnMBR under mesophilic and thermophilic conditions. Higher biogas and VFAs were produced under thermophilic conditions, which were 23% and 47% higher than those under mesophilic one, respectively. Besides, the membrane could be operated at lower TMP under thermophilic conditions. However, taking into account the energy consumption and production, operating the AnMBR under mesophilic conditions would result in a more than three-fold higher net energy production than operating under thermophilic conditions, whereas surplus energy recovery under thermophilic conditions was less than the additional energy consumption. Therefore, despite the advantages of thermophilic conditions, operating AnMBR for sludge digestion under mesophilic conditions has a higher potential to improve the energy balance in the WWTPs. As found during the review (Chapter 2), there is a lack of studies demonstrating the economic benefits of using AnMBRs for sludge treatment in the WWTP. Therefore, the feasibility of the AnMBR for sludge (primary and waste activated sludge) treatment in a conventional WWTP is evaluated in the fourth study, through mathematical modeling and simulation, on unit process and plant-wide levels. The impact of HRT and SRT as control handles on the performance of the AnMBR was assessed. The amount of COD converted into methane could be increased by increasing the SRT or lowering the HRT, the former having a higher positive impact. The nitrogen and phosphorous load in the permeate increased by increasing the SRT or lowering the HRT, while the COD concentration in the permeate was hardly affected. As for the energy balance, increasing the SRT was more efficient than lowering the HRT. Indeed, increasing the SRT caused a significant increase in energy production while lowering the HRT only slightly reduced the energy consumption and did not affect the energy production. On a plant-wide level, the integration of an AnMBR instead of the anaerobic digester decreased the operational costs of the WWTP by 27%, but led to a worse effluent quality. The latter could be remedied by post-treatment of the permeate by struvite recovery and nitrogen removal through partial nitritation/anammox, at the same time further decreasing the operational costs - with 35% compared to a conventional WWTP. Overall, applying AnMBR for sludge treatment combined with post-treatment of the permeate provides effluent quality that meets the EU regulations and implies significant operational cost savings for wastewater treatment. Finally, Chapter 5 summarizes the main findings of the previous chapters and gives perspectives for further research inspired from the thesis.