Effect of thermal pretreatment on anaerobic digestion of sewage sludge by anaerobic membrane bioreactor

Introduction The effective utilization of sewage sludge is an important part of municipal solid waste reuse and plays a pivotal role in establishing a recycling society. Anaerobic digestion is accomplished in converting organic wastes such as sewage sludge into biogas, achieving waste reduction and resource utilization at the same time. However, conventional anaerobic digestion technology has the disadvantages of slow growth of microorganisms, poor effluent quality, sensitivity to reactor pH, temperature and environmental conditions, etc. Moreover, hydrolysis of sewage sludge becomes a rate-limiting step due to complex floc structure and hard cell walls during the anaerobic digestion. The method of improving efficiency of sludge anaerobic digestion becomes an important issue. In this research, low-temperature thermal pretreatment was integrated into anaerobic membrane bioreactor (AnMBR) to stimulate the decomposition of organic matter in sludge for the sake of improving anaerobic digestion efficiency and reducing energy requirement. AnMBR combines anaerobic digestion and membrane separation technology, which offers plenty of merits including separating HRT and SRT, low sludge production and energy production. Well-known is that AnMBR retains microorganisms in the reactor through efficient membrane filtration, and improving the quality of the effluent. At present, the application of AnMBR in anaerobic digestion of high solid organic waste is extremely prevalent. Besides, thermal pretreatment is widely used in improving in anaerobic digestion efficiency through accelerate hydrolysis process. In this study, low-temperature treatment(70°C) was applied to upgrade bioconversion efficiency because high-temperature treatment entails huge energy consumption and places strict demands on devices. Based on the above consideration, the present study evaluated the performance of mesophilic anaerobic digestion of excess sludge by high solid AnMBR and the effect of low-temperature thermal pretreatment. Additionally, membrane fouling behaviour and the COD mass balance were carried out to in-depth evaluate the potential and sustainability of AnMBR system. Material and Methods The AnMBR used in this study consists of a continuous stirred reactor (CSTR) and a separate submerged membrane unit, with a total effective volume of 15L. The average pore size of the membrane is 0.1μm, and the effective filtration area is 0.1 m. The bottom of the membrane unit is equipped with an aeration pipe, which can continuously aerate the biogas from the top of the CSTR at a rate of 5 L·min in terms of cleaning membrane surface and reducing membrane fouling. A digital pressure sensor is connected to the membrane unit to monitor the transmembrane pressure (TMP) in real time. Long term experiment was divided into six phases. The substrate of phase 1 and phase 5 was steam injector treatment (70°C) excess sludge while venturi nozzle treatment (70°C) excess sludge was used during phase 2 and phase 4. The HRT of phase 1-3 was 30d and the HRT was improved to 15d from phase 4. The performance of reactor operation, biogas production, organic matter removal rate and mass balance due to the utilization of thermal pretreatment were evaluated. Besides, the concentration of sludge in AnMBR was 2.5% and 3.0% respectively during phase 1-3 and phase 4-6 in order to investigate the maximum sustainable flux at different high solid concentrations. The continuous operation management factors of AnMBR are summarized in table 1.Additionally, the resistance-in-series model (Cheng, 2020) was applied to analysis membrane fouling. Table 1. Experimental condition at different phases.