At present, the recovery and utilization of methane from anaerobic wastewater treatment systems as a source of energy are well-researched and widely adopted for a more sustainable system approach. However, not all methane produced in an anaerobic treatment system is completely recovered; subsequently, dissolved methane present in the effluent can be released into the environment and contribute to greenhouse gas accumulation in the atmosphere and reduce the system's methane yield. Many studies have already investigated and discussed the factors affecting the production of dissolved methane, as well as the techniques for its recovery. Among the recovery techniques, the use of degassing membrane contactor is most preferred for wastewater treatment application. However, reported data in the literature is limited to certain types of wastewater characteristics and anaerobic systems. Studies on membrane-based recovery of dissolved methane from AnMBR effluents are reviewed in this paper. For the case of the degassing membrane contactor, porous, or micro-porous membranes provides higher dissolved methane recovery efficiency than non-porous. However, porous membranes are more susceptible to pore wetting problem. Among the different operating conditions of degassing membrane contactors, liquid velocity, or flow rate greatly affects the recovery, wherein higher velocity decreases the recovery efficiency of dissolved methane. Consequently, research priorities aimed at development of degassing membrane to accommodate higher liquid velocity and to reduce pore wetting. Moreover, energy analysis of the AnMBR with degassing membrane system should be analyzed for performance in full-scale applications.
Introduction
Methane is a hydrocarbon compound resulting from the anaerobic degradation of organic materials. It is flammable and explosive gas, producing carbon dioxide and water vapor (Encyclopædia Britannica, 2018). The sources of global methane are a result of both natural and anthropogenic activities. The latter provides 60% of methane sources which are further classified as agriculture, energy, waste, and industrial sectors. The majority of methane produced from agriculture sector is released during the enteric fermentation in animal raising; methane from the energy sector is mainly produced from the production and processing of oil; from the waste sector it is primarily generated from the solid waste and wastewater processing; and lastly, the industrial source of methane comes from chemicals and metal productions (Karakurt et al., 2012).
As part of the focus of this study, almost 9% of the methane released into the environment comes from the activities involved in wastewater systems—from its collection, treatment, and disposal (Karakurt et al., 2012; Hu et al., 2017; Short et al., 2017). Methane is released into the wastewater through the metabolism of methanogens in an anaerobic condition of wastewater treatment system (Crone et al., 2016). Water or wastewater treatment is an inevitable part of the human community to abate the negative impacts of its disposal on the environment and living beings. However, the high energy requirement for the collection and treatment of wastewater is a major concern. This emphasizes the need for processes which will allow recovery of methane and its utilization as a source of energy for the wastewater treatment plant to improve its energy efficiency (Rongwong et al., 2018). Theoretically, about 0.35 liters of methane is produced per grams of chemical oxygen demand (COD) removed from the wastewater (Tchobanoglous et al., 2003) and about one cubic meter of methane has an estimated energy potential of 9 kWh (Crone et al., 2016). As cited in the study of Molino et al. (2013), if methane produced from the wastewater treatment system is used as automotive fuel, around 97% of potential carbon dioxide emission can be reduced compared to the use of fossil fuel, provided that the methane content of biogas is at least 90% (Harasimowicz et al., 2007). A critical review study on nine pilot-scale AnMBR systems (treating domestic wastewater) estimated that five of these systems have positive energy balance, which proved the potential of AnMBR to be an energy producer (Shin and Bae, 2018). Aside from this energy impact, recovery and utilization of methane have an environmental impact, too. According to the Intergovernmental Panel on Climate Change in 2014, methane has 28 times global warming potential than carbon dioxide (IPCC, 2014). This establishes the need for the control on the release of methane into the environment.
However, not all methane produced in a wastewater treatment system is recovered, which in turn will be discharged with the effluent in the form of dissolved methane and released into the environment. Liu et al. (2014) provided correlations among the solubility of methane in water and the temperature and salinity of the water (Figure 1). From Figure 1, the theoretical dissolved methane present in municipal wastewater effluent (with an average influent soluble COD concentration of 200 mg/L and a COD removal efficiency of at least 90%) at 30°C is around 45% of the total methane produced (Liu et al., 2014). In support of this, Smith et al. (2013) found that the percentage of dissolved methane in the effluent is 40–50% at 15°C. This dissolved methane can be utilized as an additional energy source for the operation of wastewater treatment facilities. Rongwong et al. (2018) calculated that optimum net electricity energy of 0.178 MJ could be recovered from the dissolved methane per cubic meter of effluent from an AnMBR coupled with a degassing membrane. This is still around 85% of the total energy recovered from the dissolved methane.
