Energy analysis and optimization of hollow fiber membrane contactors for recovery of dissolve methane from anaerobic membrane bioreactor effluent

This work presents an energy analysis and optimization of the hollow fiber membrane contactors for the recovery of dissolved methane (CH4) in effluents of anaerobic membrane bioreactor wastewater treatment processes. The obtained CH4 could be merged with biogas for further purification or used with a micro-turbine for electricity generation to achieve an energy self-sufficient wastewater treatment process. A mathematical model considering simultaneous CH4 and carbon dioxide (CO2) desorption was used to estimate the membrane area required to remove the dissolved CH4, as well as quality of the outlet gas from the membrane contactor. Energy balance between electrical energy obtained from the recovered CH4 and energies consumed by vacuum and liquid pumps for the operation of membrane contactor were investigated and reported as a Net Electricity obtained per m3 of effluent or simply Net E. Results revealed that a combination of a high strip gas flow rate and slightly low vacuum condition closed to the atmospheric pressure can provide the highest Net E at 0.178 MJ/m3. This value is 85.37% of the total electrical energy that can generated from a 90% recovery of dissolved CH4 using an effluent saturated with a 60 vol% CH4 biogas and flow rate at 2 m3/day. The calculation was made based on the assumptions that 1) the membrane contactor is operated in a non-wetting mode where membrane properties remain constant, 2) flux decline due to the membrane fouling is not considered and 3) the energy required for membrane cleaning and other relevant activities are not factored into the energy analysis. Based on our results, to obtain a high CH4 mole fraction at the gas outlet, a low strip gas flow rate is recommended, however, the operating gas pressure needs to be lowered by applying a vacuum condition to improve the Net E. In addition, it was found that the Net E could be improved by increasing the number of membrane fibers, and lowering the liquid flow rate. The CH4 recovery efficiency could also be optimized to obtain an optimal Net E.