Improved sludge anaerobic digestion capacity by dynamic membrane and alkaline-thermal pretreatment: Long-term continuous operation and techno-economic analysis

This study explored the application of a dynamic membrane (DM) and alkaline-thermal pretreatment for sludge digestion with a low hydraulic retention time (HRT). The formed DM enabled stable sludge digestion at an HRT of less than 12 days, and the alkaline-thermal pretreatment at 60 celcius and pH 10 for 24 h further reduce the HRT to 6 days, resulting in an 82% increase in the average methane production rate. The pretreatment not only enhanced methane yield but also increased the abundance of methanogenic archaea and their specific methanogenic activity under all HRT conditions. Additionally, microbial analysis revealed that the performance of high-rate sludge digestion was associated with the prevalence of Firmicutes and Bacteroidetes phyla in the bacterial domain, indicating that the pretreatment promoted the growth of hydrolytic bacteria. Furthermore, the pretreatment process significantly increased the relative abundance of Methanosaeta at a low HRT, indicating an increase in acetoclastic methanogenesis. The permeability and resistance of the DM as well as the pump energy consumption were investigated with various combinations of filtration flux and relaxation ratio. The optimal values for filtration flux and relaxation ratio were determined as 8.2 L/m2/h and 0.33 h/h, respectively. The net present value analysis, considering total capital investment cost, net energy production, and chemical dosage, confirmed the economic feasibility of the alkaline-thermal pretreatment. Dynamic membrane along with the alkaline-thermal pretreatment at low temperature would be a practical solution for improving the treatment capacity, methane recovery, and economic benefit of a sludge digester.