A Comparison of Conventional Heating and Microwave Irradiation for Enhanced Biogas Production from Municipal Biosolids

This study evaluates advanced anaerobic digestion incorporating thermal pretreatments (microwave at 2.45 GHz and conventional heating) as an alternative stabilization process for municipal biosolids. Identical pretreatment temperatures (80, 120 and 160°C) and heating rates (7.5 °C/min) were applied for both thermal pretreatments to systematically compare conductive heating and microwave irradiation of biosolids.Both conventional heating and microwave pretreatments indicated that in a pretreatment range of 80-160°C, temperature was a statistically significant factor (p<0.05) on biosolids solubilization. At similar temperatures, both conventional and microwave heating achieved similar COD solubilization ratios. The results were also confirmed by biopolymer (proteins, sugars and humic acids) solubilization experiments. Fourteen lab-scale digesters were operated for 5 months to optimize energy (methane) output and SRT requirements of untreated (control) and thermally pretreated digesters. All control and pretreated digesters achieved steady state at all three SRTs of 20, 10 and 5 days, corresponding to volumetric organic loading rates of 1.74 to 6.96 g COD/L/d. At the SRT of 5 days, both mesophilic and thermophilic controls stopped producing biogas after 20 days of operation with total volatile fatty acids concentrations exceeding 1818 mg/L at pH <5.64 for mesophilic and 2853 mg/L at pH 0.05) on methane production at all three SRTs.Although the t-test results showed that populations of pretreated and non – pretreated (controls) digesters were significantly different (p<0.05), the increase in the pretreatment temperature above 80°C did not yield a statistically significant effect (further increase or decrease) on the methane production. This implies that from a simple methane recovery point of view, the lowest pretreatment temperature tested (80°C) was sufficient. Energy assessment showed that digesters operated at SRT of 10 days were more favorable and maximum net energy was achieved by mesophilic control (9.6 GJ/d/tonne TS added) and conventionally pretreated digester at 80°C (8.9 GJ/d/tonne TS added) respectively.