Effect of dewatered sludge microwave pretreatment temperature and duration on net energy generation and biosolids quality from anaerobic digestion

Highlights: • Waste sludge was first dewatered and then pretreated for a more focused heating. • MW temperature had significantly higher impact on solubilization than holding time. • MW holding time enhanced solubilization but caused VFA accumulation/high energy input. • High-temperature MW pretreatment produced the high amount of extra heat with CHP unit. • Extra methane was not sufficient to cover the energy consumed by MW pretreatment. -- Abstract: The effects of microwave (MW) final temperature (80 and 160 °C) and holding time (1 and 30 minutes) were evaluated by ten anaerobic digesters (AD). MW was applied to dewatered secondary sludge with total solids of 10.5 ± 0.5% (by weight) to minimize energy input. Mixed sludge was digested under thermophilic and mesophilic conditions at sludge retention times (SRTs) of 20, 12, and 6 days. MW final temperature had significantly higher effect on solubilization than holding time. At MW of 80/160 °C, longer holding times further enhanced solubilization but increased volatile fatty acid accumulation at thermophilic SRT of 6 days. While all mesophilic digesters met the Class B requirement in terms of pathogens removal, all thermophilic digesters produced Class A biosolids. MW-ADs were able to tolerate 15–20% higher loading rates compared to controls. Up to 21% higher organics removals were achieved by MW. MW at 160 °C generated the highest amount of extra heat that could be used within/outside of plant. However energy recovered from the enhanced methane production was not sufficient to cover the electrical energy consumed by MW. Therefore, a custom-designed electromagnetic heating unit based on dielectric properties of sludge is needed to minimize energy input and maximize energy recovery for full-scale applications.