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

The effects of microwave (MW) final temperature (80 and 160 degrees 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 degrees 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 degrees 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. (C) 2018 Elsevier Ltd. All rights reserved.