Urine pretreatment enhances energy recovery by boosting medium-chain fatty acids production from waste activate sludge through anaerobic fermentation

Medium -chain fatty acids (MCFAs) represent a promising form of energy recovered from waste activated sludge (WAS) through anaerobic fermentation. However, the low WAS degradability hindered the MCFAs generation from WAS. In this study, urine pretreatment of 5 - 25 wt% was applied to enhance the MCFAs production in anerobic WAS fermentation with ethanol as an electron donor. Urine pretreatment (5 - 25 wt%) dosedependently improved the MCFAs production by 0.8 - 3.3 times (from 2.4 to 4.4 - 12.8 g chemical oxygen demand (COD)/L) and enhanced the MCFAs selectivity by 0.8 - 3.5 times (from 12.2 % to 21.4 % - 54.4 %). Urine pretreatment (5 - 25 wt%) improved WAS degradation by up to 54.8 % and enhanced electron transfer efficiency from substrates (i.e. WAS and ethanol) to MCFAs by up to 18.6 %, which contributed to the improvement in MCFAs production. According to model -based analysis, the MCFAs production potential (Pm) rose from 2.71 +/- 0.09 g COD/L with 0 % urine pretreatment to 4.51 - 13.11 g COD/L with 5 - 25 wt% urine pretreatment, while the lag phase (lambda) decreased from 5.30 d to 4.67 - 3.08 d, which may be the reason for the improved MCFAs production. Additionally, urine pretreatment significantly facilitated each step involved in the MCFAs generation, i.e. solubilization, hydrolysis, acidification, and chain elongation, by up to 600 %, 51 %, 17 %, and 42 %, respectively. This study for the first time reported an innovative method, urine pretreatment, enhances energy recovery by MCFAs from WAS through anaerobic fermentation, which potentially brings wastewater treatment plants economic and environmental profits.