Enhancement of acidification from waste activated sludge via anaerobic fermentation by free nitrous acid (FNA) pretreatment assisted by ultraviolet

In order to break the bottleneck of the limited hydrolysis performance and low short chain fatty acids (SCFAs) production efficiency from waste activated sludge (WAS) during the traditional anaerobic fermentation, this work investigated the effect of the ultraviolet (UV) assisted free nitrous acid (FNA) pretreatment on WAS disintegration and acidification, and compared with thermal (H) and ultrasonic (US) coupled with FNA pretreatment. Results revealed that UV assisted FNA co-pretreatment (FNA-UV) had a synergistic effect on disruption of both extracellular polymeric substances and cell envelope. ･OH and ･O2-, as the main reaction intermediates, their intensities in FNA-UV group were much stronger than that obtained in other pretreatment groups (FNA-US and FNA-H). Moreover, these two free radicals, with the intermediates such as ･NO, ･NO2 and ONOO-, further promoting the release of soluble organics. The contents of soluble carbohydrates and protein were 60% and 90% higher than that obtained in FNA group respectively, which served more soluble substrates for SCFAs generation. Accordingly, SCFAs concentration peaked at 4d in FNA-UV group (201.8±4.8)mg COD/g VSS with 56.8% acetic acid (HAc)), which was 67% higher than that of FNA group. Carbon balance analysis at the final stage of the fermentation showed that UV assisted FNA pretreatment played an important role in sludge reduction, release and transformation of soluble substrates, and finally SCFAs production. The functional microbial consortia analysis indicated the anaerobic fermentation bacteria and nitrate-reducing bacteria were obviously enriched in FNA-UV group, which were 23.7%~270.6% higher than that obtained in other groups. © 2022 Chinese Society for Environmental Sciences. All rights reserved.