Effect And Mechanism Study Of Ferric Chloride Combined With Nitrite Pretreatment On Acid Production And Dewatering Performance In Waste Activated Sludge

The production of short-chain fatty acids(SCFAs)via anaerobic fermentation of waste activated sludge(WAS)is often limited with poor quality of SCFAs and long fermentation time.To overcome the issues,we provided an efficient strategy by using ferric chloride(FC)to aid nitrite pretreatment.Experimental results showed that the maximal SCFAs production of 211.3 ± 3.1 mg COD/g VS was achieved with 4 mmol/L of FC integrated with 250 mg/L of nitrite pretreatment on day 5,which was 4.1-fold higher than that of the blank control(52 ± 5mg COD/g VS,day 7).Besides,the enrichment of acetic acid was observed in the combined system,which accounted for 54.6 ± 3.5% of total SCFAs,while only 31.5 ± 4.9% was detected in the blank control.Propionic acid,iso-butyric acid,n-butyric acid,n-valeric acid and iso-valeric acid accounted for 14.7 ± 1.5%,6.9 ± 1.4%,7.4 ± 1.5%,13.1 ± 1.0%,and 3.3± 1.5% of total SCFAs in the combined system and 22.8 ± 4.0%,11.9 ± 3.0%,6.7 ± 3.1%,17.6 ± 2.0%,and 9.5 ± 3.9% of total SCFAs in the blank control,respectively.Besides,the soluble proteins and carbohydrates in the FC and nitrite pretreatment reactor were higher than those in the blank control,suggesting that FC and nitrite pretreatment benefited WAS disintegration.The fluorescence spectrum results suggested that FC and nitrite pretreatment improved the biodegradability of the released organics,which provided more biodegradable substances for the subsequent SCFAs production.This was because the addition of FC induced the formation of free nitrous acid from nitrite.Besides,FC-induced iron reduction also promoted the conversion of recalcitrant organics to biodegradable organic matter.However,after pretreatment for 24 h,the cake moisture content of WAS was still very high,and the dewatering performance was not significantly improved.Microbial community structure analysis demonstrated that the functional bacteria involved in acetogenesis process such as Enterococcus,Proteiniclasticum,and Petrimonas were highly enriched due to the pretreatment of FC and nitrite,indicating this method could improve the relative abundance of SCFAs producers.The proportions of Enterococcus,Clostridium,Petrimonas and Proteiniclasticum were 10.32%,14.16%,5.16%,and 5.66%,respectively,which in the blank control was only 0.05%,0.19% and 0.20%,respectively.The remaining genera(e.g.,Petrimonas)were not detected.Besides,hydrolyzed microorganisms were detected in both the combined pretreatment reactor and the blank reactor,and their relative abundance was 1.07% in the former and 3.96% in the latter,which suggested the addition of FC and nitrite led to relatively small impact on hydrolytic bacteria in the community.Overall,this study revealed that the pretreatment of FC and nitrite promoted the formation of free nitrous acid and increased the yield of SCFAs,which provided a novel method for wastewater treatment plants to ameliorate the sewage treatment craft and rationally use the existing substances in WAS to enhance resource recovery.