Enhanced two-phase anaerobic digestion of waste activated sludge by combined free nitrous acid and manganese dioxide

Abstract

Anaerobic digestion (AD) is a mature and reliable technology for sludge treatment, but it still faces many technical problems including slow hydrolysis rate and low methane yield. Free nitrous acid (FNA) pretreatment was previously confirmed in enhancing sludge hydrolysis and extracellular polymeric substances (EPS) disruption in anaerobic digestion. It was hypothesized that, due to the manganese dioxide (MnO2) addition during methanogenic stage, direct interspecies electron transfer (DIET) facilitated the biological activities, thus boosting the methane production in FNA-pretreated two-phase AD model. The results showed that the methane production of the combination of FNA pretreatment and MnO2 addition was improved by 18.64% and 22.23%, compared with reactors that solely treated by either MnO2 addition or FNA pretreatment. The changes of microbial metabolism activity were evaluated by measuring the coenzyme F420 and electron transfer system (ETS). The activities of coenzyme F420 and ETS were increased to 156.26% and 134.71% in two-phase AD model, respectively. Meanwhile, the obvious microbial community succession was found with the enrichment of methanogens such as Methanosarcina and Methanobacterium. Overall, the combination of FNA pretreatment and MnO2 addition avoided the inhibition of FNA pretreatment on methanogenesis in the early stage, and showed positively synergistic effect on methane production. The enhancement of microbial metabolism was responsible for, promoting methane production in the two-stage AD model. This research provides an alternative strategy for efficiency improvement of anaerobic sludge digestion as well as methane production.