Enhanced hydrolysis of waste activated sludge for methane production via anaerobic digestion under N2-nanobubble water addition

Anaerobic digestion (AD) is a relatively safe and economically feasible disposal technique for waste activated sludge (WAS), in which hydrolysis of complex organic matters is the rate-limiting step. The aim of this study is to explore the efficiency of applying nitrogen gas nanobubble water (N-2-NBW) to AD of WAS and reveal the possible mechanisms. The possible effects of N-2-NBW on different processes during AD of WAS were investigated and N-2-NBW was expected to enhance the hydrolysis step. Results showed that after N-2-NBW addition, sludge particles possessed more negative charges (indicated by zeta potential) than the control with deionized water (DW)addition.The total methane production of NBW group was 402 mL/g-VSreduced, 29% higher than the control group. Moreover, mechanism investigations revealed that N-2-NBW addition not only improved the disintegration of high molecular weight compounds (proteins and polysaccharides), but also enhanced the activities of four extracellular hydrolases by 14-17%. Results from the present work showed that the enhancement of N-2-NBW addition on methane production from AD of WAS was mainly through the augmentation of hydrolysis of WAS, as little effect on methanogenesis and VS reduction was discerned. The promotion effect of N-2-NBW hydrolysis suggests that N-2-NBW addition is a promising pretreatment strategy for AD of WAS with no chemical addition at low energy consumption, thus, increasing the economic feasibility of WAS disposal. (C) 2019 Elsevier B.V. All rights reserved.