Review on enhanced hydrolysis acidification strategies for pre-treatment of refractory wastewater or VFAs production: Mechanisms and application

Abstract

Hydrolysis acidification (HA) as the economic pre-treatment technology for refractory wastewater was widely used. The final products of HA were dominated by volatile fatty acids (VFAs) which could be regarded as the valuable raw chemical materials to produce fuels. Thus, HA could be considered as a potential technology to achieve wastewater treatment and utilization of wastewater resource simultaneously. However, the HA process was limited for application due to the defects, such as low hydrolysis efficiency. Therefore, enhancing HA strategies have been developed by means of built-in electrochemical system (BES), adding exogenous additives and micro-aeration. These enhanced strategies have attracted interests due to their remarkable and positive effect on biodegradability, toxicity and VFAs concentration (especially for acetate) of wastewater. Therefore, potential mechanisms of the enhancement strategies were analyzed and summarized from aspects of effluent characteristics, enzyme activity and microbial community. This review also revealed the factors involved in the implementation of strengthening strategies by summarising process control parameters. Furthermore, the comparison and discussion toward systems are made from the aspects of enhancing mechanisms, the application of technologies on different types of wastewater pre-treatment, the VFAs application potential for fuel production and techno-economic assessment. Lastly, challenges and prospects of the enhanced HA technologies in the pre-treatment of refractory wastewater were summarized to provide a theoretical guidance for potential large-scale use. The scope of further research includes the removal mechanism of refractory organics, establishment of mathematical models to describe and predict the performance of HA systems, optimized VFAs ratio and BES system, development of online in situ monitoring system, development of recycling or fixation technologies for additive materials and antifouling membrane materials.