A review of integrated anaerobic digestion and advanced oxidation processes for distillery wastewater treatment

Sugar processing and molasses-based industries, such as alcohol and wine distilleries, generate copious amounts of distillery wastewater (DWW), which, when discharged into receiving water bodies such as rivers, lakes, and other aquatic systems, leads to severe environmental pollution. Anaerobic digestion (AD) is widely used to treat DWW and reduce its pollution load. However, AD is ineffective in removing all pollutants in DWW, particularly the colour-causing melanoidins. These biorecalcitrant compounds are not readily degraded during AD, and the treated effluent therefore remains intensely dark in colour. Consequently, melanoidins and other micropollutants are discharged into receiving waters, where they pose serious environmental and human health risks, including bioaccumulation in aquatic organisms and adverse effects on endocrine systems. To overcome the limitations of AD, its integration with advanced oxidation processes (AOPs), such as photocatalysis and ozonolysis, either as pre- or post-treatment, has been investigated. However, there remains a gap in identifying the optimal integration sequence and determining which AOPs are best suited for pre- and post-treatment of AD. This review provides an overview of DWW generation and characteristics and evaluates the application of AD for its treatment, including its limitations. It further examines the integration of AD with AOPs as either pre- or post-treatment to overcome these limitations and identifies the optimal integration order for the treatment processes. The current study critically reviews research on AD–AOP integration for DWW treatment, focusing on determining the optimal treatment sequence, standardising performance metrics against COD, colour, biodegradability, methane yield, energy input, and treatment time, and providing a process-selection framework.