Current advances and challenges for direct interspecies electron transfer in anaerobic digestion of waste activated sludge

Abstract

To realize energy self-sufficiency demand is a key component of carbon–neutral goal of wastewater treatment plants (WWTPs). Waste activated sludge (WAS), as the by-product of WWTPs, is produced in large quantities. Considering that WAS is a collection of organics, anaerobic digestion (AD) is a promising technology for methane recovery from WAS. However, conventional interspecies hydrogen transfer is vulnerable to environment changes, which can lead to instability or failure of AD. The recent finding of direct interspecies electron transfer (DIET) provides an alternative strategy to promote both the methane production potential and rate of WAS through an efficient and stable electron transfer method. This review highlights the current advances in relevant AD-based DIET for methane production from WAS, including the pathways and advantages for establishing DIET in the AD of WAS, and concludes with the current challenges in theory and application. The three main strategies for establishing DIET are conductive e-pili, conductive materials, and external voltage. These strategies not only present the advantages of shortening the lag phase and promoting the potential and rate of methane production but can also enhance intermediate degradation and provide buffering effects to inhibitors. Additionally, the role and function of microbial community structures can achieve directional regulation. Based on recent findings, theory shortages include unlocking syntrophic partners, disclosing cellular structure features, and exploring sophisticated models, while sustainable pretreatment strategies, applicable technical equipments, and comprehensive assessments may potentially limit DIET in engineering applications. This review aims to provide a comprehensive platform for establishing DIET in the AD of WAS and the carbon–neutral goal of WWTPs.