A Scoping Review of Options for Increasing Biogas Production from Sewage Sludge: Challenges and Opportunities for Enhancing Energy Self-Sufficiency in Wastewater Treatment Plants

Abstract

Treating municipal wastewater is a complex and costly process. With rising energy costs and sustainability targets, wastewater treatment plants (WWTPs) are looking for alternatives to reduce operating costs and carbon dependence. Anaerobic digestion is the most common and established technology used in WWTPs to treat sludge since it can potentially improve energy recovery and reduce sewage treatment costs, mainly due to the generation of biogas. Biogas is a renewable energy resource and can be used in several applications, including heating and producing electricity. By exploring the biogas potential, WWTPs can reduce their operating costs and energy demands. The objective of this paper is to conduct a scoping literature review in order to provide the key concepts underpinning alternatives to improve biogas production and utilisation in WWTPs. In addition, this study aims to provide an overview of the current state-of-the-art that may serve as a quick reference for the research community, WWTP operators, and engineers, including definitions and a general overview of the current state of biogas technologies around the world. Methods to increase biogas production, including co-digestion, pre-treatment, and biological hydrogen methanation, are reviewed, and the alternatives to using biogas are also summarised. This review has identified that co-digestion was the most efficient technique to improve biogas production and methane yield, while pre-treatment of sludge improved sludge biodegradability and reduced sludge treatment costs but also enhanced biogas production. Although many studies have explored different methods to improve biogas production in WWTPs, there is still a need for further investigation, especially regarding the techno-economic feasibility of these methods in full-scale facilities. The current challenges are mainly related to the need for extra investment and increased operating costs to integrate the new techniques into the current system. There is a great interest in alternatives to improve energy efficiency and self-sufficiency in WWTPs. This work provides an important review of the increasing number of recently published research papers that focus on improving biogas generation from sewage sludge in WWTPs.