Freezing/thawing pretreatment for enhanced energy and resource recovery from waste activated sludge: A critical review

Given the large quantities and considerable treatment costs, the simultaneous recovery of bioenergy and bioresources in the waste activated sludge (WAS) treatment process to reduce the economic burden and change the treatment model from energy input to sustainable treatment has gained increasing attentions. Freezing/thawing pretreatment, a green and cost-effective method to strengthen sludge floc disintegration, has been widely used in sludge management to recover resources and energy. Nevertheless, a comprehensive summary and assessment of applying this strategy is still lacking. Accordingly, to fill this gap, the progress and breakthroughs of the impacts of F/T pretreatment on energy (including methane, hydrogen and electricity) and resources (including short-chain fatty acids (SCFAs) and phosphorus) recovery during anaerobic digestion (AD), anaerobic fermentation (AF) and microbial fuel cells (MFCs) and related influencing factors are overall outlined for the first time. Particularly, the potential mechanisms of F/T pretreatment in improving the recovery of methane, hydrogen, electricity, SCFAs and phosphorus from AD, AF and MFCs are deeply revealed, respectively. Furthermore, this work is the first to systematically summarize the additional benefits of the additional benefits including promoting dewatering performance and pathogens inactivation of digestate and/or fermented sludge after applying F/T pretreatment. Finally, some shortcomings of current research on sludge treatment based on F/T pretreatment and the research directions that need to be focused on to promote its futural application are discussed. Our work aims to offer an reference and support in theory and technology for applying F/T pretreatment in energy and resource recovery from WAS.