Free nitrous acid-assisted bioresource recovery from anaerobic digestion of organic materials

Considering recent research trends, increasing resource recovery from the anaerobic fermentation of organic materials demands more efficient biodegradability of substrates, as well as suppressing the activities of undesirable microbes while promoting the activities of the desired ones. The utilization of substrate pretreatment technologies has been the predominant control strategies to accelerate the start-up process and enhance the efficiency of anaerobic digestion systems. Notwithstanding, many traditional pretreatment technologies have major drawbacks such as augmenting substrates degradability with little modulatory effects on microbial function in the process. Additionally, their high energy costs and concomitant environmental consequences necessitate the development of non- polluting, less energy intensive and sustainable pretreatment candidates. Along these lines, applications have emerged where free nitrous acid is used to augment the breakdown of organic wastes and inactivate microbes of competing pathways, resulting in enhanced bioresource yields from anaerobic fermentation. Free nitrous acid is produced from nitritation of digester liquors, ensuring its renewable supply in biological systems and making it particularly attractive for advancing circular economy. Hence, free nitrous acid utilization is a promising alternative method to beneficially exploit substrate and microbial activities in the different stages of anaerobic digestion such as hydrolysis, acidogenesis and methanogenesis for the dual benefits of sustainable waste management and enhanced resources yield. Overall, the findings of this review summarize for the first time the potential roles of free nitrous acid as a green pretreatment technology to profoundly enhance the efficiency of anaerobic digestion systems.