Short-chain fatty acid production from waste activated sludge and in situ use in wastewater treatment plants with life cycle assessment

The increasing waste-activated sludge (WAS) and shortage of carbon source (CSs) production for biological nitrogen removal (BNR) have limited sewage treatment development. However, WAS is CS-rich and contains nutrients and extracellular polymeric substances (EPS), which can be carbon banks for BNR but are difficult to attain and have low bioavailability. Thus, inner CS recycling and in situ use in wastewater treatment plants (WWTPs) are crucial. Pretreatment and bioconversion strategies have been adopted to enhance carbon release and production of bioavailable carbon products, especially short-chain fatty acids (SCFAs). Here, this work reviewed (i) advanced pretreatment methods designed to enhance the release of CSs, including proteins, polysaccharides, lipids, and DNA, by cracking EPS, disintegrating sludge flocs, and cracking cell walls; (ii) WAS-based SCFAs bioproduction to improve SCFAs yield and quality by improving the properties of the fermentation substrate and controlling environmental factors; (iii) fermentation process and recovery of CS and in situ use by removing impurities, recovering CS by dehydration, and replenishing the BNR. Furthermore, life cycle assessment (LCA) of this strategy was reviewed and refined. This review guides optimization of the design of inner CS recycling in WWTP and provides development directions.