Stepwise freezing-thawing treatment promotes short-chain fatty acids production from waste activated sludge

ABSTR A C T Waste activated sludge (WAS), as the byproducts of wastewater treatment plants, has been greatly produced. With high cost and environmental risk of WAS disposal, to explore a low-cost and environment-friendly technol-ogy has been a great challenge. Considering that WAS is a collection of organic matters, anaerobic fermentation has been selected as a sustainable way to simultaneously recover resources and reduce environmental pollution. To recover short-chain fatty acids (SCFAs) has gained great concern because of the high value-added application and high-efficiency production process. Considering the temperature in some areas of the world can reach to below 0 degrees C, this study proposed an efficient strategy, i.e., stepwise freezing and thawing treatment, to promote SCFAs production. The maximal production of SCFAs, i.e., 246 mg COD/g volatile suspended solid, was obtained with the shortened retention time of five days. Mechanistic studies showed that the solubilization of both extra-cellular polymeric substances (EPSs) and microbial cells could be accelerated, with the EPSs removal of 58.3% for proteins and 59.0% for polysaccharides. Also, the hydrolysis process was promoted to provide more substrates for subsequent acidogenisis, and the functional microorganisms, such as Romboutsia, Paraclostridium, Macellibacteroides and Conexibacter, were greatly enriched, with a total abundance of 26.2%. Moreover, compared to control, methanogenesis was inhibited at a shortened sludge retention time (e.g., five days), which benefited to the accumulation of SCFAs, but the methane production was increased by 25.2% at a longer sludge retention time (e.g., ten days). Thus, these findings of this work may provide some new solutions for the enhanced resource recovery from WAS, and further for carbon-neutral operation of wastewater treatment plants.(c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Waste activated sludge (WAS) is a byproduct of wastewater treatment plants, and finding a low-cost and environmentally friendly technology for its disposal is a challenge. This study proposes a stepwise freezing and thawing treatment to increase the production of short-chain fatty acids (SCFAs). Mechanistic studies show that this treatment method can accelerate the solubilization of extracellular polymeric substances (EPSs) and microbial cells, promote the hydrolysis process, and enrich functional microorganisms. In addition, this treatment method can inhibit methanogenesis, benefiting the accumulation of SCFAs.