A green co‐treatment to accelerate the hydrolysis of sludge during anaerobic fermentation using free nitrous acid and sodium citrate

Abstract Background Anaerobic fermentation of waste activated sludge to produce short‐chain fatty acids (SCFAs) is one of the means of sludge resource utilization. However, the protective effect of the extracellular polymeric substances (EPS) structure contained in sludge flocs can lead to a slow hydrolysis process, which inhibits the production of SCFAs. Results In this study, a new step‐by‐step pretreatment method for breaking sludge flocs was adopted. The cationic complexing agent sodium citrate (C 6 H 5 Na 3 O 7 ) was used first to break the cationic bridging structure of EPS, and then the strong oxidizing property of free nitrous acid (HNO 2 ) was used to break the cell structure. The results showed that the C 6 H 5 Na 3 O 7 dose of 0.10 g g −1 total suspended solids (TSS) was able to disintegrate EPS, and the optimal dose and pretreatment time of HNO 2 under these conditions were 2.504 mg L −1 and 24 h, respectively, when the highest soluble chemical oxygen demand (SCOD) content of 1700 ± 25 mg L −1 was reached. SCFA production in the group of 0.10 g g −1 TSS with 2.504 mg L −1 HNO 2 co‐pretreatment reached 1827 ± 40 mg COD L −1 at 8 days of fermentation. Conclusion The presence of C 6 H 5 Na 3 O 7 accelerated the degradation rate of HNO 2 during fermentation, and the pH fluctuation of the system was lower compared to the HNO 2 pretreatment alone. Moreover, the synergistic pretreatment significantly changed the bacterial community structure within the system and Petrimonas (47.62%), belonging to the hydrogen and acetic acid‐producing bacteria, was enriched, which promoted organic matter degradation and SCFA production. These findings will contribute to optimization of the sludge treatment process. © 2023 Society of Chemical Industry (SCI).