New insight into enhanced production of short-chain fatty acids from waste activated sludge by cation exchange resin-induced hydrolysis

Recently, the production of short-chain fatty acids (SCFAs) from waste activated sludge (WAS) has received increasing attention. Although several pretreatment methods had been investigated, sludge hydrolysis through depletion of multivalent cations has rarely been reported. In this work, cation exchange resin (CER) was used for enhancing WAS hydrolysis and subsequent SCFAs production at various dosages of 0 to 3.5 g/g SS. With the addition of CER, the multivalent cations and trace metal elements (e.g. Ca²⁺, Mg²⁺, Fe³⁺, Zn²⁺, etc.) were removed from sludge through CER-mediated ion exchange, leading to disruption of extracellular polymeric substance (EPS), while triggering cell lysis which was evidenced by remarkable leakage of cellular DNA. This CER-mediated cyclic release-removal of multivalent cations resulted in substantial sludge hydrolysis with release of biodegradable organic matters. At the optimal CER dosage of 1.75 g/g SS, soluble COD (SCOD) was dramatically increased to 5944 mg/L (SCOD/TCOD = 34.7%) against 1440 mg/L in the control after 2-day fermentation. Meanwhile, a considerable SCFAs of 334.5 mg COD/g VSS was produced within the first 4 days of anaerobic fermentation, which was 3.3 times higher than that in the control. It was found that the acetic and propionic acids accounted for 57.4–65.4% of SCFAs produced in CER-assisted anaerobic fermentation. Compared to pretreatment with potassium ferrate, surfactant and enzymes, a much higher SCFAs concentration was obtained in this study with CER. In fact, used CER could be recovered and reused, without harsh chemicals left in fermented sludge. This indeed created an environmentally and economically beneficial situation with considerably saved pretreatment agents.