New Insight into Potassium Ferrate Enhancing the Production of Volatile Fatty Acids by Co-Fermentation of Excess Sludge and Plant Waste

Excess sludge (ES) and plant waste (PW) are two typical organic solid wastes in urban areas, and their co-fermentation is one of the key strategies in the circular economy. This study innovatively investigated the impact of the oxidant potassium ferrate (PF) on the anaerobic co-fermentation of ES and PW for the production of volatile fatty acids (VFAs) and elucidated the underlying mechanisms. The results indicated that PF could enhance the co-fermentation of ES and PW to produce VFAs, with an optimal dosage of 0.06 g/g (based on total suspended solids), yielding a maximum VFAs production of 261 mg chemical oxygen demand (COD)/g volatile suspended solids (VSS), which is 2.1 times that of the control group. High doses of PF inhibited microbial metabolism, reducing VFAs production, but still higher than the control group. PF effectively promoted the solubilization of organic matter in the ES and PW co-digestion system, increasing the concentrations of soluble COD, soluble protein, and soluble polysaccharides, with higher PF concentrations leading to more significant solubilization of available organic matter. PF increased the content of loosely bound extracellular polymeric substances (EPS) but decreased the content of tightly bound EPS. The oxidizing nature of PF suppressed the production of biogas, with only 99.5 mL/g VSS produced in the 0.08 g/g PF group. PF stimulated the concentration of ammonium nitrogen in the fermentation liquid of the co-digestion system but decreased the concentration of soluble phosphate.