Effects of ferric chloride on waste activated sludge and slaughterhouse waste anaerobic co-digestion

Anaerobic co-digestion (AcD) of waste activated sludge (WAS) and slaughterhouse waste (SW) could achieve desirable methane production performance due to the abundant proteins and lipids in the co-substrates. Ferric chloride (FeCl3), widely used for wastewater treatment and sludge dewatering in wastewater treatment plants, accumulates in the WAS and may affect AcD performance. This study aimed to investigate the effects of FeCl3 on AcD of WAS and SW using a biochemical methane potential test. Experimental results indicated that 4.19 mg Fe3+/g volatile solids (VS) showed negligible effects on methane production, while more than 10.07 mg Fe3+/g VS could significantly inhibit methane production (p < 0.05). Mechanistic analysis revealed that FeCl3 could decrease the abundance of key functional microbes, such as syntrophic Petrimonas and DTU014, the hydrolytic bacteria Methylotenera, and the hydrogenotrophic Methanobacterium. Meanwhile, FeCl3 inhibited lipid metabolism, membrane transport, lysine degradation, pyruvate metabolism, and coenzyme F420 biosynthesis. Additionally, FeCl3 decreased the abundance of key archaeal methyl-CoM reductase. As a result, substrate conversion and methane production were inhibited. This study revealed the complex substrate transformation, microbial community succession, and metabolic pathways during AcD under FeCl3 conditions, and provided a theoretical framework for investigating underlying mechanisms of the effects of FeCl3 on AcD.