Disposal of Fenton sludge with anaerobic digestion and the roles of humic acids involved in Fenton sludge

Fenton sludge (FS) generated from Fenton process with high contents of iron and organic contaminants is regarded as a hazardous waste that requires to be properly disposed. Considering that Fe(III) compounds could stimulate dissimilatory iron reduction (DIR) and enrich iron reducing bacteria (IRB) that utilized Fe(III) as electron acceptor to oxidize organic matters, FS was introduced in anaerobic digestion (AD) reactors for treating wastewater meanwhile disposing FS. Results showed that methane production and organic matters removal significantly increased with dosing FS from 0 to 2.4 g. Also, a majority of organic matters involved in FS was mineralized, including 38.5% of PAHs removal. Humic acids (HA) with redox-activity involved in FS might affect efficiency of DIR. After extracting HA from FS, the rate and the extent of Fe(III) reduction of FS decreased by 33.2% and 13.9%, respectively. Together with analysis of the electron exchange capacity of HA, it suggested that the HA involved in FS might serve as an electron shuttle to effectively promote DIR. The increase of sludge conductivity and the enrichment of IRBs in microbial communities with dosage of FS were in agreement with the above results.