Iron electrodes activating persulfate enhances acetic acid production from waste activated sludge

Iron activating persulfate has been recently used to pretreat waste activated sludge (WAS) to enhance short chain fatty (SCFAs) production, especially acetate production. However, details of how iron activating persulfate enhances acetate production remain largely unknown and direct use of scrap iron or powder iron causes either iron precipitation or loss, which would limit its practical application. This work therefore aims to provide such supports in an iron electrode activated persulfate (IEAP) system. In the study, before anaerobic fermentation, WAS was pretreated in IEAP reactors which employed sheet iron as anode and contained persulfate. Experimental results showed that when WAS was pretreated by IEAP (100 mA and 50 mg S-2 O-8(2-)/g TSS addition) for 60 min, 249.80 mg COD/g VSS of total SCFAs and 171.41 mg COD/g VSS of acetate (similar to 86.13% of the total SCFAs) were respectively obtained at 3d and 7d, which was 136.4% and 167.2% of those obtained in the control. Further investigation found that in IEAP system, PS, SO4- and S2O82- promote cell lysis and offered propionic ecology, most of propionic acid was transformed to acetate under the action of sulfate-reducing bacteria (SRB), which can be stimulated by current, sheet iron and SO42-. Illumina Miseq sequencing analysis confirmed that the relative abundance of SRB in the IEAP reactor was higher than that in control. Economic analysis indicated that sludge anaerobic fermentation with IEAP pretreatment is a cost-effective approach for WAS to energy sources. The finding obtained in this work provided insights into acetate produce process with IEAP-pretreatment sludge fermentation and might have important implications for further manipulation of WAS treatment and individual SCFAs regulation in the future.