Acidophilic ammonia-oxidizing bacteria mediated Fenton facilitates anaerobically digested sludge dewatering and chemicals usage reduction by initiating diverse free radical reactions

Abstract

Due to the high alkalinity, fine particle size, and the relative abundance of refractory and adhesive biopolymers in anaerobically digested sludge (ADS), the application of traditional Fenton oxidation to improve ADS dewatering faces significant technical and economic challenges, e.g., high doses of chemicals (acid/Fe(II)/H2O2) consumption. Herein, we proposed an acidophilic ammonia-oxidizing bacteria (aAOB) mediated Fenton to address these challenges. Compared with the traditional Fenton, the modified Fenton with aAOB improved the rate and extent of ADS dewatering, leading to a three-fold decrease in Fe(II)/H2O2 consumption while avoiding the need to add acid for pH adjustment. Treated sludge specific resistance to filtration and normalized capillary suction time decreased by 98.9 % and 89.9 %, respectively, and the filtration-compression time was reduced by ∼ 86.5 %. Mechanistically, oxidation of NH4+ inherently present in the sludge by aAOB into NO2–/free nitrous acid (FNA)/NO3– reduced the pH from 7.8 to ∼4.0 within 2 days. This not only created the desired pH for sludge Fenton oxidation, but also facilitated the generation of more diverse and abundant active radicals (OH, O2–, NO, and NO2) in the sludge because of the additional reaction of H2O2 with biogenic FNA in addition peroxide’s reaction with Fe(II). Radical oxidation decomposed CHONS, CHONSP, and CHOP-containing biopolymers in TB-EPS and especially proteins and lipids into relatively hydrophobic small molecules with low unsaturation, and disintegrated bioflocs into disperse structure with lowered surface roughness, increased cell lysis, and looser proteins secondary structure. These changes, together with coagulation by Fe(III), helped release mechanically bound water from the ADS. These results suggested that the synergistic effects between microbial ammonia oxidation and Fenton oxidation have the potential to facilitate cost-effective dewatering of ADS.