Partial ozonation enhanced the content and aromatization of humic substances in activated sludge through chemical and microbial effects

Abstract

Influences of partial ozonation on characteristics of humic substances (HS) in activated sludge (AS) was systematically investigated. It was firstly found that partial ozonation not only reduced sludge production, but also increased content and aromatization of HS in AS. The total extractable carbon of HS (TEC) in AS subjected to partial ozonation (i.e., an increasing O3 dosage, from 25 to 125 mg O3/g VS) considerably increased, from 138.7 ± 2.4 mg/L (SBR-25) to 151.6 ± 2.7 mg/L (SBR-125), which were 1.01–1.10 times the TEC of the control (SBR-0). In which the organic carbon of fulvic acids (FAC) and humic acids (HAC) increased from 30.9 ± 4.8 and 106.3 ± 4.6 mg/L (SBR-0) to 34.7 ± 1.3 and 116.9 ± 1.4 mg/L (SBR-125), respectively. SUVA254 values of both FA and HA in SBR-25∼SBR-125 were greater than their values in SBR-0. Mechanisms were clarified from chemical and microbial perspectives. During O3 treatment, organic nitrogenous substances and aliphatic compounds in HA were more mineralized and fused by O3; FA were more formed from soluble proteins and polysaccharides than mineralized. Consequently, HA content in AS was reduced with enhanced aromaticity, while FA content was increased with maintained aromaticity after O3 treatment. Then, after the ozonated AS was returned to SBR systems, the enhanced microorganism-led formation of HA in SBR from more accessible FA played a dominating role in the final increased TEC. Enhanced relative abundance (RA) of genera Lactococcus and Thauera after partial ozonation was possibly responsible for the increased RA of K00615 (tktA) and K00873 (pyk), promoting production of intermediates including phosphoenolpyruvate and erythrose-4-phosphate (initial metabolites for shikimate pathway) from glucose, enhancing HA formation related with shikimate pathway, and finally increasing the content and aromatization of HS. The results could guide researchers on improving sludge management by decreasing sludge production yield and enhancing sludge utilization potential through partial ozonation during wastewater treatment.