Effect of Fe-based catalytic ozonation and sole ozonation on the characteristics and conversion of organic fractions in bio-treated industrial wastewater

Highlights: • Catalytic ozonation had higher removal of hydrophilic fraction than sole ozonation. • Hydrophobic fraction was first converted to transphilic fraction then hydrophilic. • Catalytic ozonation degraded fulvic acid, SMP and humic acid like substance better. • More large and medium molecular organics remained after sole ozonation. The conversion of hydrophobic (HPO)/transphilic (TPI)/hydrophilic (HPI) fractions of dissolved effluent organic matter (EfOM) in water by sole ozonation process (SOP) and catalytic ozonation process (COP) were investigated in this study. The modified iron shavings were used as catalyst in COP. The difference in total organic carbon (TOC) removal rate between SOP and COP for two types of bio-treated industrial wastewater was mainly caused by the different HPI degradation degree. The accumulation of HPI in SOP was 28.8% (in bio-treated dyeing wastewater) and 88.8% (in bio-treated paper-making wastewater) higher than that in COP, while HPO was degraded by more than 90% in both processes. Part of HPO was converted to TPI in the early stage (0–15 min) and the original HPI in the raw water was degraded at the same time. In the middle stage (15–60 min), TPI was transformed into HPI. The remaining fluorescent substances in HPI fraction in SOP were higher than those in COP, indicating that more HPI was accumulated in SOP. Among the fluorescent substances, aromatic protein II and SMP-like substances were removed more completely, while humic acid-like substances were relatively more difficult to degrade. Furthermore, COP had higher removal for fulvic acid-like substances, SMP-like substances and humic acid-like substances than SOP, while the removals of aromatic protein by COP and SOP were almost the same. COP had better removal effect than SOP for TPI and HPI organic pollutants with molecular weight (MW) more than 2 kDa. SOP and COP had no significant difference in the removal of HPO fraction and small molecular organics with MW less than 2 kDa. The remaining organic pollutants in the effluent of COP was mainly hydrophilic small molecular substances, which would be benefit for secondary biochemical process.