Fate of organic fractions of greywater in combined process of vacuum-ultraviolet (VUV/UV)/ozone pre-oxidation with enhanced coagulation

Pre-ozonation is often used as an effective method to assist coagulation in removing organic contaminants. In this study, a new process that combining ozone (O3) with vacuum ultraviolet (VUV/UV) process (VUV/UV/O3) is investigated to enhance the removal of organic matters in greywater from public buildings or household. The influence of organic characteristics (molecular weight distribution, hydrophobicity, and fluorescence) in greywater on VUV/UV/O3 enhanced coagulation has not been fully studied. Herein, organic matters of greywater were separated and classified, and fates of organic fractions by VUV/UV/O3 enhanced coagulation were clarified. Greywater contains tryptophan-like and tyrosine-like proteins, and terrestrially derived humic-like substances. The highest UV254, LAS, DOC and CODCr removal efficiency from greywater could reach up to 47.54%, 58.92%, 17.96% and 15.76% in VUV/UV/O3 enhanced coagulation, respectively. The removal efficiency of tryptophan-like and tyrosine-like in filtrates of varied molecular weight ranges by VUV/UV/O3 enhanced coagulation was 93.59%–98.13% and 89.86%–95.53%, respectively. VUV/UV/O3 enhanced coagulation was effective for the removal of bio-polymers, humic substances and low molecular weight acid ≤ 30 kDa, while it was insignificant for that of 0.22 µm - 100 kDa. Particle size of flocs increased significantly in filtrates obtained from increasing membrane pore size, indicating that macromolecular organic matter was more likely to form flocs. Finally, the VUV/UV/O3 could improve coagulation on the removal of varied organic pollutants and improve sedimentation by changing the flaky flocs to dense ones. Therefore, the results demonstrate VUV/UV/O3 is an effective pre-oxidation method to enhance the effect of coagulation on greywater treatment.