Ozonation of Reverse Osmosis Concentrate from Municipal Wastewater Reclamation Processes: Ozone Demand, Molecular Weight Distribution, UV/Fluorescence Characteristics, and Microalgal Growth Potential

To address the challenge of treating reverse osmosis concentrate (ROC) in municipal wastewater reclamation processes, this study systematically investigated changes in ozone demand, organic compound molecular weight distribution, UV/fluorescence characteristics, and microalgal growth potential during ozone treatment of ROC. The ROC contained fast-reacting substances and had an instantaneous ozone demand of 6.3 mg/L. The chemical oxygen demand (COD) and total organic carbon were partially removed, and the COD/five-day biochemical oxygen demand ratio increased slightly during the ozonation process. The molecular weight components shifted considerably during ozonation: the 300 Da–1000 Da components became dominant (51.6–72.3%), while the 1000 Da–4000 Da and <300 Da components were partially or completely removed. The maximum absorbance of the ROC peaked at 270 nm. At an ozone dosage of 84 mg/L, the UV254 and UV270 removal rates reached 76.9% and 86.5%, respectively. The three-dimensional fluorescence spectra showed that ozone effectively removed tryptophan-type aromatic proteins, fulvic acid-type substances, aromatic proteins, soluble microbial metabolites, and humic acid-type substances from the concentrate (84.6–88.9%), but only removed a minimal amount of the tyrosine-type aromatic protein (7.4%). The UV254 at different molecular weights and the fluorescence area integrals across regions declined rapidly initially, then slowed gradually, correlating with the rapid reaction of UV/fluorescence chromophore-containing substances in ROC. Studies on microalgal growth potential indicate that ozonation increased the maximum algal density (K) in ROC (48.9–91.7%), while ozone/coagulation effectively reduced K (35.1–76.6%). This occurs because ozone converts organic phosphonate antiscalants in ROC into more readily absorbable inorganic phosphorus, whereas ozone/coagulation effectively removes total phosphorus from water. These results can guide the safe disposal of ROC and facilitate sustainable reclamation of municipal wastewater.