Control of halophenol formation in seawater during chlorination using pre-ozonation treatment

The reverse osmosis process is widely used for seawater desalination, whereas the pre-chlorination step for controlling membrane biofouling results in undesirable disinfection by-products, such as halophenols (HPs) which are not yet regulated but of increasing concerns. The formation and speciation of HPs during chlorination of three filtered seawater samples (SA, SB, and SC) with various phenol concentrations (0.25, 0.5, 1.0 mg/L) were evaluated. 4-Bromophenol (4-BrP), 2,4,6-trichlorophenol (2,4,6-TClP), 2,4-dibromophenol (2,4-DBrP), and 2,4,6-tribromophenol (2,4,6-TBrP) were identified during chlorination, with 2,4,6-TBrP as the predominant HP. Ozone as a common oxidant in water and wastewater treatment was subsequently applied to assess its effect in dissolved organic matter (DOM) and its ability of reducing HP precursors in the seawater samples. An initial ozone dose of 5 mg O₃/L was capable of reducing dissolved organic carbon (DOC) in SA, and UV absorbance at 254 nm (UV₂₅₄) in SB, whereas it induced an elevation of UV₂₅₄ in SC. When ozone dose increased to 10 mg O₃/L, the DOC and UV₂₅₄ levels in all seawater samples were reduced. Ozone was more powerful on degrading DOM with molecular weight (MW) of near 1000 Da than those with MW of 20-100 Da, both of which composed the majority of DOM in the seawater samples. As determined by excitation emission matrix fluorescence spectroscopy, the most ozone-susceptible fraction of DOM was soluble microbial by-product-like substances, while the least was tryptophan-like aromatic proteins. Despite that the initial ozone of 5 mg O₃/L was less effective in DOM degradation than the higher dose, it successfully degraded HP precursors. By pre-ozonation at 5 mg O₃/L, no chlorophenol was detected during chlorination, and the mean reductions of the three bromophnols formed were above 92% in all seawater samples, with the reduction of 2,4,6-TBrP being the highest of 99.7, 99.6, and 99.1% in SA, SB, and SC, respectively.