Seawater reverse osmosis (RO) desalination has been extensively applied to replenish limited freshwater resources worldwide. However, disinfection byproducts (DBPs), which potentially pose health, aesthetic and ecological risks, may be formed during the disinfection step prior to RO. This study investigated DBP formation, fate, toxicity and changes in seawater organic matter (SOM) during seawater RO desalination pretreated with different disinfection strategies (i.e., chlorination, preformed chloramination, insitu chloramination, and in-line chloramination) under various operating conditions (i.e., pH, contact time, oxidant dosage, and temperature).

Trihalomethanes (THMs), haloacetic acids (HAAs) and haloacetonitriles (HANs) were the main DBP species formed in seawater chlor(am)...[ Read more ]

Seawater reverse osmosis (RO) desalination has been extensively applied to replenish limited freshwater resources worldwide. However, disinfection byproducts (DBPs), which potentially pose health, aesthetic and ecological risks, may be formed during the disinfection step prior to RO. This study investigated DBP formation, fate, toxicity and changes in seawater organic matter (SOM) during seawater RO desalination pretreated with different disinfection strategies (i.e., chlorination, preformed chloramination, insitu chloramination, and in-line chloramination) under various operating conditions (i.e., pH, contact time, oxidant dosage, and temperature).

Trihalomethanes (THMs), haloacetic acids (HAAs) and haloacetonitriles (HANs) were the main DBP species formed in seawater chlor(am)ination, and haloacetaldehydes (HALs) were only detected in seawater chlorination. The concentrations of DBPs formed under the baseline condition (pH = 8, contact time = 2 h, oxidant dosage = 2 mg/L as Cl₂, and temperature = 23°C) followed the order of chlorination > in-line chloramination > in-situ chloramination > preformed chloramination. In general, the concentrations of DBPs decreased with increasing pHs, but their concentrations increased with longer contact times, higher oxidant dosages and higher temperatures. The yield of unknown DBPs measured as high molecular weight (MW) adsorbable organic halogen (AOX) in the preformed chloramination was higher than those in the other strategies. In the preformed chloramination, relatively lower bromine substitution factors (BSFs) and higher iodine substitution factors were observed, and they were affected by pH, contact time and temperature. In contrast, BSFs of the detected DBPs were always > 90% under various conditions in the other three strategies. The changes in UV absorbance at 254 nm and fluorescence excitation-emission matrix volume were strongly correlated with the concentrations of high MW AOX and total detected organic halogen in the seawater chlorination, indicating that aromatic and humic fractions and fluorophores in SOM were the primary precursors of these DBPs. The integrated cytoand geno-toxicity in chlorination were always higher than that in chloramination and preformed chloramination always gave the lowest, which was much lower than others. Brominated HANs and HAAs were the main contributors of the integrated DBPinduced cyto- and geno-toxicity in the four chlor(am)ination strategies, while HALs also contributed significantly to the integrated cyto-toxicity in the seawater chlorination.

In the RO permeate pretreated by preformed chloramination, lower concentrations of known DBPs but higher concentrations of high MW AOX were observed compared to the other three strategies. The unknown chlorine- and bromine-containing DBPs in the RO permeates were characterized by Fourier transform ion cyclotron resonance mass spectrometry. Highly saturated aliphatic DBPs (e.g., C₁₈H₃₅O₅Cl), which potentially originated from fatty acids and their derivatives, were the most abundant DBPs in all the strategies except preformed chloramination. The relative intensities of unknown bromine-containing DBPs were significantly lower in the RO permeate pretreated by chlorination compared to the other strategies. In the RO brine, the concentrations of DBPs measured in the preformed chloramination were always significantly lower than those detected in the other three pretreatment strategies. The trend of the experimentally determined equivalent cytotoxicity correlated well with the concentrations of high MW AOX in the three chloramination strategies, suggesting the importance of unknown DBPs in the toxicity evaluation.