Chlorine is the most widely used disinfectant for disinfecting sewage effluents before their discharge into receiving water bodies. Hong Kong, where seawater is used for toilet flushing, has largely decided to use chlorine for disinfecting its 1.7 million cubic meters per day of saline sewage effluents. Upon chlorination, however, the extremely high levels of bromide ions in saline sewage effluents can be oxidized to hypobromous acid/hypobromite which could then react with the organics in the sewage effluents to form large quantities of brominated disinfection byproducts (DBPs). It has been demonstrated that brominated DBPs are significantly more toxic than their chlorinated analogues. The chlorinated saline sewage effluents with brominated DBPs are discharged into receiving water bodies. In Hong Kong, the receiving water body is the coastal marine water, which covers 41 gazetted swimming beaches and many ecologically sensitive habitats. Due to the shortage of fresh water resources, many coastal cities around the world may adopt the practice using seawater for toilet flushing. Therefore, detection, formation and toxicity of brominated DBPs during chlorination of saline sewage effluents have become an imminent issue that needs to be investigated and evaluated.
In this study, a precursor ion scan method with electrospray ionization triple quadrupole mass spectrometry (ESI-tqMS) with/without ultra performance liquid chromatography (UPLC) preseparation was improved. The improved method was then applied to fast selective detection of polar brominated DBPs in disinfected wastewater. Numerous polar brominated DBPs were detected in chlorinated saline sewage effluents; many of them are new brominated DBPs that have not been reported previously; three of them including 5-bromosalicylic acid (5BSA), 2,6-dibromo-4-nitrophenol (DBNPh) and bromomaleic acid (BMA) were identified with authentic standard in chlorinated saline sewage effluents.
The formation of polar brominated DBPs, especially those newly detected ones, were studied for the first time in chlorinated primary and secondary saline sewage effluents. For a secondary saline sewage effluent, the number and levels of brominated DBPs formed during chlorination varied with chlorine dose. The numbers of brominated DBPs formed at chlorine doses of 6 and 10 mg/L as Cl
2 were similar, and more brominated DBPs formed at chlorine dose of 15 mg/L as Cl
2. Over 18 nitrogenous brominated DBPs were detected in the chlorinated secondary saline effluent, and most of them reached their maximum intensities at chlorine dose of 10 mg/L as Cl
2. The application of chlorine dose at breakpoint without significant free chlorine residual generally reduced nitrogenous DBP formation in the chlorinated secondary saline effluent with well nitrification. The formation of total organic halogen (TOX) increased with chlorine dose from 6 to 10 mg/L as Cl
2, but did not increase further when chlorine dose was increased to 15 mg/L as Cl
2; also, a higher total organic bromine (TOBr) to TOX ratio was observed at a higher chlorine dose. Compared to the secondary saline sewage effluent, the primary saline sewage effluent generated less brominated DBPs in number and levels, and rarely generated nitrogenous brominated DBPs.
A primary saline sewage effluent with a high ammonia concentration was chosen for investigating the chlorination mode. As expected, the inactivation efficiency (for contact time 30 min) increased with increasing chlorine dose. To meet the temporary discharge standard (20,000 cfu/100 mL), a minimum chlorine dose of 6.0 mg/L as Cl
2 was required. When chlorine dose exceeded 4.0 mg/L as Cl
2, the formation of TOBr and TOX during chlorination increased dramatically. Accordingly, an appropriate chlorine dose should be 6.0 mg/L as Cl
2. The variation of the inactivation efficiency (for chlorine dose 6.0 mg/L as Cl
2) with contact time included three phases, a slightly descending one, a rapidly descending one, and a slow descending one. In consideration of the normalized TOBr and TOX values by the inactivation efficiency, an appropriate contact time should be 40 min. Moreover, a three-step chlorination (i.e., chlorine dose 6.0 mg/L as Cl
2 was conducted by dosing 2.0 mg/L at 0, 5 and 10 min, respectively) was found to have a higher inactivation efficiency and lower TOBr and TOX values per unit inactivation efficiency than the one-step chlorination with the same chlorine dose. The mechanism for the inactivation could be ascribed to the synergistic effect of free chlorine and monochloramine.
The chlorinated sewage effluents stimulated algal growth at low dilution factors (i.e., low effluent to seawater volumetric ratios), but inhibited algal growth at high dilution factors. Among the three kinds of sewage effluents, the chlorinated secondary freshwater effluent displayed the highest algal inhibition rate, and the chlorinated primary saline sewage effluent exhibited higher algal inhibition rates than the chlorinated secondary saline sewage effluent. The algal inhibition rate of the chlorinated primary saline sewage effluent decreased with the increase of chlorine dose, whereas the algal inhibition rate of the chlorinated secondary saline sewage effluent increased with the increase of chlorine dose. Of the three newly identified brominated DBPs, 5BSA inhibited algal growth in the pure medium, but stimulated algal growth when spiked in the corresponding chlorinated saline sewage effluent; BMA showed little effect on algal growth; DBNPh stimulated algal growth at low concentrations, but inhibited algal growth at high concentrations.
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