Three-step effluent chlorination increases disinfection efficiency and reduces DBP formation and toxicity
Graphical abstract
Introduction
Sewage disinfection aims at providing protection for humans from exposure to pathogenic waterborne microorganisms (Jacangelo and Trussell, 2002). Chlorine is extensively used as a disinfectant in municipal sewage treatment plants due to its effectiveness, low cost and ease of application (Lee and von Gunten, 2010, Drinan and Spellman, 2012, Shannon et al., 2008). In addition to inactivating pathogenic microorganisms, chlorine reacts with effluent organic matter present in sewage effluents to generate unintended disinfection byproducts (DBPs) (Rebhun et al., 1997, Shon et al., 2006, Krasner et al., 2009, Sedlak and von Gunten, 2011, Tang et al., 2012, Hatt et al., 2013, Huang et al., 2016, Cai et al., 2016).
To relieve the stress from fresh water shortage, seawater has been applied for toilet flushing in many coastal communities such as Hong Kong, the Marshall Islands, Kiribati, and Avalon (Mirti and Davies, 2005, Tang et al., 2007). However, such a practice introduces a high level of bromide ions (65 mg/L in seawater) into the corresponding sewage disposal system, which may cause the production of brominated DBPs during disinfection of the saline sewage effluent using chlorine (Ding et al., 2013, Yang and Zhang, 2013, Yang and Zhang, 2014, Sun et al., 2009). Evidence has shown that brominated DBPs generally presented significantly higher cytotoxicity, genotoxicity, developmental toxicity and growth inhibition than their chlorinated analogs (Plewa et al., 2002, Richardson et al., 2007, Yang and Zhang, 2013, Liu and Zhang, 2014, Li et al., 2016a). Total organic halogen (TOX), a collective parameter, is a good surrogate and a toxicity indicator for the overall halogenated DBPs in a disinfected water sample (Simmons et al., 2002, Kristiana et al., 2009, Gong et al., 2016, Zhu and Zhang, 2016). TOX includes total organic chlorine (TOCl), total organic bromine (TOBr), and total organic iodine (TOI).
For a specific disinfection unit in a sewage treatment plant, increasing the chlorine dose may increase the disinfection efficiency, but more chlorine will be consumed and thus the operational cost will be increased; also, more halogenated DBPs will be formed in the chlorinated saline sewage effluent (Ding et al., 2013). Thus, it is of great importance to develop and optimize the sewage disinfection process in order to enhance its cost-effectiveness and reduce any adverse impacts of the treated effluent on the receiving environment.
Ding (2010) reported that, with the same total chlorine dose, a three-step addition of chlorine (three-step chlorination) showed a greater disinfection efficiency than a one-step addition of chlorine (one-step chlorination), corresponding to an enhancement of 0.20-log. The result indicated that three-step chlorination had the potential to reduce the chlorine dose for the same level of Escherichia coli removal. However, this study of step-wise chlorination was still at a preliminary stage, as only disinfection efficiency was considered. It was not clear whether the reported better performance of three-step chlorination was reproducible. Additionally, compared to one-step chlorination, the comparative effect of three-step chlorination on effluent toxicity was not considered. Therefore, this study aimed to further investigate the comparative performance of three-step chlorination and the commonly used one-step chlorination, of a primary saline sewage effluent, in terms of disinfection efficiency, DBP formation and developmental toxicity.
Section snippets
Chemical reagents and seawater
All the chemical solutions were prepared from chemicals of reagent grade or higher. Ultrapure water (with a resistivity of 18.2 MΩ·cm) was provided by a Cascada I Laboratory Water Purification System (PALL). A chlorine stock solution was prepared by diluting a reagent grade NaOCl solution (Allied Signal), and titrated based on the DPD/FAS method (APHA et al., 2012). Methyl tert-butyl ether (MtBE) was supplied from Sigma–Aldrich. Seawater was collected from a local bay in Hong Kong with a
Disinfection efficiencies of three-step chlorination and one-step chlorination
Fig. 1a shows the E. coli concentrations in the primary saline sewage effluent samples without chlorination, with one-step and three-step chlorination with a total chlorine dose of 4.0 mg/L as Cl2 and a total contact time of 30 min. It includes four groups of bars for the sewage effluent samples collected on different days over a 19-month period. As shown in Fig. 1a, the E. coli concentration in the primary sewage effluent without chlorination fluctuated in the range from 0.8 × 107 to 2.4 × 107
Conclusions
The disinfection effects of three-step chlorination and commonly applied one-step chlorination of a primary sewage effluent were compared, in terms of disinfection efficiency, DBP formation and developmental toxicity. The results demonstrated that three-step chlorination was generally significantly more efficient in disinfecting the effluent than one-step chlorination. Compared to one-step chlorination, three-step chlorination enhanced the disinfection efficiency for E. coli by up to 0.73-log
Acknowledgements
This work was funded by the Environment & Conservation Fund and Woo Wheelock Green Fund (No. ECWW15EG07) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Projects No. 623409, No. 622412 and No. C7044-14G).
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