ReviewDisinfectant residual stability leading to disinfectant decay and by-product formation in drinking water distribution systems: A systematic review
Graphical abstract
Introduction
Drinking water utilities invest considerable resources in producing safe drinking water. Most of this effort and much of the accompanying water quality monitoring occur at drinking water treatment plants. However, after exiting a water treatment plant and before delivery to a customer's tap, there are numerous opportunities for water quality changes. These include potential reactions with disinfection residuals and concurrent formation of disinfection by-products (DBPs), presenting additional public health risks.
Physical components of drinking water distribution systems (DWDS) include treated water storage reservoirs, pipes and pumping stations. Each of these components may vary considerably in terms of water distribution capacity, manufactured materials and age or condition. The characteristics of a distribution system and the conditions under which it is operated may have important impacts on the potential for post-treatment changes in water quality. Distribution distances can be relatively short or can be in excess of 100 km with a retention time commonly varying between 1 and 7 days (AWWA, 2002; Blokker et al., 2016). Highly variable population densities and flow rates lead to corresponding variabilities in the relationship between distribution distance and time. For example, delivering water 50 km requires a retention time of 12–24 h in Beijing while in Australian cities such as Sydney and Brisbane (can be up to 20 days), 3–5 days is not unusual. Prolonged water age is also influenced by the need for water storage tanks to ensure adequate availability for firefighting and other emergencies (Australian Government, 2014; AWWA, 2002).
A free chlorine residual has been used as a disinfectant in DWDS in many countries for much of the 20th Century due to its high performance in disinfection and relatively low cost (McGuire, 2006). However, some limitations with using such a reactive chemical species as a distribution residual have been observed to include rapid decay (in some systems) and associated production of DBPs, such as trihalomethanes (THMs), haloacetic acids (HAAs) and other halogenated DBPs (Gagnon et al., 2006). Consequently, many drinking water utilities have switched to alternative disinfectant residuals, including chloramine and, to a much lesser extent, chlorine dioxide (Gagnon et al., 2006; NHMRC and NRMMC, 2011).
While chloramine is well-known to produce reduced chlorinated DBPs (Cl-DBPs), compared to free chlorine, the introduction of ammonia and chlorinated amines amplifies the risk of producing nitrogenous DBPs (N-DBPs) under certain conditions. This is of concern since some N-DBPs may present greater risks to public health, including carcinogenicity, compared to non-nitrogenous DBPs (Richardson et al., 2007). Disinfectant residual decay and DBP production may be a result of biofilms which are known to occur in treated water distribution systems contributing bacterial sources and organic matter (Lemus Pérez and Rodríguez Susa, 2017; Wang et al., 2013a). Although chloramine is more persistent than chlorine in the absence of nitrifying conditions, it decomposes more rapidly once nitrification occurs in distribution systems (Sathasivan et al., 2008). Bacteria present in distribution system biofilms release soluble microbial products (SMP), especially proteins which accelerate chloramine decay and DBP formation (Herath et al. 2015, 2018; Liu et al., 2014). Re-chlor(am)ination is commonly practised to maintain disinfectant levels in distribution systems (Bal Krishna et al. 2014, 2016) but may also lead to greater N-DBP production (Zeng and Mitch, 2016). Due to security concerns, many of these re-chlor(am)ination facilities have now switched from chlorine gas to safer disinfectants, such as sodium hypochlorite (Copeland and Cody, 2007).
The relationships between distribution system conditions and microbial activity within networks have been investigated by many researchers (Lehtola et al., 2006; Liu et al., 2017c; Ndiongue et al., 2005; Simoes et al., 2006; Tsvetanova and Dimitrov, 2012). Similarly, the relationships between distribution system conditions and chemical changes, such as disinfectant residual decay and DBP formation, have been widely reported (Kim et al., 2014; Lee et al., 2007; Liu et al., 2012; Simard et al., 2011; Zhang et al., 2017a). However, microbial and chemical changes are intricately related as has been shown by research focused on the impact of nitrification processes on disinfection residuals (Krishna et al., 2013; Lemus Pérez and Rodríguez Susa, 2017; Sathasivan et al., 2008; Wang et al., 2012; Zeng and Mitch, 2016). In addition to the interaction between microbial growth and disinfectant stability, microbial and chemical changes in disinfectant stability have similar influencing factors, such as water qualities and operating conditions. Therefore, varying these influencing factors would affect microbial growth and chemical reactions of disinfectants, while the change in microbial activities would further impact disinfectant stability. Few studies have sought to comprehensively examine both types of activities simultaneously, in order to provide a more complete understanding of the inter-relationships. The mathematical model developed by Abokifa et al. (2016) showed the importance of integrating bacterial growth dynamics models with predictive DBP formation models. Therefore, a systematic review is presented here to assess the current state of knowledge regarding disinfectant residual stability, as influenced by microbial activity as well as other independent factors in DWDS. The objective was to draw reasonable conclusions across studies that have focused on either one or the other sub-research topic. The review concludes with the identification of research gaps, which must be filled in order to provide a more complete understanding of the overall changes in water quality that may occur between drinking water treatment and delivery of that water to customer's taps.
Section snippets
Methods
A systematic quantitative literature review was performed based on the method described by Pickering and Byrne (2014) and the protocol developed by the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) (Fig. 1) (Moher et al., 2009). Research papers were searched on Web of Science (Clarivate Analytics, 2017) using an advanced topic search using keywords “drinking water OR potable water” AND “distribution system* OR reticulation system*” AND “chemical OR microbial OR
Results
The keyword search initially returned a list of 1809 articles. After manually screening, 234 articles were identified as potentially fulfilling criteria mentioned in Section 2, and 161 papers were then selected in this review after assessing full text.
Disinfectant residual stability including disinfectant decay and DBP production in DWDS are related to the chemical, microbiological and physical characteristics of water, and to the pipe materials in contact with water flow (Simoes et al., 2006;
Factors that influence disinfectant residual stability
There are many factors including water qualities and operating parameters that may directly or indirectly influence disinfectant residual stability. Water qualities refer to temperature, pH, nutrients, natural organic matter (NOM), dissolved oxygen (DO) and the presence of other potentially interacting species, such as bromide, iodide and corrosion products. There are five main operating parameters that have been considered among the literature: water age, hydraulic condition, pipe materials,
Conclusion
The influencing parameters of disinfectant stability have been qualitatively investigated in this review. These factors are not completely independent and may lead to positive feedback loops, such that microbial growth leads to a rapid decay of disinfectant residual further promoting microbial growth.
The types and dosages of disinfectants would be the most important factors affecting disinfectant stability. Disinfectant stability can be reduced by higher temperature and more microbial
Acknowledgements
This work was funded by Australian Research Council [ARC LP1601009099]. The authors are grateful to the support from Sydney Water Corporation, Seqwater, Queensland Urban Utilities (QUU), Unity Water and Logan City Council. RL is supported by Australian Government Research Training Program Scholarship (RTP) and UNSW Faculty of Engineering Top-up Scholarship. RL is thankful to Mr Joel Tan (UNSW) for proofreading this article.
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