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Published in final edited form as: Sci Total Environ. 2021 Jan 23;770:145356. doi: 10.1016/j.scitotenv.2021.145356

Optimizing Disinfectant Residual Dosage in Engineered Water Systems to Minimize the Overall Health Risks of Opportunistic Pathogens and Disinfection By-Products

Chiqian Zhang 1, Jingrang Lu 2,*
PMCID: PMC8428770  NIHMSID: NIHMS1677052  PMID: 33736415

Abstract

This Discussion argues that municipal water utilities may need to consider the health risks of both opportunistic pathogens (OPs) and disinfection by-products (DBPs) while selecting disinfectant residual dosages or levels in engineered water systems. OPs are natural inhabitants in municipal water systems and the leading cause of drinking-water-related disease outbreaks threatening public health. DBPs in water systems are genotoxic/carcinogenic and also significantly affect public health. Disinfectant residuals (such as free chlorine and chloramine residuals) dictate OP (re)growth and DBP formation in engineered water systems. Therefore, regulating the dosages or levels of disinfectant residuals is effective in controlling OP (re)growth and DBP formation. Existing effects assessing optimal disinfectant residual dosages focus solely on minimizing OP (re)growth or solely on DBP formation. However, selecting disinfectant residual dosages aiming to solely limit the formation of DBPs might compromise OP (re)growth control, and vice versa. An optimal disinfectant residual level for DBP formation control or OP (re)growth control might not be optimal for minimizing the overall or combined health effects of OPs and DBPs in drinking water. To better protect public health, water authorities may need to update the current residual disinfection practice and maintain disinfectant residuals in engineered water systems at an optimal level to minimize the overall health risks of OPs and DBPs.

Keywords: Drinking water, Distribution systems, Premise plumbing, Precursors, Regrowth, Public health

Graphical abstract

graphic file with name nihms-1677052-f0001.jpg

1. Opportunistic pathogens (OPs) and disinfection by-products (DBPs) in drinking water both significantly threaten public health

Microbial drinking water quality in municipal engineered water systems (EWSs), including drinking water distribution and premise plumbing systems, strongly affects public health. A drinking water distribution system is a large, complex network of water pipes that starts from the outlet of a municipal water utility and ends at the water entrance (or the property line) of an individual office building, house, apartment complex, or business. A premise plumbing system refers to water networks within an individual office building, house, apartment complex, hospital, or business (Falkinham, 2015; National Research Council of the National Academies, 2006; Wang et al., 2017). A drinking water distribution system transports only cold water, whereas a premise plumbing system typically includes both hot water and cold water lines and devices.

The most important aspect of microbial drinking water quality is the prevalence of OPs. OPs in drinking water significantly threaten the health of the end consumers, especially children, senior citizens, and immunocompromised populations. Dominant water-related OPs are Legionella (especially L. pneumophila), Mycobacterium (e.g., nontuberculosis mycobacteria or NTM and M. avium complex or MAC), Pseudomonas aeruginosa, Vermamoeba vermiformis, Naegleria fowleri, and Acanthamoeba (Donohue et al., 2019; Falkinham et al., 2015; Falkinham, 2015; Hamilton et al., 2016; Isaac and Sherchan, 2020; Kao et al., 2015; Lu et al., 2017; Lytle et al., 2021; Makris et al., 2014; Martinez and Janitschke, 1985; Visvesvara et al., 2007; Wang et al., 2012; Zhang et al., 2019; Zhou et al., 2018). OPs are normal or natural inhabitants in municipal EWSs rather than contaminants such as enteric pathogens, which indicate treatment failure and rarely appear in well-maintained drinking water systems. OPs survive, persist, and proliferate in drinking water pipes because of their high disinfectant resistance, proliferation in amoebae (certain OPs are amoebae themselves), and biofilm formation or association. Water-related OPs frequently cause disease outbreaks, pose significant public health risks, and cause serious socioeconomic losses (Ashbolt, 2015; El-Chakhtoura et al., 2018; Falkinham et al., 2015; Liu et al., 2017; Makris et al., 2014; Wang et al., 2017; Whiley et al., 2014b).

Among the dominant OPs in municipal EWSs, Legionella (especially L. pneumophila) is the leading cause of drinking-water-related disease (e.g., Legionnaires’ disease or legionellosis) outbreaks. From 1971 to 2006, 780 drinking-water-related disease outbreaks occurred in the United States (US) and caused 577,094 illnesses, where Legionella was an important etiologic agent (Craun et al., 2010). From 2007 to 2008, 36 drinking-water-related disease outbreaks occurred in 24 states in the US and Puerto Rico, and 33% (12) of the outbreaks were acute respiratory illnesses due to Legionella infections (Brunkard et al., 2011). From 2009 to 2010, 33 drinking-water-related disease outbreaks occurred in 17 states in the US, causing 1,040 illnesses, 85 hospitalizations, and 9 deaths (Hilborn et al., 2013). Legionella caused 19 (58%) outbreaks, 72 (7%) illnesses, 58 (68%) hospitalizations, and 8 (89%) deaths. From 2011 to 2014, more than one dozen drinking-water-related disease outbreaks occurred annually in the US, resulting in hundreds of illnesses, more than 50 hospitalizations, and six to seven deaths per year (Beer et al., 2015; Benedict et al., 2017). Legionella caused more than half of those outbreaks, more than 10% of those illnesses, approximately 90% of those hospitalizations, and all those deaths. In addition, the crude national incidence rate of Legionnaires’ disease per million persons in the US increased dramatically from 4.2 in 2000 to 16.2 in 2014 and to 18.9 in 2015 (Shah et al., 2018). On the basis of the annual hospitalizations (8,000 to 18,000 with a midpoint of 13,000 in the US) required by community-acquired Legionnaires’ disease (Marston et al., 1997), the annual hospitalization cost for water-related Legionnaires’ disease in the US is much higher than 434 million US dollars (Collier et al., 2012). Another important water-related OP that frequently appears in municipal EWSs and poses significant health risks is Mycobacterium (Donohue et al., 2019; Lu et al., 2017; Lu et al., 2016; Marciano-Cabral et al., 2010; Vaerewijck et al., 2005; Wallace et al., 1998). For instance, water-related NTM infections in the US cause more than 16,000 hospitalizations and more than 426 million US dollar economic loss per year (Collier et al., 2012).

Maintaining a disinfectant residual, commonly monochloramine or free chlorine, at a certain level (e.g., 0.5 mg Cl2·L−1) or higher throughout municipal EWSs is the customary practice called “secondary disinfection” in the US and many other countries to suppress the (re)growth of OPs and other microbes (Al-Zahrani, 2016; LeChevallier, 1999; Seidel et al., 2005; Zhang and Liu, 2019). The tradeoff of maintaining a disinfectant residual in EWSs is that the residual reacts with inorganic and organic precursors and form harmful DBPs (Makris et al., 2014; Rosario-Ortiz et al., 2016; Sadiq and Rodriguez, 2004; Wawryk et al., 2020). Trihalomethanes (THMs), haloacetic acids (HAAs), and nitrogenous DBPs are typical chlorinated/chloraminated DBPs in drinking water (Li et al., 2019; Mian et al., 2018). Researchers have detected over 600 DBP species, while more DBPs (i.e., the “missing DBPs”) need to be discovered (Hrudey, 2009; Li and Mitch, 2018; Richardson, 2002; Richardson et al., 2007; Richardson and Postigo, 2012; Richardson et al., 2002). DBPs are genotoxic and even carcinogenic, negatively affect human reproductive/developmental systems (e.g., potentially causing birth defects), and significantly threaten public health (Grellier et al., 2015; Hrudey, 2009; Laleh et al., 2020; Liu et al., 2020; Mazhar et al., 2020; Richardson and Plewa, 2020; Richardson et al., 2007; Richardson et al., 2002; Tovar and Susa, 2021). For instance, 4.9% of the total annual bladder cancer cases in the European Union for people of or over 20 years old could be caused by drinking water THM exposure (Evlampidou et al., 2020).

2. The (re)growth of OPs and formation of DBPs in EWSs have distinct or even opposite responses to disinfectant residual dosages

Disinfectant residual dosage or level is a key parameter governing the densities and (re)growth of OPs in municipal EWSs. The abundance of OPs in municipal EWSs commonly has a negative correlation with disinfectant residual dosage or level, suggesting that disinfectant residual is effective in controlling the (re)growth of OPs and other microorganisms (Gillespie et al., 2014; Li et al., 2018; Lu et al., 2014; Lytle et al., 2021; Zhang and Liu, 2019). For instance, in a dead-end of a chloraminated drinking water distribution system in South Australia where the residual was not maintained, the abundance of common OPs was statistically significantly higher than that at other points in the same system where the residual was maintained (Whiley et al., 2014a). In an Eastern Mediterranean region, total bacterial counts (37 °C) were a significant predictor of and were negatively correlated with free chlorine residual levels in household tap water (Pieri et al., 2014). Therefore, disinfectant residual decay in municipal EWSs would promote the (re)growth of microbes including OPs and increase the health risks of drinking water. Indeed, an epidemiological study based on the EWS of Cherepovets, Russia, found that free chlorine residual decay increased total heterotrophic plate counts (37 °C) and the risk of gastrointestinal illness in the residents (Egorov et al., 2002). Disinfectant consumption by pipe biofilms and corrosion scales/products is an important reason for disinfectant residual decay in municipal EWSs (Li et al., 2019; Makris et al., 2014; Zhang et al., 2008).

The correlation between DBP concentrations and disinfectant residual levels or dosages in municipal EWSs is generally positive. Notably, water utilities that maintain a “detectable” or trace level (e.g., <0.2 mg Cl2·L−1) of disinfectant residuals would face an increase in DBP concentrations or formation in their distribution systems if they lift the residual levels to numerical minimum values (Roth and Cornwell, 2018). A study found that the concentrations of THMs and HAAs in municipal drinking water from six Chinese cities both significantly increased with free chlorine dosage (Ye et al., 2009). Another study similarly found that the formation of several DBP species in natural-organic-matter-containing model solutions and monochloramine dosage had positive linear correlations (Yang et al., 2007). In residential premise plumbing systems in Quebec, Canada, the concentrations of THMs and HAAs both had a significant positive correlation with free chlorine residual levels at water entry points of houses (Chowdhury et al., 2011). The increase in THM and HAA concentrations with free chlorine (or monochloramine) dosages or levels could be because the formation of THMs and HAAs is a direct result of free chlorine (or monochloramine) residual consumption (Abou Mehrez et al., 2015; Cimetiere et al., 2010; Clark and Sivaganesan, 1998; Gang et al., 2002). Therefore, attenuating the health risks of OPs and other microbes in municipal EWSs by increasing disinfectant residual dosages or levels might exacerbate the DBP issue (Levin and Kleiman, 2003). However, exceptions do exist regarding the correlation between DBP concentrations and disinfectant residual dosages or levels. For instance, in several full-scale drinking water distribution systems, THM concentrations were high but HAA concentratione were low at points with low chlorine or chloramine residual levels (Speight and Singer, 2005).

3. We face a dilemma when selecting a disinfectant residual dosage or level

We are facing a dilemma when maintaining disinfectant residuals to ensure drinking water quality in municipal EWSs. To suppress the (re)growth of OPs and other microbes, maintaining a high disinfectant residual level might be necessary, but as discussed above, the high residual level potentially promotes the formation of DBPs. To minimize the formation of DBPs, we might choose to lower the residual dosages or levels, but OPs and other harmful microbes could thrive when the residual level is below a threshold.

The formation of DBPs in municipal EWSs is non-preventable when secondary disinfection is applied, no matter which disinfectant residual is used (Richardson et al., 2002; Zhang et al., 2018). Therefore, certain European countries (i.e., the Netherlands, Switzerland, Austria, and Germany) typically do not apply secondary disinfection (i.e., no disinfectant residuals in EWSs). Instead, those countries use alternative engineering strategies or safeguards (e.g., depleting available organic carbon in finished water, maintaining a constantly high pressure in distribution systems, and upgrading distribution systems to minimize and/or prevent leaks, breaks, and cross-connections) to control the (re)growth of OPs and other microbes (Hydes, 1999; Rosario-Ortiz et al., 2016; Smeets et al., 2009; Van der Kooij et al., 1999; Zhang and Liu, 2019). However, most major countries in the world, such as the US, Canada, the United Kingdom, Australia, Russia, and China, do maintain disinfectant residuals in water systems to reduce microbial (re)growth, and lowering disinfectant residual dosages or levels could indeed promote OP (re)growth. Therefore, adjusting disinfectant residual dosages or levels to minimize the overall or combined health risks of OPs and DBPs rather than abandoning secondary disinfectant might still be an appropriate strategy to ensure drinking water quality for most water authorities in the world.

4. We might need to comprehensively consider OP (re)growth and DBP formation while maintaining disinfectant residual levels

To better protect public health, we might need to simultaneously monitor the dynamics of OPs and DBPs in municipal EWSs and dose disinfectant residuals at an optimal level to balance the health risks of OPs and DBPs. Unfortunately, previous studies monitored and controlled drinking water quality targeting OPs alone or DBPs alone. No studies have comprehensively assessed the dynamics of OPs and DBPs in municipal EWSs and developed or proposed an effective strategy to optimize disinfectant residual dosages or levels to simultaneously control the health risks of both agents. The US Environmental Protection Agency (US EPA) sets enforceable numerical limits for the concentrations of regulated DBPs and a major OP (Legionella) in drinking water (US EPA, 1989). The US EPA also lists multiple DBPs and OPs in their Contaminant Candidate Lists (US EPA, 1998, US EPA, 2004, US EPA, 2005, US EPA, 2009, US EPA, 2016). However, regulatory numerical limits for the densities of most OPs or total OPs do not exist. In addition, even though OPs and DBPs are listed together, they are regulated separately.

Water utilities might need to optimize disinfectant residual dosages to ensure that the overall or combined health risks of OPs and DBPs are the lowest. Efforts adjusting disinfectant residual dosages to limit the formation of DBPs must not compromise OP(re)growth control, and vice versa (Ashbolt, 2004). OPs and DBPs cause distinct diseases (Li and Mitch, 2018). OPs mainly cause acute infections such as legionellosis and NTM infections, while DBPs induce cancers, central nervous system problems, and birth defects. Therefore, a comprehensive approach putting the health burdens of OPs and DBPs on the same scale (i.e., comparing apples and oranges) is highly needed. However, very few studies directly compared the health risks of pathogens (e.g., OPs) and DBPs. A pioneering work compared the health benefits of ozonating drinking water to kill Cryptosporidium parvum for preventing gastroenteritis and the health risks of ozonation (i.e., the formation of carcinogenic DBPs) (Havelaar et al., 2000). Nonetheless, a thorough approach evaluating and directly comparing the health risks of dominant OPs and typical chlorinated/chloraminated DBPs in municipal EWSs does not exist. Water treatment engineers and scientists are suggested to establish such an approach to optimize disinfectant residual dosages or levels to minimize the overall or combined health burdens of OPs and DBPs to better protect public health.

A few previous publications argued that municipal water utilities should balance the health risks from OPs and DBPs while selecting practical disinfectant residual dosages or levels. This Discussion differs from those previous publications because of two reasons. First, many previous publications (Gang et al., 2002; Gopal et al., 2007; Hrudey, 2009; Li and Mitch, 2018; Richardson et al., 2002; Singer, 1994) briefly mentioned the general idea of optimizing disinfectant residual dosages to balance the health risks of pathogens and DBPs. However, unlike this Discussion, those publications missed a thorough discussion of why such optimization is highly necessary and what essential steps are needed to determine an optimal disinfectant dosage or level. Second, multiple previous publications (Ashbolt, 2004; Craun et al., 1994a; Craun et al., 1994b; de Macedo, 1993; Fawell et al., 1997; Galal-Gorchev, 1996; Havelaar et al., 2000; International Agency for Research on Cancer, 1991; Reiff, 1995; World Health Organization, 1993) hold basically the same view that: “The health risks of water-related pathogens are far more serious than those of DBPs, which have (extremely) small health effects. Evidence for the carcinogenicity of DBPs in humans is inadequate, inconsistent, controversial, and/or inconclusive. Compared with reducing DBP formation, controlling microbical drinking water quality through (residual or secondary) disinfection must always take precedence and should never be compromised in both developing and developed countries.” However, state-of-the-art research has repeatedly proved that DBPs in drinking water are toxic and have significantly adverse health effects in humans and animals (i.e., genotoxicity, carcinogenicity, and negative effects on reproductive/developmental systems) (Hrudey, 2009; Laleh et al., 2020; Liu et al., 2020; Mazhar et al., 2020; Richardson and Plewa, 2020; Tovar and Susa, 2021). In addition, a study estimated that the annual monetized costs of water-related infectious diseases (7 billion in 1998 US dollar value) and DBP-exposure-associated cancers (1 to 16 billion in 1998 US dollar value) in the US are comparable (Levin and Kleiman, 2003). Therefore, controlling drinking water quality through secondary disinfection focusing primarily on the microbial (i.e., OP) part is biased and inadequate. OP (re)growth control and DBP formation control are equally important, and the overall or combined health risks of those two types of agents should be minimized through optimizing disinfectant residual dosages or levels.

5. Strategies for optimizing disinfectant residual dosages

As discussed above, optimizing disinfectant residual dosages or levels to minimize the overall or combined health burdens of OPs and DBPs to better protect public health might be an urgent task for water engineers, practitioners, researchers, and authorities. However, how to achieve this goal is missing at large from the current liteature. We hereby propose three essential steps for disinfectant residual dosage optimization.

  1. Determine the accurate, quantitative correlation between disinfectant residual dosages or levels and the concentrations of OPs and DBPs in municipal EWSs. In general, when disinfectant residual dosage increases, OP densities would decrease, while DBP concentrations would increase. However, numerous other factors, such as source water quality, water treatment train, water temperature, pH, water pipe materials, and the existence of biofilms, affect the dynamics of the concentrations of OPs and DBPs in EWSs. Therefore, each EWS could have a specific correlation between disinfectant residual dosages or levels and the concentrations of OPs and DBPs. Water boards are suggested to quantitatively investigate such a correlation in each of their EWSs.

    To better understand such correlations, we might need to obtain detailed spatiotemporal dynamics of disinfectant residual levels, OP densities, and DBP concentrations in municipal EWSs. Numerous studies have monitored such dynamics; however, the following critical questions still need to be answered by future studies: 1) How often this monitoring shall take place (i.e., the sampling frequency)? 2) What is the reasonable strategy to select sampling points to better reflect water quality in municipal EWSs? 3) How does booster disinfection/chlorination or the existence of booster points affect such spatiotemporal dynamics?

  2. Perform quantitative risk assessment to estimate the health burdens of exposure to OPs and DBPs in drinking water. Previous research has analyzed the health risks of a few individual OPs and DBPs. However, in real distributed drinking water, numerous species of OPs and DBPs coexist. The health risks due to the coexistence of multiple OPs (i.e., sporadic infections and disease outbreaks) and DBPs (i.e., acute effects, cancers, and reproductive/developmental system diseases) are unclear. Water engineers and researchers are suggested to rely on time-detailed and spatially resolved drinking water quality data to quantitatively assess the health risks of the coexistence of multiple species of OPs and DBPs in drinking water.

  3. Estimate the socioeconomic costs associated with the health risks of OPs and DBPs in drinking water. We might need to reasonably estimate the overall or combined socioeconomic costs of the health burdens of OPs and DBPs. Only when we can develop a quantitative correlation between the total socioeconomic costs and the concentrations of OPs and DBPs may we reasonably optimize disinfectant residual dosages or levels to minimize their overall or combined health effects.

6. Conclusions

The prevalence of OPs is the most important aspect of microbial drinking water quality. The concentrations and formation of DBPs are the most important aspect of physicochemical drinking water quality. Both OPs and DBPs in drinking water significantly threaten public health. The (re)growth of OPs and the formation of DBPs in municipal EWSs both closely correlate with the dosages or levels of disinfectant residuals. However, OPs and DBPs respond to disinfectant residuals often oppositely. A high residual level effectively suppresses the (re)growth of OPs while enhances the formation of DBPs. Oppositely, a low or “detectable” disinfectant residual level reduces the formation of DBPs but could not prevent OPs from thriving. We may need to comprehensively consider OP (re)growth and DBP formation while selecting a practical disinfectant residual dosage or level to ensure that the overall or combined health risks of OPs and DBPs are minimum.

Acknowledgments

The United States Environmental Protection Agency (US EPA) through its Office of Research and Development funded and collaborated in the research described here [Regional Applied Research Effort Program (RARE 6) and Safe and Sustainable Water Resources (SSWR 7.2.1)]. This article has been subjected to US EPA’s peer review and has been approved for publication. The authors thank Jatin Mistry at the US EPA Region 6 for providing us valuable comments and suggestions for this Discussion.

Footnotes

Declaration of competing interest

The authors declare no actual or potential competing financial interest.

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