Chlorination is the most widely used method for water treatment in humanitarian emergencies because of its simplicity, low cost, and importantly, the residual protection it provides. In refugee and internally displaced persons (IDP) camps in humanitarian crisis zones, centralized batch chlorination remains the primary approach to treating large quantities of water, while point-of-use (e.g., household) and point-of-distribution (e.g., chlorine dispensers) approaches are also utilized in specific niche roles [1
]. A number of chlorine products are commonly utilized in humanitarian response including calcium hypochlorite powders (e.g., high-test hypochlorite, HTH), sodium hypochlorite solutions (e.g., bleach), as well as tablets composed of various chlorine compounds such as sodium dichloroisocyanurate (NaDCC) or chlorine dioxide. When these products react with water, hypochlorous acid and hypochlorite ions are formed, which provide the disinfectant effect. The reliance on chlorination in humanitarian operations, including of raw surface waters during the early weeks of rapid-onset emergencies, has raised concerns among practitioners about possible health risks due to disinfection by-products [6
]. Similar concerns have also been raised about the use of various chlorine products in household water treatment programs in developing country settings [7
Disinfection by-products (DBPs) form when hypochlorous acid reacts with natural organic matter (NOM) such as humic and fulvic acids present in natural surface and ground waters [9
]. DBPs are chemically stable and will accumulate in treated water in the presence of chlorine and organic precursors. DBP formation increases with residual chlorine concentration, temperature, contact time, pH, and NOM levels [10
]. DBPs such as trihalomethanes (THMs) may be linked to cancer and non-cancer adverse health effects (i.e., reproductive system) and are therefore subject to maximum allowable concentrations from the WHO and many national regulators [11
]. The exact role of THMs versus other DBPs (over 600 species have been identified) in causing adverse health outcomes is unclear; THMs are likely a surrogate measure for other more hazardous DBPs [14
]. Current epidemiological evidence shows a consistent association between long-term THM exposure (30+ years) and risk of bladder cancer, although the causal nature of the association is not conclusive, whereas the epidemiological evidence concerning other cancer sites is insufficient or mixed [15
]. Current evidence also suggests minor effects from high THM exposure during pregnancy on fetal growth indices such as small for gestational age at birth, but is inconclusive with respect to other reproductive outcomes (e.g., low birth weight, fetal loss, preterm delivery, and congenital malformation) [15
Globally, displacement crises are becoming increasingly intractable, with populations forced to remain in refugee and IDP camps for extended periods of time, sometimes even for decades. As a consequence, health risks associated with chronic exposures, which were previously considered to be less relevant in humanitarian response situations, are becoming increasingly pertinent (see, for example, Reference [24
]). DBPs are one such concern that humanitarian responders must start thinking concretely about. To our knowledge, there has not been an evaluation of DBP levels in an emergency water supply intervention before. In response to this gap, we investigated DBP levels using field-appropriate methods in production water at an emergency surface water treatment plant serving a refugee settlement in northern Uganda. In this paper, we describe the investigation undertaken, present findings, and discuss their implications for humanitarian response. In doing so, this paper provides the first characterization of DBPs in drinking water supplies in a humanitarian field setting.
Overall, our findings did not indicate that a DBP-related health hazard is created when surface water is chlorinated at the Palorinya SWTP vis-à-vis the WHO guideline limit for chloroform. Additionally, our findings show that “standard” two-stage water treatment with clarification and chlorination in separate tanks results in much lower levels of DBPs in production water as compared to “rapid” treatment with simultaneous clarification–chlorination in the same tank. When water is subject to rapid treatment in which chlorination is done before the water is properly clarified, DBP/THM levels in production water will be elevated and may approach or exceed the WHO guideline limit. The elevated levels of DBPs under rapid treatment likely arise due to a combination of greater availability of NOM as well as higher doses of chlorine in order to achieve sufficient residual.
This finding reinforces the need to transition away from rapid water treatment to two-stage treatment as soon as is feasibly possible in emergencies. This is, of course, notwithstanding the fact that ensuring adequate chlorination to protect against waterborne pathogens remains the priority, as the health risks associated with waterborne diseases far outweigh those linked to DBPs, even in industrialized-world settings (and this difference is likely to be even more pronounced in emergency and resource-poor settings) [14
]. Efforts to control the potential health risks associated with DBPs must not compromise waterborne pathogen control.
While we cannot generalize the findings from the Palorinya SWTP to all emergency surface water treatment interventions globally, this study may represent a “worst-case scenario”, as the SWTP water intake was downstream of a slow-moving, highly vegetated, and marshy area of the White Nile. Moreover, the study took place during the rainy season when NOM levels may be elevated due to surface runoff and associated soil erosion (albeit, in the dry season, lower river flow would reduce dilution which would also act to concentrate and elevate NOM levels). Fortunately, even in the case of a marshy surface-water source, effective pre-clarification before chlorination resulted in DBP/THM levels remaining well below guideline limits, even after 24 hours of storage.
The DBP/THM levels we observed at the Palorinya SWTP correspond with levels observed by other workers investigating water chlorination programs for public health protection in developing-country settings. Lantagne et al. [7
], in their investigation of point-of-use chlorination programs in Kenya, observed TTHM levels around 150 ppb 24 h after river water (with a substantially higher turbidity than ours at 305 NTU) was directly chlorinated using 1% sodium hypochlorite solution (FRC at 24 h was 0.07–3.64 mg/L, similar to the FRC range we observed as well). This water type is comparable to the “rapid treatment” samples we observed that were directly chlorinated without clarification, which had a mean DBP/THM of 218.0 ppb (Table 4
). Interestingly, the more “clear” water types that Lantagne et al. observed in Kenya (with respect to turbidity and organics content), such as rainwater and well water, had TTHM concentrations in the range of 30 to 80 ppb, which corresponds with DBP/THM levels we observed in “standard treatment” samples which were clarified prior to chlorination (mean 85 ppb). Similarly, in point-of-use chlorination programs in Tanzania, Lantagne et al. [8
] observed TTHM levels in the range of 100 ppb at 24 h when river water (turbidity: 5.1 NTU) was directly chlorinated with sodium hypochlorite solution (FRC: 0.8–1.2 mg/L at 24 h). In Tanzania, Lantagne et al. observed substantially lower TTHM levels (10 to 60 ppb) when clearer water types such as tap, well, or lake waters were directly chlorinated. Importantly, none of the DBP levels Lantagne et al. observed in either Kenya or Tanzania exceeded the 300 ppb WHO chloroform guideline. Similarly, Nhongo et al. [40
] found THMs to generally be within WHO guideline limits in their investigation of DBP levels in the piped water supply system of Harare, Zimbabwe. Légaré-Julien et al. [34
] found THM levels in the range of 50 ppb when natural surface waters from Quebec, Canada were treated using a point-of-use coagulant-disinfectant product that included NaDCC as the disinfecting agent. On the other hand, Werner et al. [41
] found levels of chloroform and TTHMs approaching and even exceeding the 300 ppb WHO guideline at 24 h when point-of-use chlorine dioxide tablets were used to treat natural surface waters in the Northern British Isles. Although these latter two studies were not conducted in developing-country settings, the chlorine tablets they evaluated are intended for use in public health promotion programs in developing-country settings.
The DBP levels observed in this and other studies raise an important question regarding the point of reference used to assess whether a health risk exists (or not). The WHO guideline limit we and other workers have used, of 300 ppb chloroform, relates to health concerns associated with chronic exposure (30+ years). As mentioned at the beginning, there are, however, specifically vulnerable subpopulations, pregnant women namely, for whom there may be adverse reproductive effects at sub-chronic levels of exposure (on the order of weeks and months). Current evidence suggests minor effects from high THM exposures (on the order of 100 ppb with respect to TTHM concentration) during pregnancy on fetal growth indices and other reproductive outcomes. [15
]. This is, indeed, within the range we observed at the Palorinya SWTP, as well as that observed by Lantagne et al. in household chlorination programs in Kenya and Tanzania [7
]. Further investigation of DBP/THM levels in emergency water supplies, especially during the early weeks of a rapid-onset emergency when rapid water-treatment methods may be in use, and the associated risk of adverse reproductive effects for pregnant women warrants further investigation.
As the first report on DBP/THM levels in production water from an emergency water treatment system, this study is a first step toward understanding a water-related health risk that humanitarian responders will likely have to contend with more in coming years. Further field data on DBP levels in emergency water supplies are needed in order to better characterize the extent of the issue. Based on our experience in Uganda, we found the Hach THM Plus Method to be a simple, inexpensive, and effective tool for monitoring DBP/THM levels in humanitarian field settings. We were able to effectively implement the method at the Palorinya SWTP to produce credible readings, both with and without supervision once water quality technicians at the on-site water quality lab were trained on how to collect, analyze, and report DBP/THMs using the THM Plus kit. Procedurally, the method is not complicated nor labor-intensive, and local staff were comfortable doing it independently after one week of training and supervision. We recommend it for further field use.
Even with a colorimetric method however, continual monitoring of DBP/THMs introduces additional expense and labor demands to routine monitoring which is already prone to gaps in humanitarian operations. As such, in order to provide useful water quality intelligence while minimizing demands on field teams and resources, proxy measurements for DBP/THM production should also be explored. NOM precursors to THMs can be estimated relatively simply in the field by measuring surrogate parameters such as UV254
]. Field-appropriate proxy methods should be explored further.
Finally, DBP control strategies for humanitarian field settings may need to be explored if the risk to vulnerable sub-populations is determined to be significant enough. Ideally, the formation of DBPs should be prevented through effective pre-clarification before chlorination. If the common pre-clarification methods used in emergency water treatment are found to be insufficient with respect to NOM removal, enhanced coagulation or adsorption with powdered or granular activated carbon (PAC/GAC) could potentially be utilized to further reduce NOM levels [45
]. Ceramic nanofiltration membranes have also been evaluated for NOM precursor removal but are technically complex and energy intensive to operate [48
]. Once DBPs are formed, they may be removed by multiple methods including aeration techniques such as diffused-air systems or aeration towers (these may not be suitable for humanitarian field settings however); adsorption using PAC or GAC, or nitrifying bio-filters (slow sand filters) [45
]. Such systems would need to be adapted and evaluated for humanitarian field use.