Drinking Water

Healthy development of the gut microbiome provides long-term health benefits. Children raised in countries with high infectious disease burdens are frequently exposed to diarrhoeal pathogens and antibiotics, which perturb gut microbiome assembly. A recent cluster-randomized trial leveraging >4,000 child observations in Dhaka, Bangladesh, found that automated water chlorination of shared taps effectively reduced child diarrhoea and antibiotic use. In this substudy, we leveraged stool samples collected from 130 children 1 year after chlorine doser installation to examine differences between treatment and control children's gut microbiota. Water chlorination was associated with increased abundance of several bacterial genera previously linked to improved gut health; however, we observed no effects on the overall richness or diversity of taxa. Several clinically relevant antibiotic resistance genes were relatively more abundant in the gut microbiome of treatment children, possibly due to increases in Enterobacteriaceae. While further studies on the long-term health impacts of drinking chlorinated water would be valuable, we conclude that access to chlorinated water did not substantially impact child gut microbiome development in this setting, supporting the use of chlorination to increase global access to safe drinking water.

Iodine is a naturally-occurring halogen in natural waters generally present in concentrations between 0.5 and 100 µg L^-1. During oxidative drinking water treatment, iodine-containing disinfection by-products (I-DBPs) can be formed. The formation of I-DBPs was mostly associated to taste and odor issues in the produced tap water but has become a potential health problem more recently due to the generally more toxic character of I-DBPs compared to their chlorinated and brominated analogues.
This paper is a systematic and critical review on the reactivity of iodide and on the most common intermediate reactive iodine species HOI. The first step of oxidation of I⁻ to HOI is rapid for most oxidants (apparent second-order rate constant, k_app > 10³ M^-1s^-1 at pH 7). The reactivity of hypoiodous acid with inorganic and organic compounds appears to be intermediate between chlorine and bromine. The life times of HOI during oxidative treatment determines the extent of the formation of I-DBPs. Based on this assessment, chloramine, chlorine dioxide and permanganate are of the highest concern when treating iodide-containing waters. The conditions for the formation of iodo-organic compounds are also critically reviewed.
From an evaluation of I-DBPs in more than 650 drinking waters, it can be concluded that one third show low levels of I-THMs (<1 µg L^-1), and 18% exhibit concentrations > 10 µg L^-1. The most frequently detected I-THM is CHCl₂I followed by CHBrClI. More polar I-DBPs, iodoacetic acid in particular, have been reviewed as well.
Finally, the transformation of iodide to iodate, a safe iodine-derived end-product, has been proposed to mitigate the formation of I-DBPs in drinking water processes. For this purpose a pre-oxidation step with either ozone or ferrate(VI) to completely oxidize iodide to iodate is an efficient process. Activated carbon has also been shown to be efficient in reducing I-DBPs during drinking water oxidation.

Chlorine dioxide (ClO₂) applications to drinking water are limited by the formation of chlorite (ClO₂^–) which is regulated in many countries. However, when ClO₂ is used as a pre-oxidant, ClO₂^– can be oxidized by chlorine during subsequent disinfection. In this study, a kinetic model for the reaction of chlorine with ClO₂^– was developed to predict the fate of ClO₂^– during chlorine disinfection. The reaction of ClO₂^– with chlorine was found to be highly pH-dependent with formation of ClO₃^– and ClO₂ in ultrapure water. In presence of dissolved organic matter (DOM), 60-70% of the ClO₂^– was transformed to ClO₃^– during chlorination, while the in situ regenerated ClO₂ was quickly consumed by reaction with DOM. The remaining 30-40% of the ClO₂^– first reacted to ClO₂ which then formed chlorine from the DOM-ClO₂ reaction. Since only part of the ClO₂^– was transformed to ClO₃^–, the sum of the molar concentrations of oxychlorine species (ClO₂^– + ClO₃^–) decreased during chlorination. By kinetic modelling, the ClO₂^– concentration after 24 h of chlorination was accurately predicted in synthetic waters but was largely overestimated in natural waters, possibly due to a ClO₂^– decay enhanced by high concentrations of chloride and in situ formed bromine from bromide. Understanding the chlorine-ClO₂^– reaction mechanism and the corresponding kinetics allows to potentially apply higher ClO₂ doses during the pre-oxidation step, thus improving disinfection byproduct mitigation while keeping ClO₂^–, and if required, ClO₃^– below the regulatory limits. In addition, ClO₂ was demonstrated to efficiently degrade haloacetonitrile precursors, either when used as pre-oxidant or when regenerated in situ during chlorination.

Household sand filters (SFs) are widely applied to remove iron (Fe), manganese (Mn), arsenic (As), and ammonium (NH₄⁺) from groundwater in the Red River delta, Vietnam. Processes in the filters probably include a combination of biotic and abiotic reactions. However, there is limited information on the microbial communities treating varied groundwater compositions and on whether biological oxidation of Fe(II), Mn(II), As(III), and NH₄⁺ contributes to the overall performance of SFs. We therefore analyzed the removal efficiencies, as well as the microbial communities and their potential activities, of SFs fed by groundwater with varying compositions from low (3.3 μg L⁻¹) to high (600 μg L⁻¹) As concentrations. The results revealed that Fe(II)-, Mn(II)-, NH₄⁺-, and NO₂⁻-oxidizing microorganisms were prevalent and contributed to the performance of SFs. Additionally, groundwater composition was responsible for the differences among the present microbial communities. We found i) microaerophilic Fe(II) oxidation by Sideroxydans in all SFs, with the highest abundance in SFs fed by low-As and high-Fe groundwater, ii) Hyphomicropbiaceae as the main Mn(II)-oxidizers in all SFs, iii) As sequestration on formed Fe and Mn (oxyhydr)oxide minerals, iv) nitrification by ammonium-oxidizing archaea (AOA) followed by nitrite-oxidizing bacteria (NOB), and v) unexpectedly, the presence of a substantial amount of methane monooxygenase genes (pmoA), suggesting microbial methane oxidation taking place in SFs. Overall, our study revealed diverse microbial communities in SFs used for purifying arsenic-contaminated groundwater, and our data indicate an important contribution of microbial activities to the key functional processes in SFs.