The health consequences of inadequate water supplies include an estimated 4 billion cases of diarrhea and 1.87 million deaths each year, mostly among young children in developing countries.1,2
In addition, waterborne diarrheal diseases lead to decreased food intake and nutrient absorption, malnutrition, reduced resistance to infection,3
and impaired physical growth and cognitive development.4
Recently, household drinking water treatment and safe storage options have been recognized as approaches that can reduce disease risk until the longer term goal of universal access to piped, treated water can be attained.5,6
Household water treatment and storage practices can prevent disease, and thereby support poverty alleviation and development goals.
Chlorination was first used for disinfection of public water supplies in the early 1900s, and is one factor that contributed to dramatic reductions in waterborne disease in cities in the United States.7
Chlorine gas, calcium hypochlorite powder, and concentrated or locally produced liquid sodium hypochlorite have historically been the chlorine donors used for water treatment. Sodium dichloroisocyanurate (NaDCC) is an organic compound that disassociates in water to form sodium cyanurate and hypochlorous acid. Use of NaDCC tablets in Bangladesh households was associated with a significant reduction of fecal coliform bacteria in stored drinking water than in controls (2.8 colonies/100 mL versus 604.1 colonies/100 mL).8
Medentech (Wexford, Ireland) is the largest producer of NaDCC tablets worldwide, distributing more than 900 million water purification tablets in 2008 for emergency and development purposes; in total potentially treating greater than 15 billion liters of water.
Although NaDCC has been widely used for emergency response and recreational water treatment, concerns about potential health impacts from sodium cyanurate had precluded approval as a long-term drinking water disinfectant.9
In 2004, NaDCC was approved by the U.S. Environmental Protection Agency for long-term drinking water use.10
In addition, the Joint Food and Agriculture Organization of the United Nations/World Health Organization (WHO) Expert Committee on Food Additives recommended a tolerable daily intake for NaDCC for long-term drinking-water disinfection of 0–2.0 mg/kg of body weight.11,12
This recommendation was formally adopted by the WHO into the Guidelines for Drinking Water Quality in the second addendum to the Third Edition.13
Sodium dichloroisocyanurate offers some advantages over other chlorine-based disinfectants for household water treatment in developing countries, including a shelf life of five years, resistance to degradation from sunlight, single-use packaging, and low weight in distribution. These advantages, in some cases, outweigh the disadvantage of higher cost per liter treated than locally-made sodium hypochlorite solution ($0.033/liter for sodium hypochlorite and $0.08/liter for NaDCC). As access to NaDCC has expanded in developing countries, where many water sources contain suspended and dissolved organic material, some health officials and implementing organizations have expressed concern about the formation of disinfection by-products in NaDCC-treated water and the attendant risk to consumers.
In 1974, it was discovered that hypochlorous acid and hypobromous acid react with naturally occurring organic matter to create four compounds with potential human health effects: chloroform (CHCl3
), bromoform (CHBr3
), bromodichloromethane (CHCl2
Br), and dibromochloromethane (CHClBr2
These four compounds are collectively termed trihalomethanes (THMs). Initially, THM research focused on the effects of chloroform. However, further research has shown that chlorination of drinking water leads to the formation of many compounds that may or may not have mutagenic activity. More than 600 water disinfection byproducts have been identified in chlorinated tap water, including haloacetic acids.15
The THMs, and to a lesser extent the haloacetic acids, are currently used as indicator chemicals for all potentially harmful compounds formed by the addition of chlorine to water.
The WHO has established guideline values for the four THMs that are fully protective for cancer and non-cancer effects, based on epidemiologic and laboratory studies establishing a non-linear dose-response relationship between THM analyte and health impact. The guideline values are set below the expected threshold for these effects. Chloroform has been classified as possibly carcinogenic to humans, based on sufficient evidence for carcinogenicity in experimental animals but inadequate evidence in humans.16
The WHO guideline value for chloroform is 300 μg/L (or 300 parts per billion).17
Bromodichloromethane has been classified as probably carcinogenic to humans, with sufficient evidence in animals and inadequate evidence in humans.18
The WHO guideline value is 60 μg/L.19
The International Agency for Research on Cancer of the WHO has classified dibromochloromethane and bromoform as not classifiable in humans for carcinogenicity,18
and the WHO guideline values for both are 100 μg/L.19
The WHO also proposes the use of an additive toxicity guideline value, using a fractionation approach. The sum of the four actual values of the THMs divided by their guideline value should not be greater than one.19
Lastly, the WHO Guidelines specifically state that “Where local circumstances require that a choice must be made between meeting either microbiological guidelines or guidelines for disinfectants or disinfectant by-products, the microbiological quality must always take precedence, and where necessary, a chemical guideline value can be adopted corresponding to a higher level of risk. Efficient disinfection must never be compromised.”20
Most research on THMs has been conducted in water treatment plants in developed countries, analyzing THM formation potential of source waters and mitigation strategies such as the use of alternate disinfectants. The one exception is a 2008 study of household (point-of-use) drinking water treatment with sodium hypochlorite that documented THM concentrations did not exceed WHO individual analyte or additive guideline values 24 hours after sodium hypochlorite addition in waters with 4.23–305 nephelometric turbidity units (NTU) in Kenya.21
However, NaDCC was not tested. Previous research on THM formation with NaDCC has been limited. In Seine River water of 3–4 NTU, added NaDCC concentrations leading to chlorine residuals of 3.8–10 mg/L had chloroform concentrations of 2–21.7 μg/L 24 hours after treatment.22
Other THM analytes were not tested. In a study from the food industry, as added NaDCC concentration increased, leading to chlorine residuals of 6.98–210.11 mg/L, corresponding increases in THM concentrations did not occur, regardless of how the water was chlorinated before NaDCC addition.23
However, the THM concentrations increased with similarly increased sodium hypochlorite residuals. The utility of this data for household drinking water treatment is limited because the maximum household added hypochlorite concentration used would be 5 mg/L and water is unlikely to be chlorinated before NaDCC addition in developing countries.
In this report, we compare WHO THM Guidelines with THM levels formed by sodium dichloroisocyanurate and sodium hypochlorite disinfection of water from a variety of sources with varying turbidity levels in rural western Tanzania.