Provision of safe drinking water continues to be a challenge in developing countries and microbial contamination of water can lead to a number of waterborne diseases. Studies in India show that access to tap water may not guarantee that it is safe to drink. Water treatment, storage and WASH practices influence water quality.

Water treatment can harm too

Disinfection is regularly needed to protect people from the diseases caused due to pathogens in the water, and treatment with chlorine is a common practice in India. However, the process can in itself lead to formation of other contaminants such as the trihalomethanes (THMs) and haloacetic acids (HAAs) due to reaction of chemical oxidants with organic matter in the water.

Thus a balance has maintained between the benefits of the disinfectant used and the potential risks from the bye-products. The concentrations of Trihalomethanes (THMs) and Haloaceticacids (HAAs) (byeproducts from chlorine disinfection) depend on factors like source of water, pH, temperature, residence time, chlorine dose, disinfectant type, operating conditions of the treatment plant, bromide concentration, applied chlorine dose, residual chlorine, etc.

Trihalomethanes (THMs) and Haloaceticacids (HAAs) can be very harmful to humans in higher concentrations than normal. THMs are the first regulated group of bye products from disinfectants (DBPs) and include chloroform (CF), bromodichloromethane (BDCM), dibromochloromethane (DBCM), and bromoform (BF).

DBPs have been associated with cancers of the bladder, colon and rectum and adverse birth outcomes such as spontaneous abortion, (low) birth weight, stillbirth and congenital malformations in epidemiological studies and to a much lesser extent at high levels in toxicological studies.

The permissible limit set by the United States Environmental Protection Agency (USEPA) is 80 mg/L for total THMs. The World Health Organisation (WHO) has set the individual limit for CF, BDCM, DBCM, and BF as 300 mg/L, 60 mg/L, 100 mg/L and 100 mg/L respectively. The Bureau of Indian Standards has also regulated CF, BDCM, DBCM, and BF individually as 200 mg/L, 60 mg/L, 100 mg/L, and 100 mg/L respectively in 2012 (IS:10500, 2012).

Human beings use water in their daily life for drinking, bathing, showering, cooking, washing, cleaning, etc and they can get exposed to THMs through oral ingestion, dermal absorption, and inhalation. It is thus very important  to detect and monitor THMs in municipal water supplies to safeguard humans against their carcinogenic risks.

Monitoring bye products of disinfection

In India, THMs, are still not regularly monitored at the water treatment plants. For example, all the water treatment plants (WTPs) in Delhi use chlorination processes considering the regrowth of pathogens in the water supply pipes. However, there is no information on the risks associated with THM levels in the drinking water supplied to the city.

This paper 'Monitoring of Trihalomethanes and its cancer risk assessment in drinking water of Delhi City, India' from the journal Pollution discusses the findings of a study that monitors the THMs levels in the tap water coming out from the distribution system of nine WTPs in urban areas of Delhi, India, and assesses the cancer risk from drinking this water. Thus THM levels in tap water were determined, and cancer risk was evaluated for THMs through oral ingestion, dermal absorption, and inhalation for the study.

Nine water treatment plants (WTP), namely Chandrawal WTP (CWTP), Wazirabad WTP (WWTP), Haiderpur WTP (HWTP), Bhagirathi WTP (BWTP), Bawana WTP (BAWTP), Nangloi WTP (NWTP), Sonia Vihar WTP (SVWTP), Okhla WTP (OWTP), Dwarka WTP (DWTP) were selected for the study.

What's in drinking water in Delhi?

The study found that:

• The total organic carbon (TOC) was in the range of 0.57 mg/L to 12.92 mg/L with a mean of 5.48 mg/L. The samples from the command areas of Chandrawal, Wazirabad, Haiderpur, Bhagirathi, Sonai Vihar showed higher TOC concentrations than at Bawana, Nangloi, Okhla and Dwarka command areas. Higher TOC levels in the tap water indicates contamination of drinking water due to mixing of wastewater into the water distribution network.
• The THM levels varied between 11.41 µg/L to 175.54 µg/L in the distribution system, with a mean level of 77.58 µg/L. The THM concentrations exceeded the maximum permissible limit given by Indian Standards. The concentration of chloroform was maximum, followed by bromodichloromethane, dibromochloromethane, and bromoform.
• The residual chlorine was in the range of 0.14 mg/L to 2.17 mg/L, having a mean concentration of 0.94 mg/L, and was found to be greater than the permissible limits given by Indian Standards, i.e., 0.2 mg/L to 1 mg/L (IS:10500, 2012).
• The samples from the command areas of Chandrawal, Wazirabad, Haiderpur, Bhagirathi, Nangloi, Sonia Vihar showed higher residual chlorine concentrations than the samples from Bawana, Okhla and Dwarka command areas.  In the distribution network the increase in residual chlorine advocates higher THMs formation.
• For males and females, the mean value of total cancer risk (CR) was 5.09E05 5.70E-05, respectively. As the THMs levels were high, the total CR value was also more than the  negligible level of risk i.e., 1.0 x 10-6 through all exposure routes.
• The total cancer risk in the Chandrawal WTP command area was the highest (7.91E-05, 8.88E-05) and lowest at the Okhla WTP command area (2.06E-05, 2.31E05) for males and females, respectively.
• Cancer risk through ingestion was found to be the highest followed by oral inhalation and dermal absorption, which was also higher than the acceptable limit given by USEPA, i.e., 10−6.
• Females showed higher cancer risks through all exposure routes than males.

The full paper can be accessed here