Achieving safe levels of byproducts in disinfected drinking water
Since the 1900s, the disinfection of community drinking water systems has been a practice observed by professionals as a control mechanism in treating waterborne infectious diseases. Today, we have some of the best technology and monitoring protocols in the world, but are they enough and is the ‘safe’ level advice really safe for the public?
As part of Water Source's Expert Series, Viridis Consultants Director James Howey discusses problematic disinfection by-products and the need to continue to assess emerging risks.
In Australia, the Australian Drinking Water Guidelines (ADWG) outlines health related guidelines for both acute and chronic health related hazards in drinking water. The guideline limits are calculated to provide safe consumption over a lifetime. These guidelines are implemented across Australia through various state and territory legislation.
Drinking water catchments include dams, rivers, groundwater bores, barrages, and weirs, and depending on the risk profile contain varying sources of human pathogens. These pathogens can be capable of manifesting in acute illness if not adequately treated by drinking water service providers. Chronic health issues also need to be addressed and these can be present in the source water or a by-product of the treatment process. Treatment requirements can be complex and drinking water service providers have a regulatory responsibility to get it right.
In water sources, it is realistic to expect fluctuations in the concentration of pathogens due to a variety of hazardous events, including seasonal rainfall, changes in surface water temperature, deceased animals, failing wastewater systems or intensive livestock.
The level of drinking water treatment is determined through analysis of pathogen indicators and a sanitary survey. This is the first priority, but chronic health issues cannot be ignored, they must also be quantified, and preventive measures put in place.
Drinking water chlorination: pros and cons
Chlorine is the most widely used disinfectant for the treatment of a variety of pathogens. It is easily available in the market. There are two main types of chlorine used to disinfect drinking water, chlorine gas and sodium hypochlorite solution. There are pros and cons with both, selection of the right disinfectant is crucial.
Sodium hypochlorite is cheap and readily available; however, it is prone to chemical breakdown, which is exasperated by high temperatures. Chlorine gas is another option, it is chemically pure and does not have the same breakdown problem, but there are handling concerns. Chlorine gas requires storage in specially designed facilities and handling by highly trained operators, which is not always possible in some settings.
These issues are leading to a third option, salt chlorinators, due to their affordability and storage benefits; service providers can buy bags of salt that can be stored for months at a time as they don’t break down, and through an electrolysis process, chlorine can be created as required.
Trihalomethane limits tightening up for drinking water treatment
Certain waters have precursors to chlorine disinfection by-products, organic matter in the source water can react with chlorine to produce potential carcinogens, such as chloroform. The ADWG sets guideline health limits for disinfection by-products, trihalomethanes (THM), such as chloroform, have a total limit of 250 µg/L.
The ADWG requires that the treatment of pathogens is not compromised due to chronic issues, such as THMs. This, in some instances, has legitimised some water service providers to not implement control measures for disinfection by-products, which often only occur seasonally and can be expensive to manage.
However, with a growing body of research, Queensland Health has suggested that 250 µg/L could be high enough to cause health concerns for communities, and as such are taking tougher action on exceedances of THM limits.
Water service providers need to ensure that risk assessments adequately assess this risk, appropriate monitoring is undertaken, and measures are taken to mitigate the risk. These may include reducing water age in the reticulation, avoiding chlorinating unfiltered water, enhancing coagulation to remove precursors, managing chlorine levels, cleaning mains and reservoirs in the reticulation or switching to a chloramine residual in the distribution.
Increasing pressure will be applied to those water service providers operating schemes with seasonal exceedances. This should be an area for continual improvement, it is almost inevitable that guideline limits will be reduced at some point, the limit in the UK is 100 µg/L and in the US, 80 µg/L.
Chlorate is an emerging hazard when treating contaminated water
Sodium hypochlorite decomposes over time and one of the breakdown products is chlorate. Up to this point there has been no guideline value in the ADWG, although the World Health Organization (WHO) has identified a limit of 0.7 mg/L.
As chlorine decays it lowers the amount of freely available chlorine in the disinfectant and therefore more needs to be dosed exasperating the concentration of chlorate in the drinking water. The decay of sodium hypochlorite is influenced by the concentration of the solution and temperature. Bulk buying months of sodium hypochlorite solution and storing it in a hot tin shed is not a good option in the management of chlorate – neither is buying chemicals from non-quality-controlled suppliers, where the age of the solution is unknown.
As there is no limit for chlorate set in the ADWG, many water service providers are not currently monitoring it and the extent of the issue is not fully known. Queensland Health has committed to bringing in an interim limit of 0.8 mg/L, which is based on the WHO figure, adjusted for the larger average body mass in Australia (amount of chemical per kilo of body weight).
Chlorine gas is an option to eliminate the chlorate problem, especially in hot conditions and where multiple doses of chlorine are used. However, if this is not an option due to the risk profile, a salt electrolysis chlorinator may be worth considering. Although, there is still the potential of breakdown into chlorate, the solution can be created freshly, as required, at the point of use.
Chlorination is an essential barrier for waterborne pathogens to ensure the safe supply of drinking water. However, disinfection by-products are becoming an ever-increasing topic. These risks need to be considered as part of a drinking water risk assessment and addressed using a hierarchy of controls.
Chlorine is one of the oldest forms of water treatment and is still safe, but as our knowledge and water treatment technologies advance, we need to address emerging risks.