Chlorination or chloramination? What is best for the drinking water industry
It is common knowledge that chlorine and its compounds are used to disinfect water before it is fit for human consumption, and this has been the case since a cholera outbreak in Maidstone, UK, in 1897, was cured by disinfecting the water.
As science and technology have advanced though, so has our understanding of the water we drink and it’s health implications. Currently, it would appear that although chlorination of drinking water is eliminating waterborne diseases and viruses, it can actually create carcinogens in our drinking water.
Dissolved organic matter (DOM) is present in all water, fresh and marine, in various forms and concentrations. DOM comprises of terrestrial and aquatic sources of decayed organic and microbial material. DOM differs from sea to sea, river to river, lake to lake, and its composition is primarily dictated by the source of the organic matter in the catchment (dense leafy vegetation, for example, will form different DOM than a coniferous tree plantation).
Although not harmful to humans, DOM does provide an unsightly colour, and unfavorable taste and odour issues in final drinking water.However, when DOM is oxidised by halogens (commonly chlorine and it’s compounds, but ozone is also used) it can form disinfection by-products (DBPs), which have been found to have carcinogenic, mutagenic and toxic properties. In 2007, over 700 DBPs had been identified, a number which I expect to be much higher today.
Therefore, each water treated for drinking water purposes must have a treatment tailor-made for it – no 2 waters are the same – to ensure the safe removal of DOM before chlorination to kill off pathogens, bacteria and viruses.
DOM is commonly removed via a series of treatments, normally coagulation and floculation followed by ultra filtration, however, none of the removal processes are 100% accurate, and some DOM will still be found in water ready for disinfection, resulting in the formation of these DBPs.
Due to their health concerning properties, the World Health Organisation has set a maximum contaminant level for 4 DBPs under the family name of trihalomethanes (THMs). This is an annual average limit, and water companies must provide a water under this annual average limit to their consumers for it to be deemed safe. However, this limit is difficult to achieve, as increasing human inputs to water (through urban runoff, industrial waste, farming practices etc) are adding to the smaller, harder to remove DOM compounds that can survive the DOM removal practices at the treatment works.
As a result, chloramine (a combination of chlorine and ammonium) has been used to treat waters due to its shorter life span (resulting in initial killing of bacteria but lower contact time with DOM in the water, therefore less DBPs). From an outsiders point of view, this would appear great. Adhering to legislation set in place to ensure safe drinking water and forming less DBPs seems like a win-win situation, doesn’t it…?
Recent research has shown that, although chloramine can form less THMs, the addition of ammonium to the treatment process can start forming nitrogenous DBPs, which although are found in much smaller concentrations than THMs, for example, have been found to be much more carcinogenic, toxic and mutagenic.
Therefore, research into the mechanisms of chloramine, and the different types of DOM it can be used to treat, is essential to compile evidence for new legislation to be passed to ensure future drinking water is of optimum quality.
This is where I come in – part of my PhD here in Bangor, UK, is to examine the differences between chlorination and chloramination of water draining different catchments (therefore containing different DOM characters) to see if I can identify a link between catchment characteristics and DBPs formed. To date, I have devised a chloramination method that can run side by side with an already devised chlorination method to produce comparable results on the disinfection of water from catchments with different characteristics. This method will hopefully soon be published, so keep your eyes out for a chloramination paper coming soon!