Water Disinfectants & disinfectant by-products
1. What disinfectants and by-products are we talking about?
- 1.1 Why is there concern about water disinfectants?
- 1.2 How are disinfectant by-products formed?
- 1.3 What are the most relevant disinfectant by-products?
- 1.4 How can the amount of disinfectant by-products be reduced?
1.1 Why is there concern about water disinfectants?
Chlorine has been used very widely to kill germs in drinking water and fight waterborne disease. However chlorine reacts with natural material in the water to form a range of disinfectant by-products (DBPs) of public health concern. Therefore, alternative chemical disinfectants are increasingly being used, either alone, in addition to chlorine, or in combination with one another. They include ozone (O3), chlorine dioxide (ClO2) and chloramines (NH2Cl). However, each has been shown to form its own set of disinfectant by-products (DBPs).
There is thus a need to better understand the health risks associated with each mode of disinfection and to decrease the chemical risk without compromising the microbiological safety of drinking water. More...
1.2 How are disinfectant by-products formed?
Disinfectant by-products (DBPs) are formed when chemical disinfectants react in the water either with natural organic matter (NOM) or with bromide ions (Br-). Natural organic matter (NOM) comes mainly from the breakdown of plants and organic matter in soil and Br- comes from natural sources and from wastewater discharges. The reactions of disinfectants are influenced by the physical characteristics of the water, such as pH and temperature, and by treatment conditions, such as disinfectant dose. When inorganic bromide is present, it can also take part in the reaction to produce brominated by-products.
When the disinfectant is chlorine, the reaction gives rise to halogenated (addition of chlorine or bromine) by-products such as trihalomethanes, haloacetic acids, chloral hydrate, haloketones, haloacetonitriles and halogenated hydroxyfuranone derivatives. When bromide is also present in the water, a mixture of different by-products containing varying amounts of chlorine and bromine will be formed.
When the disinfectant is ozone, it oxidizes natural organic matter to form a number of by-products, with aldehydes being dominant. If bromide is present ozone can form brominated compounds similar to some of those produced by chlorine, and also inorganic bromate.
When the disinfectant is chlorine dioxide, it does not appear to form many by-products, but it does break down to form chlorite and chlorate.
When the disinfectant is chloramine, it generally leads to the formation of cyanogen chloride and significantly reduced levels of chlorine disinfectant by-products, but it can also form nitrite in the distribution system if it is not properly controlled. More...
1.4 How can the amount of disinfectant by-products be reduced?
The amounts of disinfectant by-products (DBPs) formed can be reduced by:
- reducing DBP precursors (e.g. NOM, Br- ions) which react with disinfectants to form disinfectant by-products (DBPs):
- optimizing the use of disinfectant by combining different disinfectants at different stages, i.e. at the treatment plant (primary disinfection) and in the distribution system (secondary disinfection). Ozone can be used exclusively as a primary disinfectant, chloramines exclusively as a secondary disinfectant, and both chlorine and to a lesser extent, chlorine dioxide, in either role. It is less usual for chloramines to be used as a primary disinfectant because it acts much more slowly.