Disinfection By-Products: Cause and Consequence
With recent soaring temperatures and seasonal rainfall, conditions are just right for Disinfection By-Products (DBPs) levels to rise within reticulation networks.
What are they?
As the name suggests, DBPs are by-products from the chemical reaction of disinfecting agents (most commonly chlorine) and naturally occurring organic compounds (such as humic and fulvic acids). Organic compounds are present in both surface and ground water sources, and heavy rainfall tends to increase the level present in raw water. Of the types of disinfection by-products which are produced, trihalomethanes (THMs) are usually present in the largest concentration (ADWG, 2011). Variables which affect the concentration of DBPs produced during disinfection are: the concentration of natural organic matter in raw water, the disinfectant used, concentration of dosed disinfectant, pH, temperature and disinfection contact time.
Although several different categories of DBPs exist, as chlorine is the most common disinfectant, this article will concentrate on the discussion around THMs.
What is the risk?
Studies have suggested a link between DBPs and various forms of cancer, particularly in the bladder and the rectum though more research is required to confirm this link (ADWG, 2011). In response to this, the Australian Drinking Water Guidelines stipulates a maximum health based THM concentration of 0.25 mg/L.
Though regulated DBPs are not sufficient to account for toxicity in drinking water, other unregulated DBPs may in fact be causing harm. There is as yet no concrete evidence of the mechanism or causing agent behind the cancer risk identified in studies, suggesting that there may be a missing piece of the DBP puzzle.
Reticulation networks in Australia pose a high risk in comparison to other countries due to high temperatures, long distribution systems (due to geographical sprawl) and above ground pipelines commonly being used to supply regional areas (again increasing temperatures of water). As discussed in the section above, high temperatures will result in an increase in reaction speed between disinfecting agents and precursors (ADWG 2011).
How to measure them
Methods used to measure THMs include head-space analysis, solvent extraction, purge and trap, and direct collection on resins. Solvent extraction amongst these being relatively simple, involving adding sodium chloride to the sample and THMs being extracted using a solvent, which is then analysed via gas chromatography, with limits of determination at <20ng/L (ADWG, 2011).
The ADWG recommends that actions to reduce THMs (and therefore DBPs) must be undertaken, although at a level where disinfection is not compromised, as the absence of disinfection still outweighs THMs in terms of risk. As capabilities to measure and investigate DBPs continue to develop, management strategies will also continue to evolve. Current strategies as recommended by the ADWG include:
Removing precursors (i.e. organic compounds in raw water) via ion exchange, activated carbon, coagulation & filtration, oxidation with ozone or dosing potassium permanganate.
Removing THMs once formed, including using air stripping or the use of granular activated carbon.
Using alternative disinfection methods, such as chloramination, ozonation with chlorine dioxide, though these will also product other DBPs which must then be managed.
- National Health and Medical Research Council, Natural Resource Management Ministerial Council, 2011, Australian Drinking Water Guidelines Paper 6 National Water Quality Management Strategy, National Health and Medical Research Council, National Resource Management Ministerial Council, Commonwealth of Australia, Canberra
- Liew D, Linge K, Kristiana I, Joll C, Cadee K, Charrois J, 2015, Australian DBP Research: Identifying Impacts on the Water Industry, Water Research Australia Limited, Adelaide