Water Research Australia Water for the wellbeing of all Australians November 2013 Page 1

WHY DISINFECT WATER?The immediate and acute risks to human health that may be caused by pathogens in drinking water makes some form of disinfecti on indispensible, and chlorinati on has provided a very eff ecti ve means of minimising this risk. Pathogens include harmful viruses, bacteria and protozoa that, without treatment, may lead to health impacts, such as gastroenteriti s, or in the most severe cases can result in death.

CHLORINE IS THE MOST COMMON DISINFECTANTChlorine is commonly used as the primary disinfectant in drinking water supplies across Australia, and in many regions throughout the world. While chlorine’s disinfecti on ability has provided substanti al public health benefi ts, its interacti on with natural organic matt er (NOM) and inorganic precursors (e.g. iodide and bromide) that may be present in the source water can generate disinfecti on by-products (DBPs). Importantly, diff erent disinfecti on regimes may lead to diff erent forms of DBPs.

WHAT ARE DISINFECTION BY-PRODUCTS?DBPs are formed when compounds (called ‘precursors’) present in source waters react with disinfectants used in the disinfecti on process and lead to potenti ally harmful products. While more than 600 DBPs have been identi fi ed to date, only approximately 20 of them are adequately

understood. Consistent eff orts have been made to determine identi ti es and toxiciti es of various DBPs, especially those of the trihalomethanes (THMs) and haloaceti c acids (HAAs), and to model their formati on and control their occurrence.

NATURAL ORGANIC MATTER AND DISINFECTION BY-PRODUCTSMany source waters used for drinking contain natural organic matt er (NOM), considered a major ‘precursor’ for DBP formati on. NOM consists of humic substances (humic and fulvic acids) and non-humic matt er (proteins, amino acids, sugars and polysaccharides). NOM is derived from the degradati on of aquati c biota, fl ora and fauna, and is contributed to by anthropogenic organic pollutants from point and non-point sources along the waterways. NOM is a complex mixture of organic compounds which, due to variability in molecular size, weight and structure, is diffi cult to characterise using traditi onal analyti cal methods.

In a simplisti c sense, NOM is made up of parti culate and dissolved components. The dissolved component is known as dissolved organic carbon or DOC. Of the two, parti culate organic matt er does not play a signifi cant role in DBP formati on, so control of DOC becomes the criti cal factor in reducing the chemical risk of DBPs.

DOC , along with other ‘precursor’ compounds such as iodide or bromide that may be present in source waters, can react with the disinfectant to form DBPs.

Disinfecti on by-products are formed when compounds

in source waters react with disinfectants ...

Fact Sheet Water treatment and disinfecti on by-product formati on

Water Research Australia Water for the wellbeing of all Australians

EFFECTIVE WATER DISINFECTION – GETTING THE BALANCE RIGHTOne approach to minimise DBP formati on is to remove or reduce the NOM concentrati on in the initi al stages of treatment through conventi onal processes such as coagulati on. With variati on of existi ng plant practi ces, improvements may be achievable with limited change to plant infrastructure.

In operati on, water treatment plant operators need to strike a fi ne balance between competi ng chemical (i.e. DBP) and pathogen risks in water treatment to ensure safe drinking water. This trade-off is highlighted in the fi gure below (Microbial and chemical risk trade-off analysis for disinfecti on, Sadiq and Rodriguez, 2004).

Adequate disinfecti on is criti cal to avoid acute microbial risks but if water is ‘over-disinfected’ the chemical risk of DBP formati on may increase. To provide adequate disinfecti on of potable water supplies, the typical chlorine dosage at fi nal disinfecti on is 1.5 to 2.5 mg/L, however this is greatly dependent on water temperature and contact ti me.

The amount of NOM present in the source water prior to chlorine additi on bears a direct relati onship on the amount and character of DBPs formed, but not necessarily a direct relati onship to the toxicity.

CHLORINATION CONTROLThe fi nal simple means of operati onal management for DBP formati on is through alternati ve chlorinati on strategies. Chlorine dose, pH, contact ti me and temperature are all criti cal parameters than can vary considerably in the treatment process. High chlorine doses increase concentrati ons of chlorinated DBPs. Ambient temperature

(i.e. seasonal eff ects) have also been noted, with warmer climates resulti ng in potenti ally more DBP formati on, as more chlorine must be added to ensure an acceptable disinfectant residual.

REGULATORY CONTEXT In most industrialised countries, including Australia, THMs are regulated, however more stringent DBP regulati ons are being suggested for other DBPs, including HAAs and inorganic DBPs (e.g. bromate and chlorate), due to health concerns regarding potenti al reproducti ve, carcinogenic and cell mutati on (mutagenic) eff ects. The THMs are also treated as a regulatory surrogate for other less prevalent and diffi cult-to-measure chlorine DBPs.

Although exposure to DBPs through drinking water presents a potenti ally low level chronic risk to health, exposure through other pathways such as through dietary intake, has been shown to be more signifi cant.

Some of the informati on in this fact sheet was derived from a WaterRA project (No. 1041-10) ti tled ‘Characterisati on of DBP Formati on for Water Quality Management’ by A/Prof Mary Drikas, Dr Andrew Humpage and Mr Rolando Fabris. This project was funded by Water Research Australia, Water Corporati on (WA), SA Water and Central Highlands Water (Victoria).

This series of fact sheets was initi ated by the Cooperati ve Research Centre for Water Quality and Treatment (CRC WQT) in 2003. The fact sheets are largely based on research carried out in the CRC WQT and its successors, WQRA (2008-2013) and Water Research Australia (WaterRA, 2013 - ).

Since 2008 the research has been funded enti rely by the Members of WaterRA, who comprise Australian water uti liti es, universiti es, engineering and consulti ng companies and government agencies. Our Members provide annual contributi ons in the form of money, ti me, experti se and other in-kind contributi ons. The research conducted investi gates issues of concern to the water industry and will benefi t all Australians through improved water treatment and quality.

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DBPs

Disinfecti on level

Regulatory regime

Microbial

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