Water Journal : Water Journal May 2012
contaminants of concern refereed paper technical features 78 MAY 2012 water most widely used and, therefore, the most important precursor in water treatment (Wilczak et al., 2003). This effect has been attributed to the formation of DMA during degradation of these polymers (Park et al., 2009). In addition, DMA was reported to be released from polyDADMAC on ozonation, thus increasing NDMA precursor material (Padhye et al., 2011). Natural organic material containing nitrogen functional groups and nitrogenous microbial products, for example, metabolites of algae or cyanobacteria, or biofilm material, have been identified as precursors (Chen and Young, 2008; Gerecke and Sedlak, 2003; Krasner et al., 2010). In contrast, Sacher et al. (2008) suggested that natural organic material is not a major precursor of nitrosamines, and the major precursors are contaminants associated with wastewater plant and industrial effluent. The same authors tested a range of precursor materials and concluded that NDMA was formed to a far greater extent (1--2 orders of magnitude) than the seven other nitrosamines analysed, and in most cases NDMA was the only nitrosamine detected as a by-product of chloramination of the precursor materials. Confirmed precursors include a range of agricultural and industrial contaminants, such as the herbicides diuron and chlortoluron, and the pharmaceutical ranitidine (Sacher et al., 2008; Chen and Young, 2008; Xu et al. 2012). Due to the various precursor materials associated with wastewater, drinking water sources impacted by wastewater effluent are considered at high risk of nitrosamine formation. In practice, nitrosamines are present in such low concentrations it is very difficult to identify specific precursor compounds, and often formation-potential tests, similar to those used to measure trihalomethane formation potential, are used to determine the level of precursors in a water source (Mitch et al., 2003). The test involves dosing the water sample with an elevated level of pre-formed monochloramine, usually around 60mg/L; the sample is then left in the dark for several days. The nitrosamine concentration (usually only NDMA) is measured and this is described as the NDMA formation potential (NDMAfp). The value doesn't always reflect the concentration that may be found in the distribution system, but is an indication of the maximum concentration that may be formed under extreme conditions, and is considered the "potential" of that water sample to form NDMA. A similar test was used by Labernik et al. (2010) to determine the efficacy of various treatment processes for the reduction of NDMA precursors. The authors found that coagulation in the presence of polyDADMAC increased the concentration of precursors, as did filtration through a biologically active sand filter. Morran et al. (2009) determined the level of NDMA precursor material through several conventional treatment plants. They reported that, in general, the formation potential increased through the plant for those plants using a cationic polymer as a flocculant aid, whereas a plant using alum alone for coagulation displayed minor changes of precursor concentration through the treatment plant. Sludge lagoon supernatant displayed a very high concentration of precursors for the plants using cationic polymers in drinking water treatment or sludge treatment, impacting on the raw water quality due to supernatant recycling to the head of the plant. Controlling Formation The first step in the minimisation of formation of nitrosamines is the removal of the precursor compounds. One of the most effective processes is reverse osmosis, with more than 99% removal of precursors from recycled water reported by Farre et al. (2011). Other authors report that precursors are effectively removed by activated carbon, with the combination of ozone and granular activated carbon one of the most effective combinations tested (Sacher et al., 2008). Some control of nitrosamine formation can be achieved by oxidation of water prior to disinfection, due to oxidation of the most important precursor functional groups. (Chen and Valentine, 2008; Lee et al., 2007). Ozone and chlorine dioxide were both found to be effective oxidising agents, showing similar deactivating effects for NDMA precursor material (Lee et al., 2007). Ozone alone, and in combination with hydrogen peroxide, was reported to be effective for the removal of NDMA precursors by Pisarenko et al. (2012). Pre-chlorination has been found to be particularly effective, although there is some disagreement in the literature regarding the chlorine exposure (CT) required for effective precursor inactivation -- possibly because the ease of oxidation is dependent on the structure of the precursor material (Hrabovsky, 2009). Similarly, the results of the research In Australia the most important reason for applying chloramines for disinfection is to ensure a residual throughout the system, particularly in long pipelines. In the US it is often used as an alternative to chlorine to minimise chlorinated DBPs to comply with the Stage 2 DBP rule.
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