Water Journal : Water Journal May 2012
refereed paper contaminants of concern water MAY 2012 79 undertaken on the effect of the order of addition of chlorine and ammonia during disinfection have not been entirely in agreement (Portillo et al., 2008; Schreiber and Mitch, 2005; Pehlivanoglu-Mantas and Sedlak, 2006). However, the majority of the work suggests that the application of chlorine prior to ammonia reduces NDMA formation significantly (Hrabovsky et al., 2009; Schreiber and Mitch 2005). The reasons for this effect are two- fold; the addition of chlorine prior to ammonia is essentially a prechlorination step, resulting in oxidation of precursor material, and this method of disinfection reduces dichloramine formation. As mentioned, the pH of chloramination is an operational parameter that strongly affects NDMA formation. The maximum NDMA formation is within the pH range 6 to 8 (Mitch and Sedlak, 2002; Sacher et al., 2008; Portillo et al., 2008; Morran et al., 2009). Above and below this range the formation of NDMA decreases dramatically. This could be due to the combined effect of pH on chloramine speciation and reactivity of the amine groups on the precursor materials. Another very important influence on the level of NDMA in the distribution system is the reaction time between the precursor compounds and the disinfectant. Many studies have shown a continuous slow increase in the concentration of NDMA with time in the presence of a monochloramine residual (Charrois et al., 2004; Mitch et al., 2003). As a result, it can be expected that NDMA concentrations will be higher the further the samples are taken in the distribution system, and will be higher at a particular sampling point if the detention time has increased -- for example, in winter during low flows. Removal of Nitrosamines In the production and distribution of drinking water, chloramination is the most important pathway for the introduction of NDMA and other nitrosamines into the final product. These contaminants continue to form in the distribution system, and the contamination can be increased by leaching from rubber components such as O-rings installed in new pipework. The latter contribution can occur even in the absence of chloramines. Therefore, there are few practical measures that can be applied to remove nitrosamines from drinking water. Ultraviolet (UV) irradiation is the main avenue for NDMA reduction after chloramination or within the distribution system. NDMA is very susceptible to photolysis and can be destroyed if UV is used as a secondary disinfection step after chloramination, although in the presence of a chloramine residual it will continue to form in the distribution system. In one long distribution system in South Australia researchers found that on exposure to sunlight in an open storage situated approximately seven days' detention from the disinfection point, NDMA levels decreased by up to 90% (Cook et al., 2007). In wastewater treatment it is more likely that NDMA and other nitrosamines will be present in the influent, or be formed during chlorination at some stage in the treatment process. Therefore, removal through subsequent steps in the treatment plant is possible. Many studies have been undertaken to determine the efficacy of various processes, both at the full scale and in the laboratory. Processes that have been shown to be effective include UV irradiation, UV in conjunction with hydrogen peroxide and slow sand filtration (Schmidt et al., 2006; Swaim et al., 2008; Lee et al., 2005; Lee et al., 2011; Poussade et al., 2009; Sacher et al., 2008). Activated carbon can be effective for the removal of some nitrosamines; in the case of NDMA, this has been attributed to biological degradation (Schmidt and Brauch, 2008). Ho et al. (2011) studied the removal of NDMA, NDEA and NMor from wastewater treatment plant effluent by granular activated carbon filters. No biological removal was observed over a period of 300 days, and the three nitrosamines displayed very different physical affinities for the carbon. NDEA was readily removed early in the trial, with the efficiency decreasing to 30% by the end. NMor was removed well for approximately 100 days, and NDMA was not effectively removed. Both NDMA and NMor showed evidence of desorption on any decrease in the influent concentration of the contaminant. Ozonation and ozone/hydrogen peroxide may not be effective at doses that would be used for disinfection or microcontaminant removal (Wille et al., 2011; Pisarenko et al., 2012) and reverse osmosis is at best only partially effective (Poussade et al., 2009; Pisarenko et al., 2012). WHO Guidelines The World Health Organisation has issued a drinking water guideline level of 100ng/L for NDMA, but presently there are insufficient toxicological data to determine guidelines for the other nitrosamines. The Australian Drinking Water Guidelines (ADWG) now also provide a guideline of 100ng/L for NDMA as part of the recent revisions. The current Australian guideline value for NDMA in recycled water destined for augmentation of drinking water is 10ng/L. This is based on a 10-6 increased lifetime cancer risk, whereas the WHO and ADWG guideline is based on a risk factor of 10-5 (AGWR, 2008; WHO, 2008). The Australian recycled water guidelines also contain levels for NDEA (10ng/L) and NMor (1ng/L). Five nitrosamines are on the USEPA candidate contaminant list (CCL3) and are under consideration for regulation -- The UV disinfection unit at Mannum Water Treatment Plant.
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