Water Journal : Water Journal September 2012-1
potable reuse refereed paper technical features 56 SEPTEMBER 2012 water the current PHR of 7 mg/L. It remains, however, safer to compare the highest concentrations with the guideline value. Based on its MoS of 87.5 (Table 3), bromide is assigned to group 3h (low priority). It should be noted that ozonation of high bromide water can lead to formation of bromate, although at the current time ozone is not employed as a treatment process in any of the drinking water treatment plants located downstream of the PRW discharge point for augmentation of drinking water supply into Lake Wivenhoe. Probability distribution function and the natural barrier This left 40 physical/chemical parameters that were detected with sufficient frequency to produce a reliable PDF. Most of these parameters are, in fact, aesthetic or operational parameters and, therefore, do not have a health-based guideline value (e.g., pH, total organic carbon, conductivity, total hardness). However, a few do have a health-based guideline value, such as the disinfection by-product bromodichloromethane (BDCM). Based on the data trends from 13/5/2009 to 23/1/2012, BDCM is expected to exceed the current PHR of 6 μg/L in 30% of cases (Figure 5). Most of the exceedances occurred between 18/10/2010 and 14/6/2012, and are thought to be due to the long residence time of water in the pipeline during that time of low PRW demand. BDCM is a disinfection by-product formed during chlorination, and attempts to control BDCM exceedances should never compromise proper disinfection. Also, note that the 2012 draft PHR guidelines have eliminated the guideline of 6 μg/L for BDCM and replaced it with a total THM (covering BDCM, DBCM, chloroform and bromoform) of 250 μg/L, which BDCM alone would be very unlikely to exceed (<0.1% probability). Even combined, the 99.9th percentile for chloroform (90 μg/L), bromoform (1.2 μg/L), BDCM (43 μg/L) and DBCM (24 μg/L) would only add up to a total THM of 158.2 μg/L, below the proposed guideline of 250 μg/L. This means that while BDCM is currently in Category 1 (high priority), it will drop to Category 3 (low priority) if the 2012 draft PHR standard does indeed assign a total THM guideline of 250 μg/L. Furthermore, in the event that PRW is supplied to Lake Wivenhoe to augment drinking water supply the water age will be significantly less and, as a result, the concentration of BDCM will reduce significantly and would be unlikely to exceed the current PHR standard under those operating conditions. Figure 6 shows the predicted concentration of BDCM from PRW at the Dam Wall (Figure 6). Hawker et al. (2011) predicted a 99.8% removal of BDCM in Lake Wivenhoe, mostly due to volatilisation. Using this predicted removal and a predicted percentage of PRW ranging from 1-30% at the Dam Wall from the hydrodynamic model conducted using environmental variables applicable during the monitoring window (Badin et al., 2011), BDCM is expected to be present at significantly lower concentrations at the Dam Wall, several orders of magnitude lower than the current PHR guideline value of 6 μg/L. Figure 4. Limited reliability probability distribution function for bromide. Figure 5. Probability distribution function for bromodichloromethane showing the current and proposed PHR guidelines. Figure 6. Probability distribution function for bromodichloromethane incorporating estimated attenuation and dilution. Hydrodynamic modelling scenario based on simulations using constant PRW inflow and climate and catchment forcing data from the period from 1 February 2008 to 31 December 2009.
Water Journal November 2012-1
Water Journal August 2012