Water Journal : Water Journal April 2014
112 Technical Papers The impact of DOC concentration increase on distribution system water quality was immediate. The water quality pro le shown in Figure 4 for Karoonda, located 60km from the Tailem Bend WTP, was typical for locations in the outer regions of chloraminated distribution systems. As treated water DOC concentration increased (Figure 4a), monochloramine residual decreased (Figure 4b) at Karoonda, in spite of an increase in WTP product monochloramine concentration. Reduction of monochloramine residual resulted in nitri cation (Figure 4c), as indicated by the corresponding increase in oxidised nitrogen (nitrite and nitrate) and decrease in free ammonia. In response to the change in monochloramine residual pro les in distribution systems, monochloramine decay was assessed in WTP product water. Monochloramine decay pro les were determined in WTP product water samples taken immediately after disinfection by monitoring monochloramine residual over seven days, as shown in Figure 5. The greater stability of monochloramine in Morgan WTP product water can be clearly seen, with the average three-day monochloramine demand being 1.5mg/L lower than the other WTP product waters (Table 2). A puzzling feature of the decay pro les in Figure 5 was the greater stability of monochloramine in Morgan WTP product water. All the WTPs have similar treatment processes (coagulation/ occulation/sedimentation/media ltration) and utilise the same source. At Morgan WTP, chlorine is added prior to ammonia, compared with the other WTPs where ammonia is added before chlorine. To determine if the chloramination strategy at Morgan WTP could account for differences in monochloramine decay (Figure 5), WTP product waters were chloraminated using uniform conditions (Table 1). In the laboratory the same trend (Figure 6a) was determined as described above, meaning that free chlorine contact prior to ammonia addition at Morgan WTP is not responsible for differences in monochloramine decay measured (Figure 5). Table 2. Average three-day monochloramine decay in product waters from Morgan, Tailem Bend, Summit and Swan Reach WTPs -- January/February 2011. WTP product water Three-day monochloramine demand (mg/L) Morgan 1.0 ± 0.3 Summit 2.5 ± 0.3 Swan Reach 2.4 ± 0.2 Tailem Bend 2.5 ± 0.5 1/01/2009 1/01/2010 1/01/2011 1/01/2012 1/01/2013 1/01/2014 0 2 4 6 8 10 12 14 16 18 20 % DOC Removal River Murray, Morgan Morgan WTP product water DOC concentration (mg/L) 0 10 20 30 40 50 60 70 Percent DOC removed Figure 3. Morgan WTP inlet and product DOC concentration and percent DOC removal. 0 1 2 3 4 5 6 7 8 1/01/2009 1/01/2010 1/01/2011 1/01/2012 1/01/2013 1/01/2014 0.0 0.2 0.4 0.6 0.8 1.0 1.20 1 2 3 4 5 DOC DOC (WTP outlet) NH2Cl (mg/L as Cl 2) a) c) NH3-N NO2 + NO3 NO2 + NO3 (WTP outlet) Free NH 3 (mg/L as N) and NO2 + NO3 (mg/L as N) NH2Cl NH2Cl (WTP outlet) b) DOC (mg/L) Figure 4. Water quality at Karoonda (full data points) and Tailem Bend WTP product water (open data points): a) DOC concentration; b) monochloramine residual; and c) free ammonia and oxidised nitrogen. WATER QUALITY & MONITORING WA DOWNLOAD THE OZWATER'14 SMARTPHONE APP View the conference program, plan your day's schedule, nd out essential event information and have your say on the best speakers and sessions via our conference app, available for download at www.ozwater.org.
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