technical features
water supply seven days in winter (incl uding rhe three days in the service rank). From Figure 2 a dose rare of 4.2 mg/L gives a chlorine residual of 0. 10 mg/L after seven days . Therefore a dose rare of approximately 4.3 mg/L achieved the target of 0.20 mg/L at rhe extremity of che reticulation.
refereed paper
Table 3. Typ ical Quick Reference Sheet, given to operators, for Lake King Dam waters.
Quick Reference Sheet Interpreting rhe information gathered from the T HMFP and chlorine decay experiments enabled a better understanding of the im pact of ch lorine dose rare on the T HMFP and chlorine decay behaviour. T his information was disseminated to the operators in rhe fo rm of an easy to read 'Quick R eference Sheer' shown in Tab le 3. The THM concencrarion values in Table 3 assume the chlorine is completely consumed, to provide rhe operators with a worst case scenario. The optimum dose race for Lake King Dam waters was therefore between 4.0 to 4.5 mg/L, with a target of 4.3 mg/L. Since che implementation of the operatio nal strategy as detailed in che Quick Reference Sheet, operators have been able ro achieve residual in the reticulation system an d maintai n THMs below the health gu ideli ne value.
Denmark Distribution System Den mark is located in che G reat Southern region of Western Australia, 54 km west of che regional centre of Albany (4 10 km sou ch of Perth). The town water supply is drawn fro m Quickup Dam located 6.5 km north ease of che town. Quickup Dam water is created by sedimentation, clarificat ion and filtratio n. T he created water is disinfected by chlorine gas as it leaves the Water Treatment Plan t, pumped into Horsley Road Reservoir and then pumped into Horsley Road Summ it Tank. From rhe Su mmit Tank water is gravity fed inco che reticulation system or pumped co various service tanks. In March 2002, chlorination dose rare was increased to overcome a detection of coca! coliforms in the reticulation. As a consequence, che TH M concencracions exceeded the heal ch guideline value of 250 Âľg/L on several occasio ns. W hile no t an immediate health co ncern, chis trend concerned both rhe WCWA and the Department of Health. This concern prompted an investigation inco che reaso n for che exception, and co determine shore term measures chat wou ld manage che T HM issue, while awaiti ng enhanced treatment for NOM removal.
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--
7
7 E 6 __, -; 5 = t>l)
.
"O
¡i;;
4
~
3
0
2
....
-...- 2. Jmg/L
- . . - 4.2 mg/L
-
-
6.3 mg/L
8.4mg/L
-
J
I
0 0
20
40
60
80
100
120
140
160
180
Time (hours)
Figure 2. C hlorine residual versus time for Lake Ki ng Dam waters.
(. lOWARA
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Bro"'.Vn Brothers 1 !'.,lf)'jf II Engineers Australia Pty Ltd
Journal of the Australian Water Association
Water
MAY 2007 79