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CONFERENCE: NZWWA 50th Anniversary Conference, Christchurch, New Zealand, September 2008

ABSTRACT:
Whangarei District Council (WDC) operates the Ruakaka Water Treatment Plant, which services the local area to the south of Whangarei.  The source waters are the Flygers River and the newly constructed Wilsons Dam.  Both waters contain dissolved organic carbon (DOC) in the range of 4-10 mg/L, alkalinity of between 20-40 mg/L and high levels of Iron and Manganese (0.5-3 and 0.1-0.3 mg/L respectively).
The current treatment process at Ruakaka WTP comprises coagulation, clarification, pre-chlorination, filtration followed by chlorination for disinfection.  Pre-chlorination is effective in the reduction of iron and manganese levels but also results in the formation of Trihalomethanes (THMs) in excess of the 1.0 MAV standard.
Water analysis confirmed that this is because of the Wilson’s Dam has a high component of recalcitrant DOC that is very difficult to remove using standard hydrolysed metal salt (HMS) coagulation, such as aluminium sulphate.  This residual DOC in the water reacts with the chlorine used for disinfection generate to levels of THM’s that exceed the current and proposed Drinking Water Standards for New Zealand 2005 (DWSNZ 2005) standards.
DOC was characterised on the basis of size, charge, and a solubility function for both raw water sources, and respective treated water samples.  The impact of the MIEX® Process and alum coagulation was able to be determined from the results.
Extensive concurrent trials were carried out comparing  four technologies namely Enhanced Coagulation, GAC, Ozone/GAC and the MIEX® Technology. The trial results demonstrated that the MIEX® Technology out performed GAC and Ozone/GAC with respect to the reduction of THMs.  Conventional alum coagulation was producing THM MAV in excess of 1.4.   MIEX® Technology followed by coagulation with significantly reduced alum dose (50-80 %) resulted in a THM MAV average of 0.42.
Conclusions on the performance of the  four technologies will be presented with regards to:

• Removal of different fractions/molecular weight of organics.
• Effect on Alum coagulation.
• THM precursor removal performance.
• Effect of pre-treatment with MIEX® Process on downstream processes.

The paper will also discuss and compare the   four technologies with regard to:

• Process control/performance.
• Existing conventional plant operation.

Benefits to the downstream reticulation system.

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