Water Journal : Water Journal April 2011
technical features 132 APRIL 2011 water water supply options of community perception based on opposition in the past to desalination; • Seawater, sewer mining and Melbourne Water's Western Treatment Plant (WTP) have the best yields and supply security; • Polluted/saline water is likely to require more treatment than collection and harvesting of rainwater and stormwater; • Single source water is preferred as it is difficult to match water quality of different sources in the same centralised system; • Operation of a single plant is preferred over multiple plants. Short List of Options The assessment showed water from WTP was preferable, with seven of the eight highest-scoring options incorporating WTP as the primary water source. The working group concurred it was preferable to incorporate only the highest-ranked WTP options and include other options to ensure a range of water supply sources were considered. The short list of options was, therefore, refined to include four centralised and four localised options, with a range of water sources. Table 2 summarises these options. Selection of Options for Concept Design To refine the eight options further, cost estimation and MCA were combined to give cost-benefit ratios. The MCA used non-monetary criteria to score each option. Five key criteria were used to evaluate the eight options which were further broken into 19 sub-criteria to allow more detailed analysis (Table 3). City West Water's key driver for this project was to find at least 5GL/year of alternative water; therefore, the availability of supply was given the highest weighting of all criteria. It was important to use quantitative data where possible, as qualitative data can show bias towards or against certain technologies by the person(s) scoring the criteria. The MCA tool also performs sensitivity analysis, and identifies options which could rank differently if a score is slightly altered. These scores can then be interrogated to assure bias is not being exhibited. Criteria were "sloped" where required -- for example, high BOD is considered a negative -- therefore, the slope was inverted so that the higher BOD number resulted in a lower overall score. Multi-Criteria Analysis Water supply (quantitative) WTP water ranked highest in terms of water quality as the biological oxygen demand (BOD) and other contaminants are negligible after treatment. Source water from sewer mining scored the lowest due to high BOD and TDS. The security of water was scored using the yields identified in Table 1, with water sources that are not rainfall-dependent scoring better than options which are. Operability (qualitative & quantitative) The options which used single source water and were centralised ranked higher for operability. This is due to multiple source waters being harder to treat and more treatment plants requiring more operator input. Table 2: Summary of eight short-listed options. Key features Option 2 Option 3 Option 11 Option 15 Option 18 Option 19 Option 21 Option 29 Servicing strategy Centralised Centralised Centralised Centralised Localised Localised Localised Localised & decentralised Water source Sewer mining (13.7ML/D) WTP Class C water (13.7ML/D) Sewer mining (11.6ML/D) + groundwater (2.1ML/D -- 3 bores) WTP Class C water (6.8ML/D) + groundwater (6.8ML/D -- 8 bores) Stormwater (1.4ML/D)+ groundwater x2 (5.9ML/D)+ sewer mining (6.4ML/D) WTP Class C water (6.2ML/D) + sewer mining (7.5 ML/D) Sewer mining (6.4ML/D) + groundwater x 2 (7.3ML/D) Stormwater + groundwater + sewer mining Location Princes Hwy/ Western Ring Rd ATP or WTP Princes Hwy/ Western Ring Rd ATP Truganina Swamp, Mobil, Australian Vinyls & Princes Hwy/ Western Ring Rd ATP and Princes Hwy/ Western Ring Rd Princes Hwy/ Western Ring Rd, ATP & Australian Vinyls Located at individual industrial sites Treatment train(s) MBR +RO MF/UF + RO MBR+RO MF/UF + NF/ RO MF/UF + RO MF/UF + NF/ RO MF/UF + RO MF/UF + NF/ RO Constructed wetlands + storage MF/UF + RO MBR+RO Constructed wetlands + storage MF/UF + NF/ RO MF/UF + RO MF/UF + NF/ RO Constructed wetlands + storage Transfer pipeline WTP to ATP None 23kmof DN 450 None 23kmofDN 450 None 23kmof DN 450 None None Distribution network 19km of DN 150 to DN 375 23.5km of DN 150 to DN 450 19km of DN 150 to DN 375 17km of DN 150 to DN 375 12km of DN 150 to DN 375 18km of DN 150 to DN 375 14km of DN 150 to DN 375 None, most is localised MBR: Membrane Bioreactor, UF: Ultrafiltration, NF: Nanofiltration, MF: Microfiltration, RO: Reverse Osmosis.
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