Water Journal : Water Journal April 2011
water APRIL 2011 131 water supply options Servicing Strategies Four different servicing strategies for delivery were considered for the project: 1. A centralised treatment scheme that services all customers, supplied by one or more sources of water. 2. Localised scheme (customers grouped into four areas), supplied by one or more water sources. 3. Localised/decentralised scheme, where some sites are grouped together and remotely located sites have their own plant/supply. These can be supplied by one or more sources of water. 4. Decentralised scheme with individual plants/water supplies for each customer, supplied by one or more sources of water. Long List of Options Servicing strategies and water sources were combined to develop the initial list of 30 options. These options were ranked to enable shortlisting. Five criteria were considered for the initial ranking: 1. Environmental and cultural (impact to flora and fauna, aboriginal sites, streamflows etc). 2. Source water quality (TDS, contaminants etc). 3. Social acceptance (aesthetics, noise, footprint etc). 4. Security of supply/yield (i.e., rainfall less secure than sewer mining). 5. Energy use. Only options which were considered technically feasible were included in the list of 30 options, hence feasibility was not scored. Cost was also excluded at this stage. Each water source was ranked from 0 (worst) to 5 (best). For options using more than one water source, the ranking was weighted based on the percentage of that water source used in the water supply. For example, water supply with 50% sewer mining and 50% groundwater had a ranking score calculated as: 0.5 x sewer mining base score + 0.5 x groundwater base score The key findings were: • Using stormwater from drains discharging to waterways is preferable to extracting from waterways; • Both seawater and sewer mining are likely to have higher TDS than groundwater; • Seawater desalination and extraction from the Lower Kororoit Creek are likely to be less favourable in terms Table 1: Summary of water supply yields. Source Pros Cons Yield Seawater No limit to supply. High TDS -- Reverse osmosis needed (energy-consumptive). Quality in Port Phillip Bay affected by stagnation and discharges. Brine dispersion issues -- shallow with low current. Community opposition. 5GL/yr Groundwater & aquifer storage recovery (ASR) Deep aquifer -- low total dissolved solids (TDS), water not allocated. ASR can store surface water while providing some treatment. Approximately 150--200m below ground. Several extraction points required to avoid depression cone. Shallow aquifer has risk of contamination due to local industry.1 ~3.2GL/yr Sewer mining Numerous sewers in City West Water Catchment with yields ranging from 0.5GL/yr to 3.2GL/yr. WTP has potential for high yield, but this is dependent on current flow allocation. Risk of intrusion from poor quality groundwater in shallow aquifer where most sewers are located. 5GL/yr Western Treatment Plant (WTP) Source of treated wastewater -- reduces treatment costs. WTP already has significant recycled water allocation. Discussion with MWC indicates 5GL/yr + available. 5GL/yr Altona Treatment Plant (ATP) Source of treated wastewater -- reduces treatment costs. Water is fully allocated. 0GL/yr Surface water Several sources 2,3 -- Lower Kororoit Creek (4.2GL/yr), Truganina Swamp (1.4GL/yr) and Cherry Lake (0.5GL/yr) Must be collected prior to entry into waterways and only approximately 20% can be harvested, storage required for peaks. Community opposition; wetland significance/Friends of Lower Kororoit Creek. 5GL/yr Rainwater Clean source of water (except first flush2). Further development may occur -- impacting the yield. 0.11GL/yr Industrial water Good for decentralised systems. Less distribution infrastructure. Increased costs and operational requirements due to greater number of plants required. Ranging from 0.5GL/ yr -- 2GL/yr 1 CH2M HILL (2007) 2 Based on 370mm/year annual average rainfall lowest in last 10 years (The Weather Company, 2009), the catchment size (Melbourne Water 2000) for each water source, and a runoff coefficient of 0.3, with 20% of the total yield available for use. 3 Based on 370 mm/year annual average rainfall, and runoff coefficient and catchment size from Maunsell Report (2006).
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