Water Journal : Water Journal April 2011
stormwater use refereed paper technical features 128 APRIL 2011 water Figure 12: Schematic of typical system. The Toolkit's cost estimates are primarily based on the extrapolation of data obtained from the roof water harvesting system implemented in Warrnambool. It allows for construction cost variation across Australia by applying appropriate cost factors and rainfall variation using different temporal rainfall patterns and spatial annual rainfall data from the Bureau of Meteorology. The Toolkit is for preliminary screening of options on a relative cost basis and should not be used as a design or cost estimation tool. Calculations rely on a number of assumptions and relationships derived from limited data sets. Results are suitable for preliminary screening only and should not replace a more detailed assessment by an appropriately qualified and experienced person. The Toolkit is available on a CD from Wannon Water (visit wannonwater.com. au to order a copy), and the program may be downloaded onto a PC using Microsoft operating systems. It has a self-contained help menu to guide users through its use. Using the Toolkit The flexible setup of the Toolkit allows a range of roof water harvesting systems to be defined with different arrangements for the transfer pipelines, storages and treatment plant. In Warrnambool's case, the storage basin, treatment plant and transfer pipelines already existed, so no capital cost was associated with these elements, but the operating cost is accounted for. Selecting which elements exist, or are not required, for a particular scenario is as simple as ticking a 'check box'. The transfer pipelines are able to be configured as gravity or pumped pipelines. Data Input Collection system Location of the development, scale and other collection network design parameters are input using four data input screens. Storages The capacity and configuration of storages are input on a single screen to define the inlet and outlet levels and surface area if it is uncovered. This allows evaporative losses to be calculated and included in the water balance. Pipelines The single input page allows for a gravity or pumped pipeline to be selected. The gravity system assumes the pipeline is a fully sealed system able to operate under pressure with its capacity determined by the upstream and downstream elevations. The pumped system selects the required pump size to deliver the nominated flow along the nominated pipe size. Treatment plant Again this element can be included or excluded from the Toolkit calculations depending on whether treated or untreated water supply options are being investigated. Treatment plant costs can vary significantly depending on the nature of the treatment plant, so provision has been made to override both the capital and operating costs to suit the scenario being evaluated. Data output The Toolkit calculates the volume of water spilt from the storages, transferred along the various elements and yield from the system. The yield is compared to the annual demand for these properties (based on the data input) and the percentage of annual demand supplied from the roofs calculated. It also determines the capital and operating costs of the various elements and the net present cost/ML of water harvested where the volume is also discounted using the same formula as costs. This cost is then able to be compared with other options being considered to determine if the roof water harvesting system should be considered further as an option. One of the advantages of the system is that it can be progressively built as development proceeds, resulting in a lower NPV/ML than up-front capital options. This is not reflected in the NPV calculation, meaning more detailed work is required to evaluate this effect. One of the powers of the Toolkit is being able to trial different storage, pipeline and treatment plant sizes to optimise the cost/ ML and/or the yield. Each 'run' takes a matter of seconds. Toolkit Results for Australian Major Cities The Toolkit was used to determine the yield from a regional roof water harvesting system (the same as that being demonstrated in Warrnambool) located in each of the Australian State capitals. The volume of water flowing from roofs and paved areas was calculated using the Toolkit and compared to the average annual household demand in those cities. Warrnambool data is shown for comparison in Figure 13. In outer suburbs of Sydney and Brisbane the harvestable roof water significantly exceeds the water demand of these new houses, while in Melbourne and Warrnambool the water demand is matched. It should be noted that the residential consumption data used for this comparison is sourced from the Water usage per household vs Water runoff from lot 0 100 200 300 400 500 600 Canberra Brisbane Adelaide Sydney Perth Melbourne Warrna mbool Capit al City Volume (kL) Runoff from ground s urfaces per year Potential roof water capture per year 2010 household usage Figure 13: Comparison of available roof water to residential demand. Assumptions: Allotment size 750 square metres -- 1/3rd roof area, 1/3rd paved, 1/3rd garden/lawn. Runoff coefficients: roof 0.85, paved 0.8 and garden/lawn 0.2. Water restrictions applied for some cities in 2010. Warrnambool water consumption from 2009/10. Using historical average rainfall and typical temporal patterns.
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