Water Journal : Water Journal April 2011
refereed paper stormwater use water APRIL 2011 127 Quality and Yield The works associated with the demonstration of the roof water harvesting principle in Warrnambool have currently progressed to a stage where 11 houses are connected. Water from the tiled roofs of these houses gravitates to a sump where it collects before being pumped over the bank into the storage (Brierly Basin). This has allowed water quality testing and the volumes collected to be measured. Preliminary data is provided in Figures 1 to 10. As expected, the colour and turbidity are lower for the rainwater than the river water, as are aluminium, iron and TDS. Total alkalinity and pH are higher, probably due to the carbonates leaching out of the new concrete sump. Given the softness of the Gellibrand water, this will improve the water chemistry for subsequent treatment. E. coli is of a similar magnitude for both (except for one sample), but the total plate count and total coliforms are significantly higher for the roof water samples than the storage. The samples are taken from within the sump where water can sit for some time if it has not been raining. Once pumped into the open storage it mixes with the rest of the raw water and is exposed to sunlight before being pumped to the water treatment plant. The variability on counts within the open storage is consistent with historic variability, indicating the roof water coming into the storage is not having a detrimental impact on the raw water quality. A number of samples have been tested for a suite of trace metals and other less common parameters. Lead, copper, cadmium, chromium and antinomy are all at levels equivalent to the raw water. Zinc is a little higher than the raw water, but two orders of magnitude below the drinking water limits. In the two roof samples tested, fluoride was measured at 0.36mg/L and 0.05mg/L compared to 0.05mg/L in the storage, but still well below the 1.5mg/L limit. Figure 11 shows the cumulative measured volumes harvested from the 11 roofs that were connected to the sump in June 2010 compared to the cumulative estimated demand of these properties based on 2009 meter reads. As expected, the harvested volume outstripped demand during the wetter/colder months of the year, but continued to do so during the current wetter than normal summer. The surplus is being utilised by other Warrnambool customers, resulting in less water supplied from the Gellibrand River. Legislation and responsibility Legislative responsibility for stormwater in Victoria rests with municipal councils, meaning that without the support of the Council, Wannon Water could not have pursued this water harvesting initiative. The Council will nominate the "legal point of discharge" for roof water as the roof water harvesting pipe connection point; and for surface water runoff, the kerb or stormwater connection point. Should a landowner want to install their own tank, they are still able to do so, but the overflow from the tank will need to be connected to the roof water harvesting connection point. The Victorian Safe Drinking Water Act (2003) requires Wannon Water to have a Risk Management Plan for this new water source. No significant risks were identified that could not be addressed through the design of the system. As with other raw water supplies, Wannon Water will be monitoring the water quality of this source of water to quickly identify if the quality poses a risk to the supply. Cross-connection risk will be managed by Wannon Water inspecting the connection to the roof water harvesting system, the connection to the sewer and the connection to the stormwater system. Wannon Water will also assume responsibility for the roof water harvesting network. Screening of leaves and other matter collected in gutters will be done at the Wannon Water storage, rather than expecting landowners to undertake any more maintenance of their gutters than they currently do. Flyers will be sent out regularly to remind landowners that water is being harvested from their roof and treated to form part of the drinking water supply for Warrnambool. By living in this area, they are part of a more sustainable city. Wannon Water is working with the Warrnambool City Council to make the installation of the roof water collection network a requirement of all development within this catchment -- similar to recycled water systems (3rd pipe networks) in other towns. It is supported by Section 56.07 of the Victorian Planning Schemes -- Integrated Water Management. The Toolkit With the aid of Federal funding from the "Water for the Future Program", a Toolkit has been developed to assist in assessing the financial viability of roof water harvesting systems within Australia and allow quick comparisons with other potential water supply sources. The term 'roof water harvesting system' refers to the infrastructure required to collect rainwater from individual rooftops, typically within a new residential subdivision, and then transfer, store and/ or treat this water before it is added to the town's drinking water supply. The Toolkit allows a roof water harvesting system to be defined in terms of the collection system, transfer pipelines, storage basins and a treatment plant through entering a number of key parameters. Estimates of the capital and operating costs associated with the construction and operation of the roof water harvesting system are then calculated and a time-series simulation is run to estimate the volume of water harvested by the system annually. 710mm polyethylene pipeline ready for laying to convey water (under pressure) from the subdivision to the storage basin. Figure 11: Supply and demand to date.
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