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Technical Papers
Figure 6. Trial scheme – comparison of wastewater injection and linked aquifer solutions. aquifer solutions. The prices here are whole system costs – i.e., the cost of stormwater infrastructure, network infrastructure and backup mains water. Solutions with a reliability less than 100% meet demand not supplied by stormwater/wastewater with a backup supply from SA Water priced at the current 3.23 $/kL. The network costs are added to the supply costs. Lower reliability solutions have a higher cost because the SA Water supply is much more expensive than stormwater supply and is being added to the capital costs of the network.
The graph indicates that the advantages of wastewater injection are more apparent at high reliabilities. This is because the supply cost Figure 7. Case of wastewater remains constant at $0.70/kL, while the only way to increase stormwater volume is to have a larger wetland or more bores, which are utilised less often and, hence, the supply cost increases.
CASE STUDY ASOP was applied to a complex real system case study to evaluate the
Figure 9. Aquifer storage levels with the sites connected (1 large network).
merit of a linked system. This irrigation supply system over several council regions consists of four ASR sites (two with biofilters and two with wetlands), three booster tank locations and a treed distribution network, as shown in Figure 7. Elevations vary by more than 100m across the system of irrigation nodes. The network was optimised as four separate systems and then as one linked system. Separation was introduced at council boundaries between each ASR site. The complex interdependencies of the linked system prompted further development in ASOP so that it could handle a range of aquifer failure scenarios, while also ensuring that the booster tanks were full at the beginning of each demand period. Key network optimisation components, such as aquifer linking pipes and distribution pumps, were included in the stormwater optimisation process because it was not known which aquifers would fail. The complexity of this system was reflected in optimisation runs many times longer than for the trial scheme. The length of run time limited the simulation period to five years. One optimisation process was undertaken for the connected and disconnected cases, each with the reliability target set at 100%. The connected system was able to achieve 100% reliability, while the four separate systems achieved a net reliability of 96.5%. The total cost of water in the connected system was $1.93/kL and
SEPTEMBER 2015 WATER
STORMWATER
Figure 8. Aquifer storage levels with the sites disconnected (four individual networks).
study configuration.