Water. desalination + reuse

RESEARCH | 40 | Desalination & Water Reuse | August-September 2014 tabulated in table 1 and a simplified diagrammatic representation of the scenarios is provided in figure 1. A summary of the indicative capital expenditure (capex), operating expenditure (opex) and annual costs for each of the four scenarios is presented in table 2 and figure 2. ElECtRiCity ConSumption And gREEnHouSE gAS EmiSSionS Figure 3 shows the estimated flow-specific greenhouse gas emissions for the four scenarios. The results show that emissions are dominated by direct electricity consumption (represented by scope 2 emissions) as largely coal-based electricity generation has been assumed. The results for electricity consumption are broadly in agreement with other published data. Scenario 1 has the highest electricity consumption compared with the other scenarios. Most of this consumption is for production and much of the electricity needed in production is used in overcoming the higher osmotic pressure of seawater compared to treated wastewater. It is worth noting that the increased power required for product water delivery in scenario 2 is attributable to the need to transport the water up a longer pipeline to a higher discharge elevation into a dam or other storage. This can be compared to scenario 3 in which product water is discharged to a local water distribution network along a shorter pipe length and to a lower elevation. Scenario 4 has the lowest flow-specific power requirement of the four scenarios due mainly to the absence of reverse osmosis along with the shorter pumping distances and lower elevations assumed for product delivery to areas connected to the dual pipe recycled water network. inSigHtS Depending on individual and local perspectives, a range of conclusions may be drawn from the raw scenario results summarised in figures 2 and 3. However careful consideration of the scenario attributes can provide additional insights. We look at a few examples here. The desalination scenario is more energy intensive than any of the reuse scenarios because of the higher pressures required to treat seawater. This is widely understood by industry and has resulted in many utilities purchasing renewable energy to power new desalination plants. In some cases new, full-scale, matching renewable energy facilities have been built in response to a decision to implement desalination and so reduce greenhouse gas emissions significantly. This has not occurred to the same degree with water reuse implementation. Of the reuse options, DPR and dual pipe reuse have similar unit costs that are significantly lower than IPR in the scenarios considered. There is growing confidence in technology to achieve DPR safely. And DPR produces higher quality water than dual pipe so it outstrips dual pipe in the proportion of demand that it can meet. This combination of attributes Figure 2. Estimated unit costs for the four scenarios. Table 2. Summary of indicative costs for scenarios. *Assumes financing at a coupon rate of 5% a year

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