The capital costs for a plant producing water from seawater are about twice the costs of a plant reusing secondary sewage. Both the pretreatment costs and RO costs are higher. In the case of pretreatment, this is due to the difference in recovery (75%for secondary effluent; 50% for seawater), which results in a larger seawater pretreatment system. The capital cost for the seawater RO process is higher than for the secondary effluent RO as it is operating at a much higher pressure, lower permeate flux, lower recovery, and must be made of materials that resist corrosion in seawater. Similarly, the operation and maintenance costs for producing RO water from seawater are twice the costs of reusing secondary sewage. The higher pretreatment costs are due to continuous dosage of a coagulant. The higher RO costs are due primarily to energy (the operating pressure is four times higher and the feedflow is 1.5 times higher), but also to membrane replacement. The total life cycle costs for producing RO water from secondary effluent and seawater are US$ 0.28 and US$ 0.62 m-3, respectively, a ratio of 2.21 .
Desalination and water reuse processes tend to be energy-intensive, so reduction and recovery and alternative energy sources must be sought. Energy is used (and wasted) throughout desalination and water reuse processes. There is a potential to develop or apply energy recovery devices, and to devote time and resources to reducing the energy consumption of desalination- and water reuse-specific devices. Further, the use of renewable or alternative energy sources when constructing desalination and water reuse facilities should be considered. Worthy of mention are solar electricity, geothermal, and wind energy .
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