Water desalination process options

Another interesting possibility is the use of pressure-driven membrane processes, in particular MF and UF are becoming standard and very efficient pretreatment options for sea- and brackish-water desalination. Also, for wastewater treatment, MF/UF pretreatment technology can efficiently reduce the highly fouling nature of the feed. 

Some idea of typical operation costs of desalination processes is given in Figure 4.12, plotted against the salt concentration of the water to be desalted. Although the cost data are by now a little old, the relative costs as between the various processes are still roughly right. It can be seen that, for all but the lowest concentrations, reverse osmosis is the least expensive option. 

In addition, an economic analysis of the desalination plant was performed to investigate economic viability of the SMART desalination plant. The results show that SMART is competitive with other power options, particularly with a gas fired combined plant, within a limited range of electricity generation. The calculated unit cost of fresh water production under desalination capacity of 40,000 mVday using the MED process were in the range of 0.56-0.88 /m for 80% plant availability, which is close to the results of studies performed in other countries. These results indicate that SMART can be considered as a competitive choice for seawater desalination. 

A problem common to all membrane processes is that posed by the retentate in which impurities are concentrated. In some cases, retentate can be discharged with wastewater. Other options include evaporation of the water followed by disposal or incineration of the residue, reclamation of chemicals from industrial wastewater, and disposal in deep saline water aquifers. For the special case of desalination of seawater by reverse osmosis, the retentate is returned to the ocean, which has the potential to cause problems due to excess salinity. 

Figure 7.1 shows the two major treatment options to obtain RO-quality water from sewage and seawater. The key in water reclamation is to first treat the sewage biologically and use MF/UF membrane filtration to remove suspended solids. Two membrane filtration alternatives are available for water reclamation tertiary filtration (TF) of the effluent from a conventional activated sludge (CAS) process and an integrated membrane bioreactor (MBR). For seawater desalination, pretreatment must be provided if the source is open seawater. The current practice involves multimedia filtration, but membrane filtration has also been considered. 

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