Desalination product water recovery

The pressure to be used for reverse osmosis depends on the salinity of the feedwater, the type of membrane, and the desired product purity. It ranges from about 1.5 MPa for low feed concentrations or high flux membranes, through 2.5—4 MPa for brackish waters, and to 6—8.4 MPa for seawater desalination. In desalination of brackish or sea water, typical product water fluxes through spiral-wound membranes are about 600—800 kg/m /d at a recovery ratio RR of 15% and an average salt rejection of 99.5%, where... 

In membrane distillation, two liquids (usually two aqueous solutions) held at different temperatures are mechanically separated by a hydrophobic membrane. Vapors are transported via the membrane from the hot solution to the cold one. The most important (potential) applications of membrane distillation are in water desalination and water decontamination (77-79). Other possible fields of application include recovery of alcohols (e.g., ethanol, 2,3-butanediol) from fermentation broths (80), concentration of oil-water emulsions (81), and removal of water from azeotropic mixtures (82). Membrane (pervaporation) units can also be coupled with conventional distillation columns, for instance, in esterifications or in production of olefins, to split the azeotrope (83,84). 

A brackish water having a TDS concentration of 3000 g/m3 is to be desalinized using a thin-fllm composite membrane having a flux rate coefficient, kw, of 1.5 x 10 6 s/m and a mass transfer rate coefficient, kj, of 1.8 x 10 6 m/s. The product water is to have a TDS of no more than 200 g/m3. The flowrate, Qp, is to be 0.010 m3/s. The net operating pressure (APa — AIT) will be 2500 kPa. Assume the recovery rate, r, will be 90 percent. Estimate the rejection rate and the concentration of the concentrate stream. 

Once the pretreatment study had been completed, it will be possible to decide on the type of elements to be used in the reverse osmosis unit. If the SDI of the pretreated feed is 3.0 or less, then either the spiral wound or hollow fine fiber elements can be used. The choice will depend on economics (element price) and desalination characteristics (flux and rejection). If the pretreated feed SDI is more than 3.0, then the spiral wound element should be used. When the decision as to element type is made, then it is appropriate to forward a copy of the pretreated feed water analysis to reverse osmosis element manufacturers to obtain a prediction of product water quality, recommended type of element, total number of elements required, possible problems with sparingly soluble compounds in the feedwater, allowable recovery, and price and delivery. 

El-Zanati and El-Khatib (2007) suggested an IMS consisting of a NF unit as pre-treatment section of an RO unit, while MD was used as a system to concentrate the two brine streams from both NF and RO. The integration of these systems improved the performance of the seawater desalination unit, leading to a water recovery factor of 76.2%. The water production cost was estimated at around 0.92 m. This cost was considered competitive in comparison with those of potable water produced in a RO system for seawater treatment. 

Macedonio et al (2007) evaluated the performance of different IMSs for seawater desalination. In Table 7.2 product characteristics of these systems are summarized. The results indicate an increase of water recovery of the RO unit by up to 52% when NF is used as pre-treatment the introduction of the MCr unit is able to increase the plant recovery factor up to 92.8%, much higher than that of a typical thermal system (about 10%).The overall desalination process appears also very attractive from an economic point of view, due to the high quantity and quality of produced crystals. So the sale of salt crystals (in particular MgS04 7H20) might potentially reduce overall desalination costs. 

Evaporation. In most chemical industry evaporation systems, the objective is product recovery, although occasionally the objective is concentration of an organic waste from an aqueous solution, to facihtate incineration. Similar equipment is used extensively for desalination of salt or brackish water (see also Water, supply and desalination). 

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