Desalinated water quality

Farid Benyahia (immobilized nitrifiers in wastewater treatment membrane distillation desalination water quality and energy efficiency analysis airlift bioreactors low-grade heat in membrane distillation for freshwater production bioremediation of oil spills development, design and evaluation of advanced refinery wastewater treatment processes), College of Engineering, Department of Chemical Engineering, Qatar University (QU), Doha... 

In addition to the potable uses discussed above, the desalinated water quality may be driven to even higher levels by the need of some industrial applications, especially those where ultrapure water quality is necessary. 

With a growing scarcity of freshwater available for irrigation, other sources of lower quality like brackish water, saline water, and treated wastewater become more important as additional or substituting inputs for the agricultural sector. At the same time, it is clear that a sophisticated treatment like desalination or nanofiltration under current conditions is still far too expensive to be a major solution to future irrigation water needs. Hence adaptation of farming and irrigation practices to the particular water qualities constitutes a more viable approach. 

For this reason, two pass seawater desalination process have been necessarily employed till quite recently, and the results obtained have been satisfactory to some extent with regard to water quality and practical operation. However, one pass process has advantages over two pass process for simple and compact plant, simple operation, easy maintenance and lower energy consumption. 

The use of membranes is infiltrating into the process industry, where improved water quality is needed. Power stations, petrochemical and high-tech production plants are seeking improved water quality and are using different types of membranes to meet their needs. Additional information on different aspects of desalination processes was reported by Semiat [1]. 

The critical issue for a successful RO plant is pretreatment. Long-term operating experience proves the viability of continuous MF/UF pretreatment of RO for the desalination of a wide variety of water sources. MF/UF has proven to simplify and reduce the costs of traditional pretreatment, comprised of deep-bed media filters combined with chemical treatment. MF/UF produces filtrate of a consistent quality almost irrespective of fluctuations in feed-water quality. In the last five years, RO-membrane improvements, combined with the use of membrane filtration for pretreatment, have halved the cost of advanced treatment and are now more widely used for the reuse of municipal wastewater. 

Although RO water desalination is today considered as the most cost-effective solution [4], key factors for further improvements in membrane-based desalination systems are enhancement of water-recovery factor, cost reduction, improvement of water quality, new brine-disposal strategies. All these issues can be addressed by an integrated approach. 

Karakulski K., Giyta M., and Morawski A., Membrane processes used for potable water quality improvement. Desalination 145 2002 315-319. 

Kettunen, R. and Keskitalo, P., Combination of membrane technology and limestone filtration to control drinking water quality. Desalination, 131, 271, 2000. 

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. 

J. Dlueca-Munoz, J.A. Mendoza-Roca, A. Iborra-Clar, A. Bes-Pia, V. Fajardo-Montanana, F.J. Martinez-Francisco, I. Bemacer-Bonora, Study of different alternatives of tertiary treatments for wastewater reclamation to optimize the water quality for irrigation reuse. Desalination 2008, 222, 222-229. 

One potential application is pre-treatment of seawater for RO desalination. Process modeling results showed that a FO-RO integrated system could be effective in meeting boron and chloride water quality requirements for agricultural irrigation without a two-pass RO system [64]. Since it is apparently less prone to feed side foufing, FO pretreatment would be an additional useful asset. 

D.L. Shaffer, N.Y. Yip, J. Gilron, M. Elimelech, Seawater desalination for agriculture by integrated forward and reverse osmosis improved product water quality for potentially less energy, J. Memb. Sci. 415-416 (2012) 1-8. 

Of aU the major membrane processes, RO/NF separation is the most complex both in terms of operation and controls [43]. RO (and NF) membrane systems operate in a continuous mode with minimum or no recycle. RO desalination plants can be generally quite large (see Table 3.5) for example the largest seawater RO desalination plant in Sorek, Israel has a capacity 150 million m /year. Further, for hybrid membrane systems the process control becomes even more complex. RO/NF plants require different levels of process control depending upon the quality of feed water supplied and product water quality requirements. 

Membrane desalination plants, especially seawater RO plants, are energy intensive. One option for reducing energy consumption is to use dual-purpose plants that provide both electricity and waste heat for heating RO feed water. Membrane productivity increases with feed water temperature albeit at a slight penalty in product water quality. Higher productivity, in turn, means fewer membrane elements to achieve the same product water flow rate, resulting in reduced Capex and Opex. 

The provision of water suitable for drinking is another essential to life. The quality of naturally available water varies greatly from site to site and in many places it may be necessary to remove bacteria, salts, heavy metal ions and organics desalination and chlorination are the most common processes. Moreover, it is again advantageous to monitor the water quality continuously or, at least, routinely. 

Pressure membrane separation is increasingly used for potable water treatment and in water reuse processes. Applications include softening, organic removal, and desalination of brackish well water, surface water, and seawater. For some applications, membrane processes offer the advantages of superior water quality, reduced chemical usage, less chemical waste production, and lower energy consumption. 

Brandhuber, P. Amy, G. (2001) Arsenic removal by charged ultrafiltration membrane - influences of membrane operating conditions and water quality on arsenic rejection. Desalination, 140 (1), 1 -14. 

To justify the assumptions, the estimated cost in power consumption, labor, and membrane replacement was compared with that in a seawater reverse osmosis desalination (SWRO) plant (Atikol et al., 2005). For SWRO, the reported cost for power was 0.04 US m, we estimated 0.022 US m for our system. The lower cost in energy consumption is mainly due to the low operating pressure and significantly higher water recovery in this system. The cost for pre-treatment was assumed to be lower than seawater plant due to the significantly much better water quality in the drinking water sources. The maintenance cost was adopted from the... 

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