Desalination pretreatment

Apart from the features of the membranes to be used for the needs of waste management technology, there is one more question to cope with pretreatment engineering. Even in the case of a relatively simple system for water desalination, pretreatment becomes a key problem. In engineering practice, specifically when wastewater is dealt with, the membrane systems to be used are as intricate as are the solutions, the mixtures and their combinations (along with the colloid particles included in them) which have to be treated. 

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. 

Wolf, R, Sivems, S., and Monti, S. (2005). UF membranes for RO desalination pretreatment. Paper present at EDS Conference, Cyprus, May, 2005. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

The major water desalination processes that ate currendy in use or in advanced research stages are described herein. Information on detailed modeling can be found in the Hterature cited. The major texts on water desalination written since the 1980s are those by Spiegler and Laird (47), Khan (48), which contains many practical design aspects, Lior (49) on the measurements and control aspects, Heitman (40) on pretreatment and chemistry aspects, and Spiegler and El-Sayed (50), an overview primer. Extensive data sources are provided in References 39 and 51. 

Hyperfiltration (Reverse Osmosis) is a form of membrane distillation or desalination (desalting) operating with membrane pore sizes of perhaps 1 to 10 Angstrom units. The various individual RO component technologies have improved tremendously over the last 20 to 25 years, and resistance to fouling and permeate output rates have benefited. Nevertheless, all RO plants remain susceptible to the risk of fouling, and adequate pretreatment and operation is essential to minimize this problem. 

Pearce GK (2008) UF/MF pretreatment to RO in seawater and wastewater reuse applications a comparison of energy costs. Desalination 222 66-73... 

All oxidants used must be removed in the final stage of the pretreatment process, as they are known to damage most polymer membranes used for desalination. In particular, chlorine is known to be harmful to commonly used thin-film composite polyamide membranes. 

Microfiltration (MF) and ultrafiltration (UF) membranes can be used as forms of pretreatment for nanofiltration (ISIF) or reverse osmosis (RO) desalination processes. Membrane pretreatment reduces the amount of chemicals that are required and hence reduces the environmental impact of the final discharge. MF membranes can be used to filter particles with diameters of 0.1-10 pmm and typically remove bacteria, viruses, precipitates, coagulates and large colloidal particles. UF can remove particles with diameters as small as 0.002 pm, and... 

NF has been also been studied as a potential form of pretreatment for reverse osmosis desalination processes (Hassan et al. 1998, 2000). Based on the feed water, it may be a suitable pretreatment method that allows for operation with little or even no use of antisealants. 

Hollow fine fiber modules made from cellulose triacetate or aromatic polyamides were produced in the past for seawater desalination. These modules incorporated the membrane around a central tube, and feed solution flowed rapidly outward to the shell. Because the fibers were extremely tightly packed inside the pressure vessel, flow of the feed solution was quite slow. As much as 40-50 % of the feed could be removed as permeate in a single pass through the module. However, the low flow and many constrictions meant that extremely good pretreatment of the feed solution was required to prevent membrane fouling from scale or particulates. A schematic illustration of such a hollow fiber module is shown in Figure 3.47. 

Raw seawater requires considerable pretreatment before it can be desalinated (Figure 5.22), but these pretreatment costs can be reduced by using shallow seafront wells as the water source. The SDI of this water is usually quite low, and little more than a sand filter may be required for particulate control. However, sterilization of the water and addition of antisealants will still be necessary. 

K. Marquardt, Sea Water Desalination by Reverse Osmosis, GVC/VDI Gesellschaft Verfahrenstechnik und Chemieingenieurwesen Seawater Desalination-Water Pretreatment and Conditioning, VDI Verlag, Diisseldorf (1981). 

Pretreatment. Most feed streams are mixtures with varying characteristics. In many cases there are foulants present that can be minimized by pretreatment. For example, seawater fed to RO desalination plant usually contains turbidity and micro-organisms, which can be partially mitigated by prefiltration (media or membranes) and... 

Figure 10.4 presents an estimated cost breakdown of desalinated water produced in a typical plant. The main component is, of course, the capital and financial cost, comprised of the cost of the main equipment items feed tanks, pretreatment filtration units, pumps, pressure exchangers and piping, controls, membranes and membranes housing, post-treatment and product tanks. 

The design of a desalination plant is usually a site-specific task. Pretreatment is the most important local design. It is also envisaged that when operators are insufficiently trained, the design and investment will be based invariably on exaggerated safety factors. Well-trained and experienced operators can increase desalination plant production by identifying and debugging bottlenecks. 

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. 

Huehmer, R. and Henthorne, L. (2006) Advances in RO pretreatment techniques innovations and applications of sea-water and marginal water desalination. IDS 8th Annual Conference Technion, Haifa. 

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. 

An UF system utilizing hollow-fiber (FIF) membranes has been successfully used as pretreatment prior to seawater reverse osmosis (SWRO) desalination without any chemical treatments [8]. The quality of UF permeate was good and satisfied the need of SWRO feed water [8]. 

An integrated membrane desalination system based on NF, RO, and MD has been proposed [11]. In this system a NF unit has been used as pretreatment, while the M D contributed to concentrate the two brine streams from both NFand RO (Figure 12.2). 

Microfiltration, ultrafiltration, and nanofiltration are becoming standard in feed pretreatment for water desalination, wastewater treatment and fruit-juice concentration. 

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