Seawater pretreatment

Seawater is pretreated before reaching the reactor and generally involves screening and filtration to remove suspended particles such as silts, sand, and marine creatures followed by decarbonation. Decarbonation is achieved by adding concentrated sulfuric acid to the seawater to lower the pH to 4 see reaction (3.9). The seawater is then passed over a wooden (creosote- and tar-treated) desorption tower where it is aerated to remove carbon dioxide. 

The disadvantage of this method is that it tends to form supersaturated solutions of calcium carbonate that make removal of the carbonate difficult. 

A variety of methods are used to speed up crystal formation, such as passing the seawater through a sand filter where the grains of sand act as numerous nucleation sites for the precipitation of calcium carbonate. Another method involves the use of contacting the seawater with suspended seed particles that act in a similar manner to the grains of sand. As calcium carbonate precipitates, the particles grow in size to a point where they can be removed and sent to waste. 

In addition to softening the incoming seawater, some operations use chlorine gas to kill any living organisms present in the raw seawater. This helps prevent the clogging of pipe work by marine growths. 

The overliming reaction is therefore a delicate balance between B203 content and CaO content. Using this method, it is possible to reduce the B203 level in dead-burned seawater magnesia to below 0.03%. 

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Seawater pretreatment, 16 25 Seawater reverse osmosis systems,... 

A. C. Epstein, Proceedings 39th Annual International Water Conference, Pittsburgh, Pennsylvania, Seawater Pretreatment, available NITS (1979), p. 590. 

Vial, D., Doussau, G., and Galindo, R., Comparison of three pilot studies using Microza membrane for Mediterranean seawater pretreatment, Desalination, 156, 43, 2003. 

G. Di Profio, X. Ji, E. Curcio, E. Drioli, Submerged hoUow fiber ultrafiltration as seawater pretreatment in the logic of integrated membrane desalination systems. Desalination 2011,269,128-135. 

Al-Amoudi, A.S. and Farooque, A.M. 2005. Performance, restoration and autopsy of NF membranes used in seawater pretreatment. Qesalination 178 261-271. 

It is recommended that pilot testing be used in developing design criteria for the site-specific conditions and MF/UF product selection. Both pressure-type MF/UF systems, where the membranes are encased in pressiue vessels, and vacuum-type systems, where the membrane are immersed in tanks open to atmosphere and use fUtrate/permeate pumps to create the driving force, may be used for seawater pretreatment. 

As previously mentioned, the key SWRO project construction expenditures are associated with building the plant intake, the pretreatment system, procurement and installation of the plant pumps and piping, the SWRO membranes and pressure vessels, the energy recovery system, the water posttreatment facilities, and the concentrate disposal system. It is difficult to compare the investment and construction costs of existing desalination projects because projects may differ significantiy in one or more of the cost-related parameters listed above. Based on previous experience, however, it can be estimated, for example, that the seawater pretreatment costs are in the range of 6-8 US cents/m and the costs of water conditioning and boron and chloride removal are between 4 and 8 US cents/m. ... 

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. 

An alternative pretreatment for seawater is acidification of the bicarbonate followed by degasification to remove the carbon dioxide generated. The precipitation step for the seawater process is given by (76) ... 

Chlorine is desirable as a bulk pretreatment biocide for inlet water, but its subsequent removal upstream of the membrane is absolutely necessary ana difficult. NaHSO,3 is a common additive to dechlorinate before membranes. It is customarily added at 3-5 mg/1, an excess over the stoichiometric requirement. NH3 is sometimes added to convert the chlorine to chloramine, a much less damaging biocide. Heavy metals present in seawater seem to amplify the damaging effects of chlorine and other oxidants. 

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

Only Crm coprecipitates quantitatively with hydrated iron (III) oxide at the pH of seawater, around 8. To collect CrVI directly without pretreatment, e.g., reduction to Crm, hydrated bismuth oxide, which forms an insoluble compound with CrVI, was used. Crm is collected with hydrated bismuth oxide (50 mg 400 ml-1 seawater). To collect CrVI in seawater a pH of about 4 was used. Both Crm and CrVI are thus collected quantitatively at the pH of seawater, around 8. 

Huang and Shih [616] used a graphite furnace atomic absorption spectrometer with a stabilised platform furnace involving atomisation from a graphite surface pretreated with vanadium to determine down to 24 ppt of zinc in seawater. 

Nygaard et al. [752] compared two methods for the determination of cadmium, lead, and copper in seawater. One method employs anodic stripping voltammetry at controlled pH (8.1,5.3 and 2.0) the other involves sample pretreatment with Chelex 100 resin before ASV analysis. Differences in the results are discussed in terms of the definition of available metal and differences in the analytical methods. 

Bond et al. [791 ] studied strategies for trace metal determination in seawater by ASV using a computerised multi-time domain measurement method. A microcomputer-based system allowed the reliability of the determination of trace amounts of metals to be estimated. Peak height, width, and potential were measured as a function of time and concentration to construct the database. Measurements were made with a potentiostat polarographic analyser connected to the microcomputer and a hanging drop mercury electrode. The presence of surfactants, which presented a matrix problem, was detected via time domain dependent results and nonlinearity of the calibration. A decision to pretreat the samples could then be made. In the presence of surfactants, neither a direct calibration mode nor a linear standard addition method yielded precise data. Alternative ways to eliminate the interferences based either on theoretical considerations or destruction of the matrix needed to be considered. 

Sipos et al. [789] have described a procedure for the simultaneous determination of copper and mercury in seawater down to the ng/1 range using differential pulse ASV at a gold electrode. Pretreatment is necessary, and comprises UV irradiation to release the trace metal bound to dissolved organic matter. 

For many of the organic materials in seawater, some form of chemical pretreatment is necessary before analysis is possible. The obvious cases are the hydrolysis of polysaccharides and proteins before the analysis for monomeric constituents, and the formation of volatile derivatives to permit analysis by gas chromatography. These methods will be discussed further in Chap. 9. 

The SDI- - of a water is the accepted criterion of its quality for RO conversion and some SDI s of water around the world are given on Table II. The manufacturers usually require values of waters reaching permeators to have an SDI below 3 if they are to warranty their membrane design life. This requirement of seawater for RO conversion is accomplished through pretreatment. The quality of the raw seawater determines the need for, and the specific type of pretreatment required to produce the water quality requisite to satisfy a specific permeator manufacturer s requirements. 

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. 

Figure 5.22 Flow scheme showing the pretreatment steps in a typical seawater reverse osmosis system [50]...

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