Performance, brackish water

Electrochemical synthesis, 645 cell types, 648,653,654 energy requirements, 653 fuel cells, 646, 655 overvolfage, 654 reactions, 646 Electrodialysis, 508, 517 equipment, 513, 514 performance, brackish water, 515... 

Polymaleic acid (PMA). The use of chemicals based on PMA and some derivatives has become standard practice for very brackish waters and seawater distillation processes around the world, where the TDS may reach 50,000 ppm TDS, or where total hardness levels exceed 500 to 1,000 ppm CaC03. Its use in RO systems is growing. However, PMA has limited dispersing properties and may need to be formulated with a dispersant chemical to provide satisfactory performance with some RO designs. It is claimed that PMA is also a successful silica deposit control agent and therefore may be incorporated into formulations where this is a problem. 

Kvestak and Ahel investigated the biotransformation kinetics of A9PEO in the Krka estuary in Croatia [35]. Static die-away tests were performed with autochthonous bacterial cultures originating from the two compartments of the stratified estuary the upper fresh/brackish water layer and the lower saline water layer. Experiments were performed at three different temperatures, and at two concentrations. Samples were taken daily and all separate ethoxylates (1-16) were quantified by normal phase HPLC-FL analysis. No other metabolites were analysed. 

The reverse osmosis performance of the two membranes under typical brackish water conditions is shown in Figure 2 (I, reference membrane III, with bentonite). At a rejection of 85 % the flux is almost doubled (from 2000 to nearly 4000 l/m d), the effect becoming smaller when going to higher rejections. Maximum brackish water rejection of the bentonite membrane is 97 % as against 98 % for the reference membrane. 

Membrane Properties. The reverse osmosis performance of the bentonite-doped membrane under brackish water conditions is compared to that of the reference membrane in Figure 5 (I, reference membrane II, with organophilic bentonite). At low salt rejection the bentonite membrane again shows a higher initial flux than the reference membrane, the performance of the two becoming identical at the high rejection limit. 

TABLE 15.6. Performance of Electrodialysis Equipment on Treatment of 3000 ppm Brackish Water... 

The greatest use of membranes is for reverse osmosis desalination of seawater and purification of brackish waters. Spiral wound and hollow fiber equipment primarily are applied to this service. Table 19.6 has some operating data, but the literature is very extensive and reference should be made there for details of performance and economics. 

The Office of Saline Water is directing a large number of investigations into the feasibility of new processes for producing fresh water starting with sea or brackish water as a source. It is desired that these plants last for 20 years or more. This paper points up ways in which the economic waste resulting from corrosion in saline water plants can be avoided. The article is based on a review of the corrosion literature and on consultations with marine experts in the field. Of the many materials for distillation plants, steel is the most important. It can be used to handle sea water below 250° F., if proper steps are taken such as the removal of all air (oxygen) from solution. For severe service and better performance metals like titanium, Hastelloy C, Monel, cupro-nickels, aluminum, aluminum brass, or Admiralty brass are used. 

In brackish waters, the same general corrosion principles may apply as for sea water. Experience has also shown that there may be considerable variation from plant to plant in the performance of metals, even where the cooling water comes out of the same general source such as a deep bay or estuary. There are coastal sites where the salinity may show very marked seasonal fluctuation. Protective film formation is essential for long lifetime in condenser tubing. Tubes installed at the season of the year when conditions are most favorable to form protective films tend to give longer service. 

Dynamically formed membranes were pursued for many years for reverse osmosis because of their high water fluxes and relatively good salt rejection, especially with brackish water feeds. However, the membranes proved to be unstable and difficult to reproduce reliably and consistently. For these reasons, and because high-performance interfacial composite membranes were developed in the meantime, dynamically formed reverse osmosis membranes fell out of favor. A small application niche in high-temperature nanofiltration and ultrafiltration remains, and Rhone Poulenc continues their production. The principal application is poly(vinyl alcohol) recovery from hot wash water produced in textile dyeing operations. 

The comparative performance of high-pressure, high-rejection reverse osmosis membranes, medium-pressure brackish water desalting membranes, and low-pressure nanofiltration membranes is shown in Table 5.2. Generally, the performance of a membrane with a particular salt can be estimated reliably once the... 

The operating pressure of brackish water reverse osmosis systems has gradually fallen over the past 20 years as the permeability and rejections of membranes have steadily improved. The first plants operated at pressures of 800 psi, but typical brackish water plants now operate at pressures in the 200- to 300-psi range. Capital costs of brackish water plants have stayed remarkably constant for almost 20 years the rule of thumb of US 1.00 per gal/day capacity is still true. Accounting for inflation, this reflects a very large reduction in real costs resulting from the better performance of today s membranes. 

Teskeredzic, E., Teskeredzic, Z., Tomec, M. and Modrusan, Z. (1989). A comparison of the growth performance of rainbow trout (Salmo gairdneri) in fresh and brackish water. Aquaculture 11,1-10. 

Atomic force microscopy (AFM) studies contribute also to the improvement of the NF membranes, especially for desalination of brackish water. AFM characterization of a series of commercial NF and RO membranes of different polymer types for brackish water desalination had not been attempted, so far. Thus, as reported by Hilal [79], it is imperative to study the properties of these membranes and to show that the characteristics obtained from AFM correlate to the process behaviour. This is expected to provide substantial new insights into the influence of NF/RO membrane properties on performance, providing a database for the selection of NF membranes to account for the complexities of brackish water. 

Brackish water membranes also come in various types with different performance ratings. These specific types are discussed below. 

Figures 9.8 and 9.9 shows how Beta affects flux and salt passage (rejection), respectively, for two different brackish water concentrations (assumes membrane will deliver 20 gfd at 400 psi with a rejection of 99% at Beta equal to one (no concentration polarization)).8 From the Figures, it is shown that at Beta values greater than about 1.1, the water flux and salt passage (rejection) are significantly affected by Beta. Also shown is that the effect of Beta on performance is more pronounced at higher TDS feed water than with lower TDS feed water.

Brackish water membranes can be shipped from the manufacturer wet or dry. Wet membranes have been performance tested at the factory. However, testing is usually conducted for shorter (hours) versus longer (days) periods. As noted in Figure 14.2 (Chapter 14.3.2), there is a period of time after start up during which membrane performance is not stable due to compaction. Flux and rejection both decrease during this period. Unless a membrane is wet tested until stable performance is achieved, the performance specifications for that membranes based on the wet test are not accurate.7... 

Colorimetric methods Silicon is determined by the molybdenum blue spectrophotometric method after solubilization in H2O, in alkaline solutions or in concentrated HF. A flow analysis procedure for the measurement of soluble silicon with respect to the total Si concentration is used. The proposed method is applied to samples of rain water and of aerosols on filters [45]. Simultaneous determination of orthophosphate and silicate in brackish water is performed by the same technique. Molybdate/ antimony, ascorbic acid, and oxalic acid reagents are added to the samples and spectra are recorded in the wavelength range 410-820 nm after a total reaction time of 30 min [46]. 

Electrodialysis is a membrane-based process which can be used for separation, removal, or concentration of ionic species present in aqueous solutions. These operations are accomplished by the selective transport of ions through an ion exchange membrane under the influence of a direct current. One of the earliest applications of electrodialysis was the desalting of brackish water. However, since the 1970s, extensive studies have been performed on the application of electrodialysis for waste-water treatment, especially in the electroplating and metal-finishing industries. 

The performance of the present approach becomes more obvious in Figure 2a, showing the concentration dependence of Oq. It yields only a weak salinity dependence of the plate spacing and is the only model which is consistent with the observations. The predicted concentration dependence cannot be validated by the scattered data, yet is supported by some limited plate spacing observations in brackish water... 

Properties of FT-30. The properties of FT-30 membranes have been reviewed in several publications. Therefore, only the salient features that relate to the chemistry of the barrier layer will be considered here. Reverse osmosis performance of FT-30 under seawater and brackish water test conditions was described by Cadotte et al (48) and by Larson et al (51). In commercially produced spiral-wound elements the FT-30 membrane typically gives 99.0 to 99.2 percent salt rejection at 24 gfd (40 L/sq m/hr) flux in seawater reverse osmosis tests with 3.5 percent synthetic seawater at 800 psi (5516 kPascaJJand 25°C. 

The properties of FT-30 membranes have been reviewed in several publications, including reverse osmosis performance under seawater and brackish water test conditions.60"62 In commercially produced spiral-wound elements, the FT-30 membrane typically gives 99.1 to 99.3% salt rejection at 24 gfd flux in seawater desalination at 800 psi and 25°C. In brackish water applications, FT-30 spiral elements can be operated at system pressures of as low as 225 psi while producing water at 22 to 24 gfd. Similar flux levels are possible with the TFC-202 and LP-300 membranes, as mentioned earlier. But it is notable that those membranes achieve such high fluxes through use of extremely thin surface barrier layers about only one-tenth the thickness of the FT-30 barrier layer. 

A detailed study of SPSF desalination membranes was carried out by Brousse and coworkers43. Sulfonation was effected by chlorosulfonic acid on a commercial material (Polysulfone P 1700, Union Carbide), and the products as well as their sodium salts were cast from highly polar solvents. Their performance was compared to that of noncharged cellulose-acetate membranes, largely being used for desalination of brackish water. 

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