Brackish water RO

These objectives are similar to those of the previous brackish water RO evaluation program, with greater emphasis being placed on power recovery, due to the higher energy consumption of seawater desalting. 

NF is used when high molecular weight solutes have to be separated from a solvent. It is effective in the production of drinking water, especially in the case of water softening. Compared to RO, a lower retention is found for monovalent ions. But very recently [9], it has been found that NF separates the ions of the same valency for a selective defluorination of brackish water. RO and UF have shown, respectively, solution-diffusion and convection mass transfers. In NF, a synergism between both can be observed but strongly depends on the operational conditions (pH, ionic strength, flow rate, transmembrane pressure) and on the membrane material used. 

The first commercial brackish water RO(BWRO) was on line at the Raintree facility in Coalinga, California. Tubular cellulose acetate membranes developed and prepared at UCLA were used in the facility. Additionally, the hardware for the system was fabricated at UCLA and transported piecemeal to the facility.9... 

Due to the added resistance of the membrane, the applied pressures required to achieve reverse osmosis are significantly higher than the osmotic pressure. For example, for 1,500 ppm TDS brackish water, RO operating pressures can range from about 150 psi to 400 psi. For seawater at 35,000 ppm TDS, RO operating pressures as high as 1,500 psi may be required. 

Table 4.10 Total Water Cost for Brackish Water RO...

The performance and characteristics of new RO membranes operating at ultralow pressure (ULP) (7-10 bar) are presented [61]. The total cost comparison based on a 4 kmVday plant revealed that TFC ULP provides about 10% savings over the traditional TFC HR (traditional brackish water RO membrane). Generally, the use of TFC ULP contributes to energy saving, but the capital cost increases due to the higher membrane cost. 

Membrane fouHng is a membrane system phenomena. The type of feed water determines the severity of folding. For example, seawater RO membranes are mainly fouled by organic and particulate matter, whereas brackish water RO membranes are primarily fouled by dissolved but sparingly soluble salts. Substances that foul membranes are Hsted... 

In order to avoid excessive concentration polarisation at the membrane surface, permeate recovery per membrane element should not exceed 18%. In the case of brackish water RO systems, the average recovery per 100 cm (40-in.) long membrane element is usually about 9%. The overall recovery for a staged system with pressure vessels containing six elements is usually as follows [46] ... 

A non-thermal ZLD process for treating brackish water RO reject is electro dialysis/ electrodialysis reversal (ED/EDR). The salt rejection is 60—70% but recovery approaches 97% with multiple stages [3]. The benefits of ED over MVC are lower capital cost, lower energy consumption and much greater flow capacity MVC range IS 250-3000 mVd [15]. 

Energy consumption and performance data of fuel ceU integrated seawater RO (SWRO) and brackish water RO (BWRO) systems are given in Tables 5.4 and 5.5. 

The salient features of SWRO and brackish water RO (BWRO) desalination processes that determine plant design and influence capital and operating costs are summarised below. Several RO desalination plant case studies are subsequently discussed to illustrate systems design and operation. 

Most brackish water RO plants require extensive pre-treatment as discussed in Chapter 2. Feed water quality ofbrackish waters in the US southwest is given in Table 3.4. The intensity of pre-treatment depends on the quality of feed water as detailed in T able 3.6. Feed water constituents that affect pre-treatment design are summarised below [62,63] ... 

Figure 3.38 (a) Process design flow schematic of a brackish water RO membrane system. Welton-Mohawk site, (b) Process design flow schematic of a brackish water RO membrane system. Rio Grande site. 

Figure 5.2 Process flow diagram and mass balance of a high-recovery brackish water RO plant. The brine RO increases the overall RO system water recovery from 75% to 88%.

The brackish water RO unit was based on the following assumptions [8] ... 

Table 5.5 Brackish water RO unit design parameters... 

The capital cost of a seawater RO unit is between 600 and 800/rn /day while for brackish water RO unit it is between 250 and 400/m /day [3]. The capital costs of entire seawater plants are five times h her than brackish water plants partly due to a more extensive pre-treatment system required as well as larger pumping and piping required to move larger volumes of feed water and RO concentrate (because of lower RO PWR). The capital cost of reclaim water membrane plants is typically one-half of seawater RO plants [9]. Typical breakdown of operating costs of SWRO plants by components is as follows [3,4] ... 

Cases I and II are based on high recovery brackish water RO, i.e. primary RO + brine RO (PRO + BRO). Cases III covers intermediate TDS (23,000 ppm) water. Cases IV and V are based on SWRO. 

Raising the RO feed water temperature from 20 to 30°C reduces the plant energy usage by 10%. Overall costs could also be decreased if the brine concentrates are processed to recover valuable products. A few such cases are discussed in Chapter 3. Deployment of high recovery brackish water RO systems, i.e. a primary RO (PRO) plus a brine recovery RO (BRO) increases the overall PWR from 70—75% to 85—90%. A10% reduction in energy consumption is achievable when the BRO unit is replaced by a NF unit [44]. The NF feed pressure is about 30% lower than the BRO feed pressure. 

Jawor, A. and Hoek, E.M.V. 2009. Effects of feed water temperature on inorganic fouling of brackish water RO membranes, 235(1-3) 44—57. 

In general, boron rejection of RO membranes decreases with decreasing membrane permeability. In addition, it is affected by pH and various parameters, such as temperature and salt concentration. For example at pH 10 the rejection of seawater and brackish water RO membranes is about 99% and 93%, respectively (Fritzmann et al, 2007). 

The RO desalination plant realized in Ashkelon (Israel) uses a combination of seawater and brackish water RO membranes to reduce boron concentration in the produced water. In the hybrid process designed by Busch et al (2003), the permeate from a seawater RO membrane is treated by a boron selective ion-exchange resin, achieving a reduction of the boron concentration of more than 99% while other ion concentrations are kept constant. This... 

Integrated process for boron removal in sea water desalination. SWRO = sea water RO BWRO = brackish water RO BSR = boron selective ion-exchange resin Cg = boron concentration. (Adapted from Busch et al., 2003.)... 

Stage produces a stream with a boron concentration of 0.1 mg L and contributes from 10% to 25% of a final blend having a boron concentration of about 0.4 mg L k A second brackish water RO system produces a permeate with a typical boron concentration of 0.2-0.35 mg and contributes from 50% to 60% of the final blend. The remaining contribution to the final blend (20%) is given by the permeate from the first RO stage (Fig. 7.1). 

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