Desalination product water quality

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. 

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. 

SWRO system. For example, a partial second-pass configuration is nsed at the 95,000-m / day Tampa Bay seawater desalination plant. The second pass at this facility is designed to treat up to 30% of the permeate produced by the first-pass SWRO system as needed in order to maintain the concentration of chlorides in the plant product water always below lOOmg/L. The partial second pass at the Tampa Bay seawater desalination plant was installed to provide operational flexibility and to accommodate the wide fluctuations of source water salinity (16,000-32,000mg/L) and temperature (18-40°C). Typically, the product water quality target chloride concentration of 100 mg/L at this plant is achieved by only operating the first pass of the system. However, when source water TDS concentration exceeds 28,000 mg/L and/or the source water temperature exceeds 35°C, the second pass is activated to maintain adequate product water quality. The percent of first-pass permeate directed for additional treatment through the second pass is a function of the actual combination of source water TDS and temperature and is adjusted based on the plant product water chloride level. 

Hybrid RO System Configurations The two-pass and two-stage RO system configurations may be combined to achieve an optimum plant design and tailor desalination plant operation to the site-specific water source water quality conditions and product water quality goals. 

The entire volume of permeate from the first pass of the Point Lisas SWRO system is further treated in a second-pass RO system to meet the final product water quality specifications. The second-pass system also consists of two stages—each equipped with BWRO membranes. The Point Lisas seawater desalination plant has the same number of first-pass and second-pass RO membrane trains. 

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]. 

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. 

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... 

This provides mechanical stability to the fiber array in the RO element. The fibers at one end of the element are precisely cut so that product water can be discharged from the bore of the fibers. Toyobo RO modules for seawater desalination have configurations in the doubleelement type as shown above. Because the permeate water of each element in a pressure vessel can be obtained directly, the quality of each permeate water can also be measured directly. This feamre allows easier maintenance of an RO plant (Kannari, 1995). 

Source water quality has a key influence on the suitability of using seawater desalination for industrial water supply. The water quality parameters that have a significant impact on the desalination system design, operations, and cost of water production are the concentration of TDS, chlorides, turbidity, silt density index (SDl), organic content, nutrients, algae, bacteria, temperature, boron, sUica, barium, calcium, and magnesium. 

Membrane performance tends to naturally deteriorate over time due to a combination of material wear-and-tear and irreversible fouling of the membrane elements. Typically, membrane elements have to be replaced every 5-7 years to maintain their performance in terms of water quality and power demand for salt separation. Improvements of membrane element polymer chemistry and production process have made the membranes more durable and have extended their useful life. Use of elaborate granular media pretreatment technologies and ultra and microfiltration (UF and MF) membrane pretreatment systems prior to RO desalination is expected to allow extending the membrane useful life to 7 years and beyond, thereby reducing the costs for their replacement and the overall cost of water. Detailed guidelines for designing SWRO plants are provided elsewhere (AWWA, 2007). 

Single-stage SWRO systems are widely used for production of drinking water. However, these systems have found limited industrial application mainly because of the water quality limitations of the produced permeate. Even if using the highest rejection RO membrane elements commercially available today (nominal minimum rejection of 99.75%), the single-stage SWRO desalination systems typically cannot consistently yield permeate with TDS concentration lower than 200 mg/L, chloride level of less than 100 mg/L, and boron concentration lower than 0.5 mg/L. 

Salt content of the desalinated water depends on the SWRO plant recovery ratio and the quality of the membranes. The product may contain NaCl at a level below 100 mg/L and up to 1000 mg/L. That can be reduced by a second-stage membrane treatment of permeate as explained in the previous sections of this chapter. Using two- or three-stage SWRO systems to remove boron to lower levels will further reduce the sodium chloride salt content as weU. As the water quality is improved, however, the produced water would have very low mineral content (especially calcium hardness) and therefore would be more aggressive and may dissolve matter from the piping system and increase pipe material corrosion rate. 

Electromembrane processes such as electrolysis and electrodialysis have experienced a steady growth since they made their first appearance in industrial-scale applications about 50 years ago [1-3], Currently desalination of brackish water and chlorine-alkaline electrolysis are still the dominant applications of these processes. But a number of new applications in the chemical and biochemical industry, in the production of high-quality industrial process water and in the treatment of industrial effluents, have been identified more recently [4]. The development of processes such as continuous electrodeionization and the use of bipolar membranes have further extended the range of application of electromembrane processes far beyond their traditional use in water desalination and chlorine-alkaline production. 

In this chapter the possibility of integrating different membrane unit operations in the same industrial cycle or in combination with conventional separation systems is analysed and discussed. Many original solutions in water desalination, agro-food productions and wastewater treatments are reviewed highlighting the advantages achievable in terms of product quality, compactness, rationalization and optimization of productive cycles, reduction of environmental impact and energy saving. 

Macedonio et al (2007) evaluated the performance of different IMSs for seawater desalination. In Table 7.2 product characteristics of these systems are summarized. The results indicate an increase of water recovery of the RO unit by up to 52% when NF is used as pre-treatment the introduction of the MCr unit is able to increase the plant recovery factor up to 92.8%, much higher than that of a typical thermal system (about 10%).The overall desalination process appears also very attractive from an economic point of view, due to the high quantity and quality of produced crystals. So the sale of salt crystals (in particular MgS04 7H20) might potentially reduce overall desalination costs. 

---