Product water quality

Figure 9.11 The effects of applied pressure, feed temperature, and water recovery on membrane flux and product water quality of RO membranes [12],...

The first step in the design of an industrial reverse osmosis plant is to determine the amount of water to be treated, peak demand, 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. 

It is anticipated that under certain conditions all three processes are capable of producing a high quality drinking water. For microfiltration this requires a pretreatment step. The optimal process or process combinations may depend on the raw water characteristics and the desired product water quality. 

The rejection of small colloids and organics obviously addresses the issue of the treated product water quality, but also has implications for further treatment steps - for example, when MF is used as a pretreatment to nanofiltration. The rejection and flux values that can be achieved lead to questions of... 

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. 

Operate the RO units systems at 50% recovery instead of70-80%. This reduces the frequency of membrane cleanings substantially, increases reliability, and results in consistent product water quality. In addition, the reject water is better suited for reuse apphcations such as for cooling tower make-up. It can also be used as feed for WFI vapour compression stills and clean steam-generators, provided the silica content is less than 15 mg/1. 

Table 3.10 Typical feed water and product water quality targets for high-grade industrial and potable water...

The RO unit is the pivotal process since water production by membrane separation declines with time mainly due to fording and other factors discussed in Chapter 2. The RO system must be run under conditions that minimise decline in flux while maintaining high product water quality. The pre-treatment system must be stable and reliable to ensure the RO unit operates continuously without frequent shutdowns for cleaning to restore flux and rejection. A stable RO membrane performance, in turn, is required to ensure the pohshing system produces water that meets the product water specifications without frequent shutdowns for regeneration of ion-exchange resins. 

The RO unit design is based on meeting the specified product water quality and flux. RO unit service run is based on permeate conductivity (% rejection), productivity... 

Product water quality standards As water quality standards become more stringent and limits on contaminants keep decreasing in specific value, membranes need to improve their rejection capabilities of all species (e.g., boron, which has become important for potable water considerations). ... 

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. 

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