Stand-by flush

The stand-by flush is used intermittently when the RO skid is off line in stand-by mode. It can also be used during extended lay-up of the skid. The objective is to remove particles and salts that have collected on the membrane surface while the membranes are idle. This minimizes the potential for membrane fouling and scaling while the membranes are at rest. 

The frequency for this type of flush is application and environment dependent. Flushing may be programmed to occur every 15 minutes to once every 2 hours, depending on how concentrated the pretreated feed water is and how high the temperature is. Higher feed water concentrations and higher temperatures require shorter 

The stand-by flush is used intermittently when the RO skid is off line in stand-by mode. It can also be used during extended lay-up of the skid. The 

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System flushes are typically used when an RO system goes off-line, comes back on-line, and during stand-by mode. The purpose of the off-line and stand-by flushes is to rid the feed/concentrate side of the membrane of either high concentrations of feed water species or to stir up materials that may have settled on the membrane during down time. The on-line flush (when the membranes come back on line) is to reduce the conductivity in the RO permeate before sending the permeate on to further processing or to the ultimate use. Flush water is typically sent to drain. 

Figure 13.2 Permeate concentration as a function of flush time after membrane stand-by (250 ppm micro Siemens per centimeter feed conductivity). Courtesy of Jonathan Wood and UltraPure Wafer Journal, wwiu.ultrapureiuater.com, March, 2009.

The on-line flush takes place when the RO skid is returned to service. The objective of the flush is to remove particulates and salts that have settled on the membrane surface while the skid was off line as well as to bring down the concentration of the RO permeate. Figure 13.2 shows the concentration of the RO permeate as a function of flush time after stand-by (assumes a feed water conductivity of 250 micro Siemens per centimeter). Typically, pretreated feed water is used for the flush there is no need to use higher-quality water for this flush. Pressure is generally at nominal operating condition, and any permeate that is generated is sent to drain. 

A) 2-Methyl-5-Methoxy-3-lndolylacetic Anhydride Dicyclohexylcarbodiimide (10 g, 0.049 mol) is dissolved in a solution of 2-methyl-5-methoxy-3-indolylacetic acid (22 g, 0.10 mol) in 200 ml of THF, and the solution Is allowed to stand at room temperature for 2 hours. The precipitated urea is removed by filtration, and the filtrate is evaporated in vacuo to a residue and flushed with Skellysolve B. The residual oily anhydride is used without purification in the next step. 

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