Off-line flush

The off-line flush is conducted when the membranes are brought off line for any reason. The flush is used to displace the high concentration of solids in the feed/concentrate side of the membrane with lower-concentration water (feed water under minimal pressure so as 

The flush can be programmed into the PLC and thus occurs automatically when the RO skid shuts down or it can be manually initiated. Note that not all RO systems are equipped with this flushing feature. Without this feature, fouling and scaling of membranes will be exacerbated, particularly if the RO unit cycles on and off on a regular basis. 

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Ultrafiltration systems should never be taken off line without thorough flushing and cleaning. Because membrane modules are normally stored wet, the final rinse solutions should contain a bacteriostat such as 0.5% formaldehyde to inhibit bacterial growth. 

Operations that are conducted while the RO system is off line are just as important as on-line operations in keeping an RO system functioning well. Off-line operations covered in this chapter include system flush, membrane cleaning, and membrane lay-up. 

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. 

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... 

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. 

Should a large tank not be an option, the alternative is multiple skids, some of which will sit idle. The key is to rotate skids on and off so that no one skid experiences most of the down time start-ups. It is also recommended that an off-line or shut-down flush be employed, and that the motor be equipped with a soft start. For the example above, two 350-gpm skids would be required. Both skids would be on line during the 500-gpm draw. A product tank with enough capacity to handle the 200-gpm overage would be required. 

First, when membranes come off line, they should be flushed with either permeate water or low-pressure feed water (see Chapter 13.1.1). This will reduce the concentration of ions and any suspended solids on the feed side of the membrane, thereby minimizing the potential for fouling or scaling the membrane while idle. The next step(s) depends on how long the membranes will be off line. 

Short-term idling Short-term idling includes membranes that are off line for no more than 48 hours. An automatic flush event should occur at least once every 24 hours. No other steps need to be taken to preserve the membranes. 

It is valuable to provide connections on the cooling water system, that may be used for flushing to dislodge any accumulations of deposits that might occur. The opportunity to isolate heat exchangers by the inclusion of bypass systems, must be included so that off-line cleaning can be carried out without the need to shut down whole sections of the total cooling water circuit. 

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