Recovery individual membrane modules

Figure 9.12 Individual membrane module recovery and rejection as a function of position in a 2-stage RO system with 6 modules per pressure vessel.

Recovery through individual membrane modules changes, based on the position of the module in the pressure vessel. Most spiral wound membrane modules operate with individual module recoveries ranging from 10% to 15%, with an average of 11% to achieve 50% recovery in a single, 6-module pressure vessel stage. The module at the feed end of the pressure vessel typically exhibits the lowest recovery of all modules in the pressure... 

The current ultrafiltration market is approximately US 200 million/year but because the market is very fragmented, no individual segment is more than about US 10-30 million/year. Also, each of the diverse applications uses membranes, modules, and system designs tailored to the particular industry served. The result is little product standardization, many custom-built systems, and high costs compared to reverse osmosis. The first large successful application was the recovery... 

Recovery (sometime referred to as "conversion") is a term used to describe what volume percentage of influent water is "recovered" as permeate. Generally, RO system recoveries range from about 50% to 85%, with the majority of systems designed for 75% recovery. (Individual spiral wound membrane module recoveries vary from about 10% to 15%—see Chapter 4.3). A system recovery of 75% means that for every 100 gpm influent, 75 gpm will become permeate and 25 gpm will be retained as concentrate. 

System design plays a role in determining acceptable recovery by an RO. Flow rates per pressure vessel, recovery per module, and BETA values must all be taken into account when considering acceptable recovery by the RO system (see Chapters 9.4, 9.5, and 9.6). The higher the recovery of the RO system, the closer concentrate flow rates and individual module recoveries come to reaching limits recommended by membrane manufacturers. 

Figure 5.6 shows an RO array with concentrate recycle. A concentrate recycle is usually used in smaller RO systems, where the cross-flow velocity is not high enough to maintain good scouring of the membrane surface. The return of part of the concentrate to the feed increases the cross-flow velocity and reduces individual module recovery, thereby reducing the risk of fouling. 

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