Page 25 - ro membanes

8 1. INTRODUCTION TO SALINE WATER PRETREATMENT
ratio between permeate and feed flows of the entire RO system is termed an RO system re- covery rate.
As the recovery rate increases, the magnitude of concentrate polarization increases as well. For example, for SWRO systems using standard membrane elements, operation at recovery rate of 50% would typically result of approximately 1.2e1.5 times higher salinity concentra- tion at the membrane surface than that in the source seawater. Beyond 75% recovery, the con- centration polarization factor would exceed 2, which would have a significant impact on the efficiency of the membrane separation process.
In addition, at recovery rate above 75% and ambient salinity pH, many of the salts in seawater would begin precipitating on the membrane surface, which would require the addi- tion of large amounts of antiscalant (scale-inhibitor) and would make SWRO desalination impractical. Since scaling is pH-dependent, an increase in pH to 8.8 or more, which often is practiced for enhanced boron removal, may result in scale formation at significantly lower SWRO system recovery (50%e55%).
To limit concentration polarization within reasonable limits, RO-membrane manufacturers recommend maintaining the maximum recovery rate per membrane element in a vessel within 10%e20%. As a result, with a typical configuration of six to eight elements per vessel, and taking under consideration the actual flux of the individual elements in the vessel, a sin- gle RO system is practically limited to a maximum of 50%e65% recovery. For brackish water desalination systems the typical maximum recovery rate is 85%e95%.
The concentration polarization phenomenon described above and its effect on membrane productivity (flux) decline is inherent not only to RO membranes but also occurs on the sur- face of ultrafiltration (UF) and microfiltration (MF) membranes used for saline water pretreat- ment. In this case, concentration polarization is accumulation of rejected particles (rather than salts) near the membrane surface causing particle concentration in the boundary layer that is greater than that in the raw seawater fed to the pretreatment system (which in turn results in UF/MF flux decline).
1.2.3 Membrane Fouling and Flux Redistribution
Membrane RO elements of a typical SWRO system are installed in vessels often referred to as membrane pressure vessels. Usually, six to eight SWRO-membrane elements are housed in a single membrane vessel (see Fig. 1.4).
Under typical RO-system membrane configuration, all of the feed water is introduced at the front of the membrane vessel and all permeate and concentrate is collected at the back end. As a result, the first (front) membrane element is exposed to the entire vessel feed flow and operates at flux significantly higher than that of the subsequent membrane ele- ments. With the most commonly used configuration of seven elements per vessel and ideal uniform flow distribution to all RO elements, each membrane element would produce one- seventh (14.3%) of the total permeate flow of the vessel.
However, in actual SWRO systems, the flow distribution in a vessel is uneven and the first membrane element usually produces over 25% of the total vessel permeate flow, while the last element only yields 6%e8% of the total vessel permeate (see Fig. 1.4). The decline of permeate production along the length of the membrane vessel is mainly due to the increase in feed salinity and associated osmotic pressure as the permeate is removed from the vessel