Page 21 - ro membanes

4 1. INTRODUCTION TO SALINE WATER PRETREATMENT
External fouling involves accumulation of deposits on the surface of the membranes by
three distinct mechanisms:
• formation of mineral deposits (scale);
• formation of cake of rejected solids, particulates, colloids and other organic and/or inor-
ganic matter;
• biofilm formationdi.e., growth and accumulation of colonies of microorganisms on the
surface of the membranes, which attach themselves by excretion of extracellular materials.
Although the three membrane-fouling mechanisms can occur in any combination at any given time, typically external membrane fouling of RO membranes is most frequently caused by biofilm formation (biofouling).
Internal fouling is a gradual decline of membrane performance caused by changes in the chemical structure of the membrane polymers triggered by physical compaction or by chem- ical degradation. Physical compaction of the membrane structure may result from long-term application of feed water at pressures above these the RO membranes are designed to handle (usually 83 bars for SWRO membranes) and/or by their prolonged operation at source water temperatures above the limit of safe membrane operation (typically 45C).
Chemical degradation is a membrane-performance decline resulting from continuous exposure of membranes to chemicals that alter their structure such as strong oxidants (chlo- rine, bromamine, ozone, permanganate, peroxide, etc.) and very strong acids and alkali (typi- cally pH below 3 or above 12).
While external fouling can be almost completely reversed by chemical cleaning of the membranes, internal fouling usually causes permanent damage of the microvoids and poly- meric structure of the membranes, and therefore, is largely irreversible.
1.2.2 Concentration Polarization
A very important factor that may have significant impact on the extent and type of mem- brane fouling is concentration polarization. This phenomenon entails formation of a bound- ary layer along the membrane feed surface, which has salt concentration significantly higher than that of the feed water introduced to the feed/concentrate spacers of the RO membranes (Fig. 1.2).
The boundary layer is a layer of laminar feed water flow and elevated salinity that forms as a result of the tangential source water feed flow in the RO-membrane feed/concentrate spacers and of permeate flow in perpendicular direction through the membranes on the two sides of the spacer (Fig. 1.3).
In Fig. 1.3, Cb is the salt concentration at the boundary layer (e.g., feed saline water con- centration); Cs is the salt concentration at the inner membrane surface, which typically is higher than that in the feed flow; and Cp is the salt concentration of the freshwater on the low salinity (permeate) side of the membrane.
Two different types of flow occur in the boundary layers of the feed/concentrate spacers (Fig. 1.3): a convective flow of freshwater from the bulk of the feed water though the membranes and diffusion flow of rejected solutes (salts) from the membrane surface back into the feed flow. Since the semipermeable RO membranes are designed such