Page 57 - ro membanes

40 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
deterioration of membrane permeate flux and RO system productivity, without significant impact on salt rejection. For comparison, colloidal fouling typically results in a marked dete- rioration in the RO system’s salt rejection over time. In addition, colloidal fouling also causes rapid permeate flux decline over time. Such flux/RO system productivity decline is caused not only by the accumulation of a flow-resistant cake layer of colloidal particles on the surface of the membranes, but also by the back-fusion of salt ions within the colloidal cake, which in turn results in elevated salt concentration of the permeate. In contrast, fouling caused by NOM is accompanied by almost constant salt rejection over time.
Because of its complex nature, NOM often creates diverse interactions with other foulants and with the surface of the membrane elements. As indicated previously, hydrophobic humic substances are typically one of the most common foulants in source seawater, especially when the seawater is under the influence of river discharge. The high content of calcium in seawater reduces the solubility of humic acids and increases their aggregation, which in turn accelerates the accumulation of these NOM compounds on the surface of the RO membranes.
The calcium complexation of NOM in this case often forms a gel on the surface of the mem- branes, which is very difficult to remove. Therefore, source seawater originating from an area of river confluence into the oceandespecially if the river water has high content of NOMd would be very difficult to treat and use for membrane desalination.
As indicated previously, biofouling of the feed channels and spacers of spiral-wound membranes typically results in a significant increase in the membrane DP over a very short period of time (one to several weeks). As biofilm-forming bacteria colonize the RO membrane surface, they often block sections of the feed channels and spacers between the membrane leaves over time, so that the flow pattern within the membrane elements changes and the feed flow is completely blocked in some portions of the feed channels and increased in others (Fig. 2.6).
FIGURE 2.6 Flow channeling.