Technical Paper -- Micro-Media Filtration: An Alternative To Membrane Filtration
To ensure trouble free-operation of ion exchange units, resin manufacturers recommend removal of suspended solids to negligible levels, since any solids not removed ‘up-front' will generally accumulate within the ion exchange resin beds, which are themselves excellent filters. In fact, the high electrostatic surface charge or zeta potential present on ion exchange resins enhances the ability of ion exchange resins to remove even extremely fine, charged colloidal particles1. The adverse effects of solids accumulation in resin beds include the distribution of water and regenerants within the resin bed and ultimately the pressure loss across the bed causing deterioration in the quality and quantity of the treated water. Even small amounts of solids adhering to the surface of the resin beads may adversely affect exchange kinetics, long before pressure drop increases are observed.
With conventional co-current ion exchange systems, a small amount of solids accumulating within the resin bed can often be removed by regular back-washing. There has been a trend over the past decade or so to replace conventional co-flow ion exchange demineralizers with countercurrent packed-bed. While the advantages of these newer technologies cannot be denied, their pre-treatment requirements are more stringent than traditional technology2. This is because packed-bed systems are not backwashed on a regular basis. To do so would obviate the advantages offered by counter-current regeneration. In one case where a packed bed ion exchange design was retrofitted to an existing conventional co-flow system, problems due to iron fouling occurred as often as every 3 months instead of every 6 months with the co-flow system3 until the pre-treatment system was upgraded.
Although various methods have been developed to help alleviate this situation4,5, the general consensus is that packed bed ion exchange systems are more prone to fouling with suspended solids than conventional systems.
The past two decades have seen a dramatic increase in the use of reverse osmosis demineralizers. RO systems are even less forgiving in terms of pre-treatment than ion exchange6. The close spacing of spiral wound membranes results in trapping of suspended solids inside the modules. This is exacerbated by the fact that, like ion exchange resins, RO membranes bear a surface charge which may cause fine solids to be attracted to the membrane surface. Once fouling begins, cleaning of the membranes becomes very difficult and the system may not return to original performance levels once fouling has occurred.
According to one manufacturer, "membrane fouling in RO systems is as allpervasive and inevitable as the common cold" 7. In fact, many of the failures experienced by these systems can be traced back to inadequate pre-filtration.
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