Reverse Osmosis Pre-Treatment Options

As part of the implementation of a nanofiltration (NF) or reverse osmosis (RO) membrane plant (pilot-scale or full-scale), the feed water quality must be considered.  The feed water should not contain particles that could plug the NF/RO membranes, as plugging may permanently damage them.  In addition, the water chemistry of the feed water must be evaluated to determine whether scaling of the membranes could occur.  If particles are present in the feed water and if there is a potential to precipitate salts on the membrane, pre-treatment of the feed water would be required prior to NF/RO treatment.

There are several options to pretreat the feed water depending on your specific fouling issue. Conventional pre-treatment of a NF/RO system consists of adding chemicals, sulfuric acid and scale inhibitor, to prevent precipitation of compounds such as calcium carbonate.  Advanced pre-treatment must be considered to remove particles from the feed water if present.  Membrane manufacturers typically warrantee membrane performance when the turbidity of the feed water is less than 1 NTU and the Silt Density Index (SDI) is less than 3.  In order to achieve those goals, pretreatment may be required with treatment options including conventional sand filtration, diatomaceous earth filtration, microfiltration, and ultrafiltration. Selecting the right pre-treatment process depends on the technical feasibility, seasonal water quality variations (if any), and the footprint of the plant, as well as capital and O&M costs.