Industrial RO water treatment

How reverse osmosis works

Osmosis is the natural movement of water through a semi-permeable membrane from a dilute solution to a concentrated one. Reverse osmosis applies pump pressure greater than the osmotic pressure to drive water the other way, from the concentrated feed side to a low-salinity permeate side, while most dissolved salts, organics and particles are retained. The retained material leaves in a continuous reject or concentrate stream. Membranes are sensitive to fouling and scaling, so an RO plant is always more than the membranes. Pre-treatment typically includes multimedia or sand filtration, activated carbon or chemical dosing to deal with chlorine, antiscalant dosing, softening where hardness is high, and cartridge filtration before the high-pressure pump. Clean-in-place facilities allow periodic membrane cleaning.

Where it fits

RO is typically considered where total dissolved solids in the supply are too high for the end use: boiler feed, cooling tower make-up, process water for food, beverage and pharmaceutical operations, rinse water, or product water for humidification and misting. Where only hardness is the problem, softening may be enough, and where suspended solids are the issue, filtration or ultrafiltration may address it. The reject stream must have an acceptable disposal or reuse route, which can be a deciding factor.

Design inputs

Design starts with a full feed-water analysis, including dissolved solids, hardness, alkalinity, iron, manganese, silica, turbidity or silt density index, organic content and temperature. Permeate flow, operating hours and peak demand set the plant size, while the target quality defines the number of stages or passes. Recovery is limited by scaling tendency. Power, footprint, storage and the skill level of operating staff also influence the configuration.