Reverse Osmosis Pretreatment

Why a pre treatment?

Reverse Osmosis Thin Film Composite membranes are subject to fouling by many substances:

Fouling	Cause	Appropriate Pre-treatment
Biological fouling	Bacteria, microorganisms, viruses, protozoan	Chlorination
Particle fouling	sand, clay (turbidity, suspended solids)	Filtration
Colloidal fouling	Organic and inorganic complexes, colloidal particles, micro-algae	Coagulation + Filtration Optional: Flocculation / sedimentation
Organic fouling	Natural Organic Matter (NOM) : humic and fulvic acids, biopolymers	Coagulation + Filtration + Activated carbon adsorption Coagulation+ Ultrafiltration
Mineral fouling	Calcium, Magnesium Barium or Strontium sulfates and carbonates	Antiscalant dosing Acidification
Oxidant fouling	Chlorine, Ozone, KMnO4	Oxidant scavenger dosing: Sodium (meta)bilsulfite Granulated Activated Carbon

NOM, particles and colloids can be removed by so-called "conventional treatment" consisting of coagulation followed by media filtration for low turbidity water . Additional steps such as flocculation and sedimentation are added in case of very turbid shallow seawater.

The non-conventional pretreatment for NOM , particles and colloids is Ultrafiltration.

An antiscalant solution should be dosed before the reverse osmosis membranes to disperse calcium carbonate and sulfates precipitates in order to avoid scaling.

Fine filtration (5-micron) is required as a last step before the RO membranes to prevent any debris, sand particles or piping material to damage the membranes.

--> See also ULTRAFILTRATION as a reverse osmosis pretreatment <--