RO Pre-Treatment Filtration — Specifying 5-Micron Protection
Two threats cause most premature RO membrane failure: particulate fouling and chlorine oxidation. Both are preventable with a correctly specified upstream cartridge train.
Particulate Fouling
Suspended solids accumulate on membrane surfaces, creating fouling layers that increase pressure requirements and reduce permeate flow. Damage is cumulative and cannot be reversed.
Chlorine Oxidation
Free chlorine oxidizes the polyamide layer in thin-film composite membranes, degrading polymer structure and allowing dissolved solids to pass through. This damage is permanent.
- Sediment (5–10 micron) — Removes suspended particulates
- Carbon Block — Removes free chlorine
- RO Membrane — Receives clarified, chlorine-free water
Inverting the sequence — carbon before sediment — loads the carbon block with the particulates the sediment stage was there to remove.
Most TFC RO manufacturers specify 5-micron minimum. Absolute-rated pleated cartridges provide more consistent protection than nominal-rated spun cartridges. For high turbidity water, use a 10–20 micron first stage followed by a 5-micron absolute second stage.
A carbon stage running above its chlorine reduction flow rating is passing water through without removing chlorine, regardless of what the pressure gauges read.
Size pre-treatment to peak RO feed flow, not nominal operating flow. Install pressure gauges on every housing; initiate cartridge changeout at approximately 15 PSI differential or manufacturer's terminal DP threshold.
Frequently Asked Questions
What micron rating is required to protect an RO membrane?
Most TFC RO manufacturers specify a 5-micron minimum sediment prefilter. Absolute-rated pleated cartridges give more consistent protection than nominal-rated spun cartridges; for high-turbidity water, use a 10–20 micron first stage followed by a 5-micron absolute second stage.
What are the two biggest causes of premature RO membrane failure?
Particulate fouling (suspended solids building up on the membrane surface, cumulative and irreversible) and chlorine oxidation (free chlorine degrading the polyamide layer in thin-film composite membranes, also permanent). Both are preventable with correct pre-treatment.
What order should sediment and carbon block filters go in before an RO membrane?
Sediment (5–10 micron) first, then carbon block, then the RO membrane. Reversing the order — carbon before sediment — loads the carbon block with particulates the sediment stage exists to remove.
How do I know if a carbon block filter is still removing chlorine?
A carbon stage running above its rated chlorine-reduction flow rate passes water through without removing chlorine, regardless of what the pressure gauges show — flow rate must be sized to peak RO feed flow, not nominal flow.