When a borehole feeds reverse osmosis — for boiler feed, process water or high-purity duties — the membrane fouls long before its time unless the feed is properly prepared. The two usual culprits are dissolved iron and a population of sub-micron colloids that a conventional filter never removes.
Matched to the measured borehole chemistry
Borehole turbidity is often dominated by 1–3 µm colloids; a large fraction can be sub-micron. Granular media cuts at 10–20 µm, so these particles pass straight through and load the membrane.
RO membranes need a Silt Density Index typically ≤ 3. Plain filtration barely moves the SDI on colloidal borehole water — ultrafiltration is what reliably reaches it.
Even trace dissolved iron oxidises on the membrane and forms a tenacious fouling layer, so Fe/Mn removal sits upstream of the polishing stage, never after it.
A UF barrier removes the sub-micron colloids granular media cannot, delivering a consistent low SDI to the RO regardless of raw turbidity swings.
The failure mode we are asked to fix most often is an RO that fouls within weeks behind a single sand filter. The diagnosis is almost always the same: the dissolved iron was never oxidised, and the sub-micron colloids were never removed — so both arrived at the membrane. The correct sequence is fixed by the physics: oxidise (aeration) → remove Fe/Mn (catalytic multimedia) → polish to low SDI (ultrafiltration) → antiscalant → RO. Put the polishing stage before the iron removal and it blinds; skip the UF and the SDI never drops. Getting the order and the media right is the difference between a membrane that lasts its design life and one that is cleaned or replaced continually.
The borehole hub: the full train and example arrangements by application.
Read MoreA related stage of the borehole treatment train.
Read MoreA related stage of the borehole treatment train.
Read MoreA related stage of the borehole treatment train.
Read MoreReynolds & Bauhm assesses the actual water and arranges the multimedia, aeration and membrane train that meets your target — essential, recommended and premium options against one analysis.
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