Borehole turbidity is usually dominated by 1–3 µm colloids that slip straight through granular media and drive the Silt Density Index that fouls reverse-osmosis membranes. This guide explains what colloids are, how SDI is measured, and the filtration that actually removes them.
SDI reduction and fine-particle removal that stops membrane fouling.
Ultrafiltration for sub-micron colloid and SDI reduction ahead of RO.
Oxidation and catalytic filtration — often the source of colloidal floc.
Surface loading, bed depth and media selection for the duty.
Most borehole water looks clear, yet a nephelometer reads measurable turbidity and an SDI test pins it instantly. The culprits are sub-micron and low-micron colloids — clay platelets, silica fines, oxidised iron floc and organic matter — that stay suspended indefinitely and pass through any granular bed. For a reverse-osmosis plant these colloids are the single most common cause of premature membrane fouling, so controlling them is the difference between a plant that holds flux and one that needs constant cleaning.
Below the cut of conventional media
A granular sand or multimedia bed strains particles down to roughly 10–20 µm by depth filtration. Borehole colloids are an order of magnitude smaller — 1–3 µm and finer — and carry a surface charge that keeps them dispersed and stable. They do not settle, they do not strain out, and they re-stabilise after any shear. The result is water that passes a coarse filter with the turbidity and SDI essentially unchanged, then deposits a gel-like cake on the lead RO membranes within weeks. Removing them needs either a genuinely fine barrier (ultrafiltration) or coagulation to grow the colloids into filterable floc first.
Clay, silica and organic colloids at 1–3 µm sit far below the 10–20 µm cut of granular media.
A negative zeta potential keeps colloids mutually repelled and permanently dispersed without coagulation.
Under-oxidised or poorly filtered Fe/Mn floc adds its own colloidal load downstream.
Colloidal cake raises differential pressure and cuts flux — the classic SDI-driven failure.
The standard test that predicts RO foulability
The Silt Density Index (SDI, ASTM D4189) measures how quickly a 0.45 µm membrane blinds under a fixed 2.1 bar. Water is passed through the test membrane and the time to collect a set volume is recorded at the start and again after 15 minutes; the percentage drop in flow gives SDI₁₅. Membrane manufacturers typically require SDI ≤ 5 and prefer SDI ≤ 3 for a long, stable membrane life. It is the colloidal fraction — not the nephelometric turbidity — that the SDI captures, which is why a "clear" borehole can still read an unacceptable SDI.
| SDI₁₅ | Foulability | Suitability for RO |
|---|---|---|
| ≤ 3 | Low | Preferred — long membrane life |
| 3 – 5 | Moderate | Acceptable with monitoring |
| > 5 | High | Pre-treatment required before RO |
A staged barrier matched to the measured water
Aeration oxidises dissolved iron and manganese so they form a floc that can be filtered rather than passing as colloids.
A low coagulant dose neutralises colloid surface charge so the fines aggregate into filterable micro-floc.
A dual- or triple-media bed removes the oxidised floc and coagulated colloids, cutting the bulk of the load.
A UF membrane provides an absolute ≈0.02 µm barrier, delivering SDI ≤ 3 reliably regardless of feed swings.
Online or routine SDI testing confirms the feed stays inside the membrane manufacturer’s limit before the RO.
With colloids removed and SDI controlled, the RO holds flux, runs longer between cleans and meets its design life.
An absolute sub-micron barrier that delivers SDI ≤ 3 ahead of RO regardless of feed variation.
View EquipmentLayered beds that combine coarse and fine depth filtration to remove oxidised floc and coagulated colloids.
View EquipmentIn-line coagulant dosing that neutralises colloid charge so sub-micron fines grow into filterable floc.
View EquipmentMultimedia and UF arranged specifically to protect downstream reverse-osmosis membranes.
View EquipmentWhy Choose Reynolds & Bauhm
The complete pre-treatment train that protects borehole RO plants.
Read MoreUltrafiltration for sub-micron particle and SDI reduction.
Read MoreOxidation and catalytic media that prevent colloidal floc carryover.
Read MoreThe complete borehole and groundwater treatment overview.
Read MoreWe measure the SDI and colloid load on your actual borehole, then arrange the coagulation, multimedia and ultrafiltration train that brings SDI within the membrane limit and keeps it there.
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