Quick answer: Blinding is fine particles, scale, oil or biological growth lodging inside the weave until flow falls. The signature is cycle time creeping up while everything else looks normal. Identify which of the four causes you have before washing, because acid clears scale and does nothing for oil.
Blinding is not wear
Worn cloth is abraded and thin; blinded cloth may look almost new. What has happened is that the pores between the yarns have filled with material that washing has not removed, so less filtrate passes for the same pressure. The press takes longer to fill, cake comes out wetter, and throughput falls quietly week by week.
Because the cloth looks fine, blinding is often diagnosed as a pump problem, a conditioning problem or an undersized press. Track filtration time weekly against a fixed reference and the real cause becomes obvious.
The four causes and the wash that clears each
| Cause | How to recognise it | Wash that clears it |
|---|---|---|
| Fine particles in the weave | Gradual onset from commissioning; worst with staple or fine multifilament cloth | High-pressure water; often needs a coarser or monofilament weave to prevent recurrence |
| Scale (calcium, iron) | Hard water, lime dosing or iron in the feed; deposit feels gritty and is pale | Acid wash |
| Oil and grease | Food, slaughterhouse, refinery, palm oil duty; cloth feels slick and cake will not release | Alkaline or solvent wash |
| Biological growth | Warm, organic-rich effluent; slimy feel and sometimes an odour | Alkaline wash or a permitted biocide |
Using the wrong wash wastes a shutdown and convinces people that the cloth is finished when it is not. Feel the cloth and consider the process before choosing a chemical.
The specification trap
The single most common cause is a weave that is too fine for the slurry. The trade-off is unavoidable: the surface that gives the clearest filtrate is also the one that blinds soonest.
- Monofilament — smooth single strands. Best cake release, best blinding resistance, washes clean easily; lets more fines through.
- Multifilament — bundled fibres. Better filtrate clarity, medium blinding resistance. The general industrial default.
- Staple / spun — napped surface. Best clarity, worst blinding resistance and poorest cake release.
If your filtrate is exceptionally clear and your cloth blinds every few weeks, you are probably over-specified. A monofilament face with a finer backing layer is the usual practical compromise. The full selection logic is on the filter press cloth page.
Fix the upstream cause, not just the cloth
Blinding is frequently a conditioning problem wearing a cloth costume. Badly flocculated fines drive straight into the weave; well-flocculated fines are captured in the cake where they belong. Before changing cloth specification, check:
- Polymer dose and type. Under-dosing leaves fines free. Over-dosing can re-stabilise particles and make it worse. Run a jar test against current influent rather than adjusting by trial and error on the press.
- Polymer preparation. Solution made too concentrated, or not given time to hydrate, performs far below its rating.
- Upstream solids carryover. A clarifier passing fines sends them all to the press.
- Water hardness in the wash water itself, which can scale the cloth you are trying to clean.
Fixing conditioning often extends cloth life more than changing the cloth does, and it costs nothing to test.
Build washing into the schedule
Wash on a schedule rather than waiting for symptoms. Cloth washed regularly at low effort stays serviceable far longer than cloth washed heavily once performance has already collapsed. Related: how often filter press cloth should be replaced and how the dewatering stage fits the whole plant.
Frequently asked questions
What does filter cloth blinding actually mean?
Fine particles, scale, oil or biological growth have filled the pores between the yarns, so less filtrate passes at the same pressure. The cloth can look almost new. The symptom is cycle time creeping upward while everything else appears normal.
How do I tell blinding from a worn-out cloth?
Worn cloth is thin, abraded, or damaged at the neck and seams. Blinded cloth looks intact but filters slowly. Wash it: if performance returns, it was blinded. If washing changes little, the weave is finished.
Which wash should I use?
Match it to the cause. Acid clears scale from hard water or lime dosing. Alkaline or solvent clears oil and grease. High-pressure water clears embedded solids from monofilament. Using the wrong one wastes a shutdown and proves nothing.
Why does my new cloth blind faster than the old one?
Usually the replacement is a finer weave than the original, or upstream conditioning changed so fines are no longer flocculated. Compare the new specification against the old before assuming a quality problem with the cloth.
Can polymer dosing cause blinding?
Yes, in both directions. Under-dosing leaves fines free to drive into the weave. Over-dosing can re-stabilise particles and worsen floc quality. Run a jar test against your current influent rather than adjusting the dose on the full-scale press.
Will a coarser cloth fix blinding?
Usually yes, at the cost of filtrate clarity. That trade-off is unavoidable: the weave that gives the clearest filtrate blinds soonest. A monofilament face with a finer backing layer is the common compromise.
Send your slurry description, current cloth specification and symptoms and we will advise on weave and washing: ask our engineering team.