TL;DR
Elevated permeate turbidity means a broken fiber or a failed o-ring is allowing mixed liquor to bypass the membrane. Run an integrity test to identify the damaged cassette.
What you might see
- permeate turbidity above target (typically above 0.2 NTU for MBR effluent)
- suspended solids present in permeate sample
- permeate total suspended solids (TSS) permit violation risk
- loss of integrity test pressure during routine module integrity test
Likely causes
Broken hollow-fiber membranes allowing suspended solids to pass directly into the permeate
Failed or displaced permeate manifold o-ring allowing bypass
Membrane fiber damage from chemical cleaning at excessive concentration or incompatible chemical
Air pressure integrity test failure indicating widespread fiber damage in one or more cassettes
Required tools
- Permeate turbidity meter or NTU analyzer
- Low-pressure air supply for integrity testing
- O-ring replacement kit (cassette-specific sizes)
- Chemical-resistant gloves and splash goggles
Safety first
- Mixed liquor is a biological liquid. Wear nitrile gloves and safety glasses during any cassette handling or inspection.
- Confirm both isolation valves are closed before opening cassette fittings under pressure.
Procedure
- 1
Measure permeate turbidity at the outlet of each membrane cassette module separately (if isolation valves allow) to locate the damaged module.[1]
- 2
Run a low-pressure air integrity test on the suspect cassette per the OEM procedure. A pressure decay above the acceptance criterion confirms broken fibers.
- 3
If the test fails on one cassette, isolate that cassette from service.
Warning: When isolating a membrane cassette from a running MBR system, verify that the cassette isolation and permeate isolation valves are both closed before opening the cassette for inspection. Mixed liquor under pressure can spray when fittings are opened. - 4
Inspect the permeate manifold o-ring seals on the isolated cassette. Replace any cracked or displaced o-rings.
- 5
If o-rings are intact, the damage is likely broken fibers. Check the membrane cleaning log for any events where chemical concentration or contact time exceeded OEM limits.
- 6
Request an OEM cassette refurbishment or replacement if the cassette fails the integrity test after o-ring replacement.
- 7
Return the repaired or replacement cassette to service and repeat the integrity test to confirm pass before restoring to the filtration cycle.
Sources
Veolia Evoqua Memcor MBR Wastewater Treatment (Clarifier / MBR) general technical documentation, Veolia
MBR membrane integrity testing, broken fiber detection, general water treatment references (general)
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