What is a pressure decay test on a microfiltration or ultrafiltration unit used for?
- To measure how much the membrane has fouled since the last chemical cleaning
- To find the highest feed pressure the membrane housings can safely withstand
- To check membrane integrity by watching how fast applied air pressure is lost
- To check how much dissolved air must be released from the feed water
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C. To check membrane integrity by watching how fast applied air pressure is lost
In this direct integrity test the wetted membranes are pressurized with air and isolated. Intact fibers hold the air; a broken fiber or leaking seal lets it escape, so the pressure falls faster than the allowed rate. Fouling is tracked by transmembrane pressure and permeability, not by this test.
Reference: LT2ESWTR, 40 CFR 141.719(b) direct integrity testing; EPA Membrane Filtration Guidance Manual
An ultrafiltration plant runs at constant flux. Over several weeks the transmembrane pressure after each backwash keeps climbing and no longer returns to its earlier baseline. What is needed?
- A higher flux setpoint so the membranes are flushed more thoroughly
- A chemical clean-in-place to remove fouling that backwashing cannot
- A pressure decay test, since rising pressure means broken fibers
- A shorter backwash, since the membranes are being overcleaned
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B. A chemical clean-in-place to remove fouling that backwashing cannot
Routine backwashes remove loosely held solids. When the post-backwash pressure keeps creeping up, foulants such as organics, iron or biological growth have built up and need a chemical cleaning with acid, caustic or chlorine as the manufacturer directs. Broken fibers would not raise the pressure; they are found by integrity testing.
Reference: AWWA M53 Microfiltration and Ultrafiltration Membranes, fouling and cleaning
In a two-stage reverse osmosis system, where does mineral scale such as calcium carbonate or calcium sulfate usually form first, and why?
- In the first elements of the first stage, where the feed pressure is highest
- In the permeate tubes of every stage, where the purified water collects
- In the cartridge prefilters, where the feed water first slows down
- In the last elements of the second stage, where salts are most concentrated
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D. In the last elements of the second stage, where salts are most concentrated
As water passes through as permeate, the salts left behind grow more concentrated along the train, so sparingly soluble minerals exceed their solubility at the tail end. Antiscalant, acid feed or a lower recovery prevents it. Fouling by particles and biological growth, by contrast, tends to show up in the lead elements.
Reference: AWWA M46 Reverse Osmosis and Nanofiltration, scaling control