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
A utility with a limited budget must decide which aging assets to replace first. In an asset management program, how are assets ranked?
- By age alone: the oldest asset in the system is always replaced first
- By cost alone: the least expensive replacements are always done first
- By risk: the likelihood of failure and the consequence of failure
- By location: assets nearest the treatment plant are replaced first
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C. By risk: the likelihood of failure and the consequence of failure
Asset management weighs how likely each asset is to fail (from its condition and history) against what the failure would cost in service, safety and money. An old pipe in good condition serving a few homes can rank well below a newer but deteriorating main feeding a hospital, so age alone is a poor guide.
Reference: EPA, Asset Management: A Best Practices Guide (five core questions framework)
A clearwell has an operating volume of 400,000 gal and a baffling factor of 0.5. At a peak hourly flow of 2,800 gpm the free chlorine residual at the outlet is 1.1 mg/L. What CT is the plant achieving?
Answer in mg·min/L.
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78.6 mg·min/L
Theoretical detention time = 400,000 gal ÷ 2,800 gpm = 142.9 min. T10 = 142.9 × 0.5 = 71.4 min. CT = 1.1 mg/L × 71.4 min = 78.6 mg·min/L.
Reference: Surface Water Treatment Rule, CT concept; EPA Guidance Manual for Disinfection Profiling and Benchmarking (baffling factors)