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)
What minimum removal of Cryptosporidium did the Interim Enhanced Surface Water Treatment Rule require of systems that filter?
- 2-log (99%)
- 1-log (90%)
- 3-log (99.9%)
- 4-log (99.99%)
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A. 2-log (99%)
Filtered systems must achieve at least 2-log Cryptosporidium removal, which is credited to plants that meet the tightened turbidity limits. The 3-log and 4-log figures are the Giardia and virus requirements from the original rule.
Reference: 40 CFR 141.170 and 141.173 (Interim Enhanced Surface Water Treatment Rule)
An activated sludge plant has heavy Nocardia-type foam and has been running at a 22-day MCRT. Which process change is most effective for controlling it?
- Decrease wasting to raise the MLSS and thicken the mixed liquor
- Increase wasting to lower the MCRT, and remove the foam
- Raise the return sludge rate to move the foam into the clarifiers
- Raise the dissolved oxygen so the foam organisms are oxidized
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B. Increase wasting to lower the MCRT, and remove the foam
These foam formers grow slowly and are favored by long sludge ages and by grease. Lowering the MCRT washes them out faster than they grow, and skimmed foam should be removed rather than recycled, because returning it reseeds the tank. Holding more solids lengthens the sludge age and makes the foam worse.
Reference: WEF Manual of Practice OM-9 Activated Sludge, foam control