What causes water hammer in a distribution system?
- A sudden change in the velocity of the water
- Air dissolving in the water at high pressure
- A slow rise in demand during the morning peak
- Corrosion deposits narrowing an old main
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A. A sudden change in the velocity of the water
Moving water has momentum. Stopping or starting it suddenly, by slamming a valve or hydrant shut or by a pump tripping, sends a pressure wave through the pipe that can break mains. Operating valves and hydrants slowly prevents it.
Reference: Hydraulics: water hammer (pressure surge)
A circular clarifier is 40 ft in diameter with a water depth of 16 ft. What is its volume in gallons? Use area = 0.785 × D² and 7.48 gal/ft³.
Answer in gal.
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150320 gal
Surface area = 0.785 × 40² = 0.785 × 1,600 = 1,256 ft². Volume = 1,256 × 16 = 20,096 ft³. Gallons = 20,096 × 7.48 = 150,318 gal, about 150,320 gal.
Reference: Cylinder volume: V = 0.785 × D² × depth; 7.48 gal/ft³
What oxygen range is acceptable for entering a confined space without supplied air?
- 19.5% to 23.5%
- 16.0% to 19.5%
- 20.9% to 25.0%
- 12.0% to 21.0%
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A. 19.5% to 23.5%
Normal air is 20.9% oxygen. OSHA treats an atmosphere below 19.5% as oxygen-deficient and one above 23.5% as oxygen-enriched, which is a fire hazard. Entry outside this range requires ventilation or supplied-air breathing apparatus.
Reference: OSHA 29 CFR 1910.146: permit-required confined spaces (hazardous atmosphere)
Why is grit removed early in the treatment process?
- To lower the BOD load on the aeration basin
- To stop odors forming in the primary clarifier
- To prevent pump wear and deposits in tanks
- To reduce the chlorine demand of the effluent
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C. To prevent pump wear and deposits in tanks
Grit is sand, gravel and other heavy inorganic material. It abrades pumps and piping and settles in channels and digesters, taking up volume. Because it is inorganic, removing it does little for BOD or odor.
Reference: Preliminary treatment: grit removal
Where should a coagulant such as alum be added for the best results?
- At the outlet of the sedimentation basin, ahead of the filters
- In the clearwell, together with the disinfectant
- At the rapid (flash) mix, where it is dispersed within seconds
- At the last stage of the flocculation basin
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C. At the rapid (flash) mix, where it is dispersed within seconds
Coagulant reactions are complete in seconds, so the chemical must be spread through the water instantly by intense mixing. Added later, where mixing is gentle or absent, it would not contact the particles evenly.
Reference: Conventional treatment: rapid (flash) mixing