The most commonly adopted detention period for grit chambers may vary from
- A10 to 40 seconds
- B45 to 90 secondsCorrect
- C90 to 120 seconds
- D120 to 135 seconds
Solution
Concept
Grit chambers hold sewage for a short time, about 1 minute, so heavy grit settles but light organic matter stays in suspension.
Formula
td = L / Vh
Step-by-step solution
1. A grit chamber removes heavy inorganic particles like sand and gravel from sewage. 2. If detention is too long, organic matter also settles and causes odour and decay. 3. If detention is too short, grit does not settle. 4. So the usual detention time is short, about 45 to 90 seconds (typical value about 60 s). 5. So the answer is (B) 45 to 90 seconds.
Explanation
The grit chamber must settle only heavy grit and not organic solids. So the detention period is kept short, about 45 to 90 seconds, with about 60 seconds being typical. The flow velocity is kept near 0.3 m/s.
Why the other options are wrong
A: 10 to 40 seconds is mostly too short for the grit to settle fully. C: 90 to 120 seconds is longer than needed, and organic matter may start settling. D: 120 to 135 seconds is too long, and organic solids will settle with the grit.
Shortcut
Grit chamber is about 1 minute (about 60 s), so pick the range around 60 s: 45 to 90 s.
Common mistake
Mixing up the grit chamber (seconds) with the primary settling tank (hours, about 1.5 to 2.5 h).
Correct answer: B) 45 to 90 seconds