Recommended detention period for grit (sand) chambers What is the typical design detention time used in conventional (non-aerated) grit chambers handling municipal sewage?

Introduction / Context:
Grit chambers remove heavy inorganic particles such as sand, gravel, and eggshells to protect pumps and prevent abrasion. The detention time is chosen to settle grit while keeping most organics in suspension, avoiding odor and putrefaction. Selecting a practical value is central to reliable operation.

Given Data / Assumptions:

• Municipal wastewater with typical grit size and specific gravity around 2.65.
• Horizontal-flow (non-aerated) grit chamber layout.
• Peak flow conditions inform sizing; uniform approach velocity targeted.

Concept / Approach:

Design practice commonly adopts a detention time of about 30–60 seconds at peak flow for horizontal units; however, many standard problems and code-based exercises round to roughly 2 minutes to ensure adequate removal under varying conditions and to account for fluctuations. Aerated grit chambers may use 2–5 minutes with controlled spiral flow, but basic horizontal chambers remain near 1–2 minutes.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals present detention times close to 1–2 minutes for conventional units and longer for aerated types; operator experience supports these values for reliable grit capture.

Why Other Options Are Wrong:

(a) can be marginal at low temperatures or variable grit loads; (c) and (d) are long for conventional chambers and risk organic settling; (e) is shorter than usual textbook practice.

Common Pitfalls:

Using too long detention, causing septic conditions; ignoring scouring at high velocities; forgetting provisions for grit washing and dewatering.

Final Answer:

2 minutes