In municipal wastewater treatment, what is the typical clear spacing between bars for perforations on a coarse screen (i.e., the first stage of preliminary treatment handling rags and large debris)?

Introduction / Context:
Coarse screens form the first barrier in preliminary sewage treatment, protecting pumps and downstream units from rags, plastics, and large floating matter. Choosing an appropriate bar spacing is essential to balance debris removal with headloss and maintenance.

Given Data / Assumptions:

• Coarse screens target large solids before grit chambers and primary sedimentation.
• Clear spacing refers to the gap between adjacent bars.
• Typical practice sets a representative standard value within the common coarse range.

Concept / Approach:
Industry references commonly cite coarse screen spacings in the broad range of about 25 mm to 100 mm. For municipal plants, a representative standard design value near the middle of this range (often 40–50 mm) is widely used because it effectively captures large debris without causing excessive headloss or frequent blinding.

Step-by-Step Solution:
Identify the category: coarse screening (not fine screens or micro-screens).Select a typical design spacing within standard practice for municipal works.A commonly adopted value for coarse screens is 50 mm.

Verification / Alternative check:
Comparing different manuals shows coarse screening spacings clustered near 40–50 mm for municipal duty. Plants handling unusually large debris may go wider; those wanting tighter capture may adopt smaller spacing but at the cost of higher maintenance.

Why Other Options Are Wrong:

• 20 mm and 30 mm: drift toward fine or medium screening, not typical for coarse screens.
• 40 mm: plausible but 50 mm is more standard as a representative coarse spacing.
• 100 mm: too wide for reliable debris interception in most municipal plants.

Common Pitfalls:
Confusing clear spacing (gap) with bar thickness; mixing fine screen values with coarse screen design; ignoring headloss implications of very tight spacing.

Final Answer:
50 mm.