In standing baffle walls used in primary sedimentation or grit chambers, bottom openings of size 15 cm × 15 cm are typically provided at what centre-to-centre spacing for proper underflow and avoidance of clogging?

Introduction / Context:
In sanitary engineering, baffle walls are provided in grit chambers and sedimentation tanks to control flow patterns and direct settled or settling solids. The opening size and spacing influence velocity distribution, headloss, and the tendency to clog.

Given Data / Assumptions:

• Opening size considered: 15 cm × 15 cm (square).
• Application: standing baffle wall in preliminary/primary treatment.
• Objective: ensure adequate underflow without excessive headloss or debris accumulation.

Concept / Approach:
The spacing of orifices in a baffle wall is chosen so that the average approach velocity remains low (to avoid scouring) yet sufficient to prevent deposition within the opening. Empirical guidelines used in design manuals specify centre-to-centre spacing close to 2 times the opening dimension for small baffle orifices in municipal plants.

Step-by-Step Solution:
Step 1: Identify typical opening dimension a = 0.15 m.Step 2: Apply rule-of-thumb spacing S ≈ 2 * a for small orifices in baffles.Step 3: Compute S = 2 * 0.15 m = 0.30 m = 30 cm.Step 4: Select the closest standard construction spacing: 30 cm c/c.

Verification / Alternative check:
Field practice and standard drawings for small plants commonly show 150 mm square openings at about 300 mm c/c, balancing constructability and hydraulic performance.

Why Other Options Are Wrong:

• 22.5 cm c/c: Too close; increases headloss and clogging risk.
• 15 cm c/c: Essentially touching; impractical and prone to blockage.
• 50 cm c/c: Too wide; yields non-uniform flow and dead zones.
• 45 cm c/c: Wider than typical guidance; may reduce effectiveness of underflow distribution.

Common Pitfalls:
Designers sometimes only size the opening, ignoring spacing and approach velocity. Ignoring trash/float control can lead to rapid fouling even if spacing is hydraulically adequate.

Final Answer:
30 cm c/c