Ventilating shafts along sewers are generally provided at approximately what regular interval under standard municipal practice?

Introduction / Context:
Ventilating shafts supplement manholes and ventilating columns to maintain air exchange within sewer networks. By placing shafts at regular intervals, designers reduce the accumulation of corrosive gases and balance internal air pressure with the atmosphere.

Given Data / Assumptions:

• Conventional gravity sewers in flat to gently sloping terrain.
• Passive ventilation (no mechanical blowers).
• Regular interval design to simplify layout and construction.

Concept / Approach:
Common guidelines call for ventilating features approximately every 150 m, with adjustments for branch ends, high points, and points of size or slope change. The 150 m spacing tends to prevent stagnation of air pockets and supports odour control strategies.

Step-by-Step Solution:
Identify typical interval from practice: about 150 m.Match to the given options and select 150 m.Note that tighter spacing may be applied near sensitive areas or where turbulence is high.

Verification / Alternative check:
Many municipal specifications and study examples use 150 m as a standard design value for vents/shafts, with permitted variation based on local conditions.

Why Other Options Are Wrong:
100 m: conservative and costlier; used only when local odour/corrosion history justifies it.200–500 m: may permit gas accumulation; not preferred for general conditions.

Common Pitfalls:

• Overlooking venting at summits or at terminal ends of laterals.
• Assuming that manholes alone always provide sufficient ventilation.

Final Answer:
150 m