Sanitary engineering — minimum recommended diameter for a domestic sewer laid at 1 in 100 gradient: Choose the most appropriate nominal diameter for a house sewer (building drain) at a longitudinal gradient of 1:100 under normal domestic conditions.
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A100 mm
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B150 mm
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C200 mm
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D175 mm
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E—
Answer
Correct Answer: 100 mm
Explanation
Introduction / Context:Design of small sewers balances self-cleansing velocity, economy, and constructability. For building drains within premises, codes and practice specify minimum diameters ensuring adequate flow without frequent blockages at practical gradients.
Given Data / Assumptions:
- Domestic wastewater with typical peaking factors.
- Gradient provided is 1 in 100 (1%), common in plots with limited depth.
- Pipe material compatible with standard fittings (PVC/stoneware).
Concept / Approach:
Empirical and code-based guidelines recommend a minimum of 100 mm for house sewers. With 1% slope, self-cleansing velocities are achievable for normal domestic discharge, making 100 mm the economical and acceptable choice. Larger diameters are used for branch/outside sewers and manhole-to-manhole lines (often 150 mm minimum).
Step-by-Step Solution:
1) Check minimum code diameter for house sewers: typically 100 mm.2) Check gradient: 1 in 100 is within workable range for 100 mm lines.3) Select the smallest compliant diameter to ensure economy and maintain self-cleansing criteria.Verification / Alternative check:
Numerous departmental SORs and plumbing handbooks cite 100 mm as minimum for WCs/soil lines; outside lines or longer runs often upgrade to 150 mm especially between chambers.
Why Other Options Are Wrong:
- 150–200 mm are conservative and costlier for internal house sewers at 1% slope.
- 175 mm is non-standard in many catalogs and offers no clear advantage here.
Common Pitfalls:
- Oversizing small domestic lines reduces velocity, increasing deposition risk.
- Ignoring minimum cover and ventilation leading to odors and surcharge.
Final Answer:
100 mm.