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.

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:

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.