Minimum gradient for self-cleansing: For a 150 mm diameter sanitary sewer under Indian practice, the typical minimum gradient adopted to achieve self-cleansing velocity is approximately:

Introduction / Context:
Self-cleansing velocity criteria ensure that solids do not settle in sewers at low flows. Designers use recommended minimum gradients by diameter to achieve these velocities without causing undue excavation or scouring risks.

Given Data / Assumptions:

• Pipe diameter: 150 mm (house connections and small laterals).
• Conventional Indian practice for minimum gradients.
• Typical target self-cleansing velocity around 0.6–0.75 m/s.

Concept / Approach:
Empirical tables relate minimum gradients to diameters so that, at minimum design flows, the resulting mean velocity meets self-cleansing thresholds. For 150 mm diameter, a common recommended gradient is about 1 in 100, balancing hydraulic performance with constructability.

Step-by-Step Solution:
Identify recommended gradient for 150 mm from typical design tables: approximately 1 in 100.Check that this gradient yields mean velocities near the self-cleansing range at expected DWF.Select 1 in 100 from the options.

Verification / Alternative check:
Worked examples and standard schedules often list 1/100 for 150 mm sewers, with variations for local conditions and roughness values.

Why Other Options Are Wrong:
1 in 60: steeper than necessary, potentially increasing excavation cost.1 in 120: may be marginal for self-cleansing under low flows.None/1 in 300: not representative or too flat for reliable self-cleansing.

Common Pitfalls:

• Applying one gradient to all diameters; tables vary with size.
• Ignoring roughness increase over time due to slime and sediment.

Final Answer:
1 in 100