Sanitary engineering — common causes of sewer clogging in urban drainage systems

Introduction / Context:
Sewer networks transport wastewater reliably only when internal flow velocities are sufficient to prevent deposition. Clogging leads to surcharge, odour, sanitary hazards, and costly maintenance. Understanding the typical causes helps in design (self-cleansing velocity), operation (grease traps), and public awareness campaigns.

Given Data / Assumptions:

• Conventional gravity sewers conveying domestic and trade wastewater.
• Inlet misuse (solid wastes in manholes) can occur.
• Hydraulic conditions may vary across dry-weather flow and peak flow.

Concept / Approach:

Clogging arises when the shear stress at the invert is too low to re-entrain settled particles or when immiscible substances accumulate. Low discharge reduces velocity, increasing the risk of silting. Grease adheres to pipe walls and traps solids. Inappropriate disposal of solids (textiles, plastics) causes localized blockages. Therefore, multiple mechanisms can simultaneously contribute to clogging in real sewers.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Field maintenance logs routinely cite combined effects (silting during low-flow seasons, grease from kitchens, and foreign objects) as drivers of blockages.

Why Other Options Are Wrong:

Each single-cause option is incomplete; clogging is multifactorial.

Common Pitfalls (misconceptions, mistakes):

Assuming satisfactory performance at design flow guarantees self-cleansing at all times; neglecting grease control and community education.

Final Answer:

all the above