Septic Tank - Free MCQ Practice Test with solutions, GATE Civil Environmental

This is a highly anaerobic tank, where both sedimentation and digestion takes place in one chamber, where raw treatment of sewage can directly be done. Hence, it is also called the primary unit.

• Reduction of solids and organics  is occurred by settling and anaerobic processes, but the treatment is moderate.
• Effluent creeps into the ground and is transmitted through a sewer to the centralized treatment plant.

This is a highly anaerobic tank, where both sedimentation and digestion takes place in one chamber, where raw treatment of sewage can directly be done. Hence, it is also called the primary unit. It is an anaerobic method of on site treatment. In this tank, dispersion problem occurs.

Septic tank is a highly anaerobic tank. In a septic tank, both sedimentation and digestion takes place in one chamber, where raw treatment of sewage can directly be done. It is also called the primary unit.

Design parameter for Septic tank:

• Minimum width should not less than 0.9 m
• Minimum depth should not less than 0.75 m
• L/B ratio : 2 to 3
• Detention period : 12 to 36 hours
• Sludge Accumulation Rate : 30 l / capita - year

Hence, correct option is B.

Detention time for different treatment unit type is given below.

• For Primary sedimentation tank, Detention time is 2-2.5 hour
• For Grit Chamber, Detention time is 30-60 seconds
• For Sludge digestion, Detention time is 20-30 days
• For Activated sludge process, Detention time is 4-6 hour
• For Primary sedimentation tank, Detention time is 2-2.5 hour
• For Septic tank, Detention time is 12-36 hour
• For Oxidation pond, Detention time is 2-6 weeks