Question

Important functions of a septic tank includes:

(A) Storage of sludge and scum

(B) Removal of solids from the sewage

(C) Decomposition of solid sewage under aerobic conditions

(D) Decomposition of solid sewage under anerobic conditions

Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (B) and (D) only.
• C. (A), (C) and (D) only.
• D. (B), (C) and (D) only.

Hint:

Remember a septic tank is completely closed, with no air pumped in, and think about what kind of decomposition that allows.

Answer 1

The Correct Option is B

Step 1: A septic tank is a closed underground tank used for on site treatment of household sewage, it has no mechanical air supply built in.

Step 2: As sewage flows slowly through the tank, its speed drops enough for suspended solids to settle out of the liquid stream, so the tank does remove solids from the sewage, statement (B) is true.

Step 3: The solids that settle collect as sludge at the bottom, while fats and lighter particles rise and form a scum layer at the top, and the tank holds both of these in place over time, so statement (A), storage of sludge and scum, is true.

Step 4: Because the tank is sealed and gets no forced air, whatever bacterial breakdown of the settled solids happens, happens without oxygen, that is anaerobic decomposition. So statement (D) is true, and statement (C), aerobic decomposition, is false since aerobic treatment needs a separate aerated unit.

Step 5: The correct statements are (A), (B) and (D), option 2.