The correct answer is D. The depth of tank is kept equal to its width.
A septic tank is a watertight tank that is buried underground and used to treat wastewater from a home or small business. The tank consists of two chambers: a settling chamber and a digestion chamber. The settling chamber allows solids to settle to the bottom of the tank, while the digestion chamber allows bacteria to break down the organic matter in the wastewater.
The depth of a septic tank is typically 1.5 to 2 meters, while the width is typically 2 to 3 meters. The depth of the tank is important because it allows the wastewater to settle and the bacteria to have enough time to break down the organic matter. The width of the tank is important because it allows the wastewater to flow through the tank without causing clogging.
If the depth of the tank is equal to its width, then the wastewater will not have enough time to settle and the bacteria will not have enough time to break down the organic matter. This can lead to problems such as sewage backups and the contamination of groundwater.
Here is a brief explanation of each option:
- Option A: A gap of 7.5 cm between the bottom of the covering slab and the top level of scum is provided. This is correct because it allows for the scum to float on top of the wastewater and prevents it from being drawn into the outlet pipe.
- Option B: The outlet invert level is kept 5 to 7.5 cm below the inlet invert level. This is correct because it creates a siphon effect that helps to draw the wastewater out of the tank.
- Option C: The minimum width of septic level is 90 cm. This is correct because it allows the wastewater to flow through the tank without causing clogging.
- Option D: The depth of tank is kept equal to its width. This is incorrect because it does not allow the wastewater to settle and the bacteria to have enough time to break down the organic matter.