ANS:D - the depth of tank is kept equal to its width.

"the depth of tank is kept equal to its width." In septic tank design, it's not typical for the depth of the tank to be kept equal to its width. Septic tanks often have a depth that is greater than their width. The depth of the tank is designed to accommodate the necessary volume of wastewater and allow for settling and treatment processes to occur effectively. The other statements provided are generally correct and represent common design considerations for septic tanks:

• A gap of 7.5 cm between the bottom of the covering slab and the top level of scum is provided to allow for proper ventilation and to prevent odors.
• The outlet invert level is indeed kept 5 to 7.5 cm below the inlet invert level to facilitate the separation and settling of solids within the tank.
• The minimum width of a septic tank is commonly around 90 cm, although variations may exist depending on specific design requirements and regulations.