ANS:D - all the above.

Detritus tanks are an essential component of wastewater treatment plants, designed to remove heavy solids and grit from the influent wastewater before it enters subsequent treatment processes. Let's address each statement regarding detritus tanks:

1. Flow velocity is kept 0.09 m/sec: This statement is accurate. In detritus tanks, a controlled flow velocity is maintained to allow heavier particles such as sand, gravel, and other inert materials to settle out of the wastewater. By keeping the flow velocity relatively low, these particles can settle at the bottom of the tank, facilitating their removal.
2. Detention period is kept 3 to 4 minutes: This statement is generally true. The detention period refers to the amount of time that wastewater remains in the detritus tank before it is discharged or further treated. A detention period of 3 to 4 minutes is commonly employed in detritus tanks to ensure sufficient time for sedimentation to occur. During this time, the heavier particles settle to the bottom of the tank while the clarified wastewater flows out.
3. Organic and inorganic materials are separated: This statement is not entirely accurate. While detritus tanks primarily target the removal of heavy inorganic materials such as sand, grit, and other inert solids, they may also remove some organic materials that have settled out of the wastewater. However, detritus tanks are not specifically designed to separate organic from inorganic materials; their main function is to remove heavy solids and grit. Organic matter typically undergoes further treatment in subsequent processes such as primary sedimentation tanks and biological treatment units.

• Flow velocity is kept 0.09 m/sec
• Detention period is kept 3 to 4 minutes