Question

why should oils not be thrown down the drains?

Answer 1

Oil/ghee with slit forms a thick layer inside the sewer and block the pipes.

Oil/ghee clogs the pores in the soil. This reduces the water absorbing capacity of the soil.

Answer 2

Cooking oil and fats should not be thrown down the drain. They can harden and block the pipes. In an open drain the fats clog the soil pores reducing its effectiveness in filtering water. ... So do not throw them down the drain.