In a filtration plant, water is filtered using layers of
(a) sand and clay
(b) clay and fine gravel
(c) sand and fine gravel
(d) sand, fine gravel and medium gravel

Correct Answer: (d) sand, fine gravel and medium gravel

Explanation: In wastewater treatment, waste¬water is allowed to enter a grit and sand removal tank, where the speed of wastewater is reduced in order to allow sand, grit and pebbles to settle down.