Explanation: The detention period of combined aerator lies between 3 to 4 hours and the minimum detention period is 3 hours.

Explanation: In combined aeration, both diffused air aeration and mechanical aeration take place using both diffuser plate and paddles.

Explanation: The detention period of a Sheffield mechanical aeration tank is 15 hours. Its return sludge is 15-20% of sewage flow.

Explanation: The cones, with spiral vanes rotates at a high speed of 60rpm that results in the formation of bubbles in a simplex aerator resulting in the aeration of sewage.

Explanation: Dorroco aerator is a type of combined aerator which is efficient in comparison to diffused air and mechanical aerator.

Explanation: Simplex aerator is a type of mechanical aerator where a square or rectangular tank with a hopper bottom is used.

Explanation: Carbon dioxide gas is produced in the aerobic digestion of sludge by the oxidation of organic matter by bacteria into CO2.

Explanation: The process involved in the treatment of sludge are thickening, stabilization, dewatering and dying.

Explanation: Drying of the sludge is the final stage in the sludge treatment process where the most stable form of sludge is obtained.

Explanation: In the sludge digestion process, the organic solids are converted into more stable form. It makes the dewatering and drying process of sludge simpler.

Explanation: In sludge thickening, the solid content of sewage is increased by gravity flotation and clarifier.

Explanation: Dewatering is the process of removal of water from the sludge. It can be done by drying or subjected the sludge with pressure.

Explanation: In sludge conditioning, the sludge is treated with chemicals in order to separate the water from the sludge and improving the dewatering characteristics of the sludge.