Correct Option
The correct option is Only three
Explanation
Wastewater treatment involves physical, chemical, and biological processes to remove physical, chemical, and biological contaminants from wastewater. The treatment takes place in a Wastewater Treatment Plant (WWTP) through a specific sequence of stages designed to progressively clean the water.
Statement-wise Analysis
- Pair 1: Bar Screens : Removal of large objects like rags - Correct
Wastewater is passed through bar screens at the initial stage of treatment. These vertical bars physically block and remove large solid objects such as rags, sticks, cans, plastic packets, and napkins to prevent damage to pumps and downstream equipment.
- Pair 2: Grit Tank : Removal of floatable oil and grease - Incorrect
The primary function of a grit and sand removal tank is to reduce the velocity of the incoming wastewater to allow heavy inorganic solids like sand, grit, and pebbles to settle down by gravity. Floatable impurities like oil and grease are typically removed later in a sedimentation tank or clarifier using a device called a skimmer.
- Pair 3: Aeration Tank : Growth of aerobic bacteria - Correct
In the aeration tank, air is pumped into the clarified water. This oxygen supply facilitates the rapid growth of aerobic bacteria. These bacteria consume human waste, food waste, soaps, and other unwanted organic matter remaining in the water.
- Pair 4: Clarifier : Settling of sludge - Correct
A clarifier is a settling tank used after the aeration stage. Here, the water is allowed to stand undisturbed so that solids, including the microbial mass (activated sludge), settle at the bottom. The clear water from the top is then removed.
Key Takeaway
Wastewater Treatment Sequence: Bar Screens (large solids) → Grit Tank (sand/pebbles) → Primary Clarifier (settling solids/skimming oil) → Aeration Tank (biological breakdown) → Secondary Clarifier (activated sludge settling).