The correct option is (a).
Explanation
Wastewater treatment involves physical, chemical, and biological processes to remove contaminants. The physical treatment stage, often referred to as primary treatment, focuses on the removal of large debris and suspended solids through mechanical barriers and gravity settling.
Statement-wise Analysis:
- Statement 1 is Correct: Bar screens are the initial step in the physical treatment of wastewater. They consist of vertical bars designed to catch and remove large solid objects such as rags, sticks, cans, plastic packets, and napkins. This prevents blockages and damage to pumps and equipment in subsequent stages.
- Statement 2 is Incorrect: In the grit and sand removal tank, the speed of the incoming wastewater is decreased, not increased. The flow velocity is reduced specifically to allow heavy inorganic particles like grit, sand, and pebbles to settle at the bottom due to gravity.
- Statement 3 is Incorrect: Sedimentation tanks (clarifiers) are designed to facilitate the settling of solids like faeces (sludge) at the bottom and the floating of lighter impurities like oil and grease (scum). To achieve this, the water flow is kept very slow. Increasing the flow speed would create turbulence, keeping solids suspended and preventing effective sedimentation.
Key Takeaway:
Physical wastewater treatment relies on mechanical screening and the reduction of flow velocity to facilitate the gravitational settling of grit and sludge.