Correct Option
The correct option is (c).
Explanation
Wastewater treatment involves multiple stages to remove contaminants. Primary treatment relies on physical processes like sedimentation and flotation to separate suspended solids and floating matter from the sewage. Mechanical devices are employed in tanks to automate the removal of these separated impurities.
Statement-wise Analysis
- Statement 1 is Incorrect. Skimmers are not used exclusively in grit removal tanks. While grit chambers are designed to remove heavy inorganic solids (like sand and gravel), skimmers are a standard and essential component of primary sedimentation tanks (clarifiers). In clarifiers, they are required to handle the significant amount of floating organic matter that separates during the settling process.
- Statement 2 is Correct. The specific design purpose of a skimmer is to remove floatable impurities. Substances such as oil, grease, fats, soaps, and plastics have a specific gravity lower than water, causing them to rise to the surface as "scum." The skimmer collects this layer.
- Statement 3 is Correct. Skimmers operate on the surface of the water in the clarifier. As the water flows slowly through the tank to allow solids to settle, the skimmer (often a rotating arm or a mechanical blade) sweeps across the top surface to push the floating scum into a collection trough for separate disposal.
Key Takeaway: In wastewater treatment, skimmers are mechanical devices operating on the surface of sedimentation tanks (clarifiers) to remove floating scum (oil and grease), whereas scrapers are used at the bottom to remove settled sludge.