The correct answer is C. Dual action of aerobic bacteria and anaerobic bacteria.
Facultative stabilization ponds are a type of wastewater treatment system that uses a combination of aerobic and anaerobic bacteria to break down organic matter. Aerobic bacteria require oxygen to survive, while anaerobic bacteria can live in the absence of oxygen. In facultative stabilization ponds, both types of bacteria are present and work together to treat the wastewater.
Aerobic bacteria are the primary type of bacteria that break down organic matter in facultative stabilization ponds. They do this by using oxygen to oxidize the organic matter, which produces carbon dioxide and water. Aerobic bacteria are most active in the upper layers of the pond, where there is plenty of oxygen.
Anaerobic bacteria are less active than aerobic bacteria, but they can still break down organic matter in the absence of oxygen. They do this by using fermentation to produce methane and other gases. Anaerobic bacteria are most active in the lower layers of the pond, where there is less oxygen.
The dual action of aerobic and anaerobic bacteria is essential for the effective treatment of wastewater in facultative stabilization ponds. Aerobic bacteria break down the most easily degradable organic matter, while anaerobic bacteria break down the more resistant organic matter. This process results in the complete removal of organic matter from the wastewater, making it safe to discharge into the environment.
The other options are incorrect because they do not accurately describe the process of wastewater treatment in facultative stabilization ponds. Option A is incorrect because it states that sewage is treated by aerobic bacteria only. However, as discussed above, both aerobic and anaerobic bacteria are necessary for the effective treatment of wastewater in facultative stabilization ponds. Option B is incorrect because it states that sewage is treated by algae only. Algae are photosynthetic organisms that can remove some of the organic matter from wastewater, but they cannot completely remove it. Option D is incorrect because it states that sewage is treated by sedimentation. Sedimentation is a physical process that removes suspended solids from wastewater, but it does not remove organic matter.