ANS:A - anaerobic decomposition

The treatment of sewage in septic tanks primarily relies on anaerobic decomposition. Let's explore each of these processes in detail:

1. Anaerobic Decomposition: Anaerobic decomposition occurs in the absence of oxygen. In the context of septic tanks, sewage undergoes anaerobic digestion by microorganisms such as bacteria and archaea. These microorganisms break down organic matter present in the sewage into simpler compounds like methane, carbon dioxide, and organic acids. This process occurs within the anaerobic environment of the septic tank, where oxygen is limited. Anaerobic decomposition is the dominant mechanism in septic tanks and is responsible for reducing the volume of sewage and breaking down organic solids.
2. Aerobic Decomposition: Aerobic decomposition, on the other hand, occurs in the presence of oxygen. While aerobic processes can contribute to sewage treatment in certain systems like activated sludge treatment plants or aerobic digesters, it is not the primary mechanism in septic tanks. In septic tanks, the oxygen supply is limited due to the enclosed nature of the tank, leading to anaerobic conditions. Therefore, aerobic decomposition is not the main process responsible for sewage treatment in septic tanks.
3. Parasitic Decomposition: The term "parasitic decomposition" is not a recognized term in the context of sewage treatment. It may be a misinterpretation or confusion with other terms related to decomposition processes. In general, decomposition refers to the breakdown of organic matter by microorganisms, and parasites are organisms that live in or on another organism (the host) and derive nutrients at the host's expense. Parasites are not directly involved in the decomposition of sewage within septic tanks.

In summary, the treatment of sewage in septic tanks primarily occurs through anaerobic decomposition, where microorganisms break down organic matter in the absence of oxygen. Aerobic decomposition, which occurs in the presence of oxygen, is not the main mechanism in septic tanks. The term "parasitic decomposition" does not apply to sewage treatment processes in septic tanks.