What is involved in the secondary treatment of sewage?

In the secondary treatment of sewage, bacterial action plays a crucial role in breaking down organic matter. This process typically involves the use of microorganisms to decompose the organic pollutants present in the wastewater. During this stage, sewage is exposed to various types of bacteria, which feed on the organic substances, converting them into simpler compounds. This biological treatment effectively reduces the biochemical oxygen demand (BOD) of the wastewater, which is a measure of how much oxygen is required for the microorganisms to break down the organic material.

The use of bacteria is essential for the efficiency of secondary treatment methods, which can vary from activated sludge processes to trickling filters or bio-towers. Each method relies on the natural processes of microbial digestion, aiding in the purification of the water before it undergoes further treatment or is released back into the environment.

In contrast to the correct answer, the other options do not correctly describe the secondary treatment process. Chemical addition primarily pertains to primary treatment, where chemicals may be used to precipitate or reduce the volume of solids. Disposal into the ocean does not constitute a treatment method, as it is more of a final disposal option without the necessary purification. Filtration through soil is more characteristic of natural treatment systems, such as constructed wetlands, rather than a