Which treatment stage is primarily responsible for breaking down organic matter biologically?

• A. Primary treatment
• B. Secondary treatment
• C. Tertiary treatment
• D. Preliminary treatment

Answer: (B)

The stage that is primarily responsible for breaking down organic matter biologically is secondary treatment. During this phase, microorganisms such as bacteria are utilized to decompose and metabolize organic pollutants present in wastewater. This biological process is essential, as it significantly reduces the biochemical oxygen demand (BOD) of the effluent, indicating fewer organic materials that would otherwise contribute to water pollution if discharged into the environment.

In secondary treatment, various methods can be employed, including activated sludge processes, lagoon systems, and trickling filters, all of which rely on the natural activities of these microorganisms. The result is processed water that is substantially cleaner, making it suitable either for further treatment or direct discharge into receiving waters.

In contrast, primary treatment focuses on the physical separation of solids from liquids and does not involve biological processes to break down organic matter. Tertiary treatment is a subsequent step intended to polish the effluent and remove remaining contaminants but does not primarily target biological decomposition. Preliminary treatment is an initial stage aimed at removing large solids and debris, ensuring that the wastewater is adequately prepared for subsequent treatment phases.