Which part of the treatment process uses biological processes?

• A. Primary
• B. Tertiary
• C. WAS
• D. Secondary

Answer: (D)

Biological treatment uses microorganisms to break down organic matter. In the secondary stage, wastewater is exposed to bacteria and other microbes in aerated tanks or biofilm systems, letting them digest dissolved and suspended organics. This microbial activity converts organic material into new biomass, carbon dioxide, and water, significantly reducing BOD and suspended solids. Primary treatment relies mainly on physical removal of settleable solids, and tertiary treatment focuses on polishing the water with additional physical/chemical steps rather than biological digestion. Waste Activated Sludge is related to managing the biomass in secondary processes, not a separate biological stage itself.