Which stage of wastewater treatment is indicated by the removal of 45-55% of BOD?

The stage of wastewater treatment indicated by the removal of 45-55% of Biochemical Oxygen Demand (BOD) is secondary treatment. This stage focuses on the biological processes that reduce organic matter in wastewater. During secondary treatment, microorganisms consume the organic pollutants, significantly lowering the BOD levels in the effluent.

Typically, preliminary treatment occurs before secondary treatment and involves screening and grit removal, which do not significantly impact BOD levels. Primary treatment further removes larger solids and some organic matter, usually achieving around 20-30% BOD removal. Tertiary treatment is the final step, which aims to polish the effluent and can lead to much higher percentages of BOD removal, often exceeding 90%. Thus, secondary treatment is characterized by its effectiveness in utilizing biological processes to achieve the specified percentage of BOD removal, making it the correct answer.