Final answer:
Microbes are involved in the secondary and tertiary stages of wastewater treatment. In the secondary stage, aerobic bacteria break down organic matter, while tertiary treatment involves further nutrient decomposition by bacteria and filtration. Thus, the answer is B) secondary and tertiary.
Step-by-step explanation:
Microbes play a crucial role in the secondary treatment of wastewater, where the main aim is to remove dissolved and suspended biological matter. During this stage, aerobic bacteria break down organic materials such as sugars and fats, a process typically carried out in controlled environments like fixed film systems or suspended growth systems. This part of the treatment requires oxygen to support the bacterial activity, which is provided by mixing air with the sewage.
Although microbes are most actively involved in secondary treatment, they are also used in tertiary treatment, where further decomposition of nutrients occurs, again involving bacteria and filtration processes. However, primary treatment mainly involves the physical settling or flotation to remove organic solids, fat, and grease, without the active role of microbes.
Therefore, the correct answer to which steps of wastewater treatment involve microbes is B) secondary and tertiary.