Bacteria are the most ____ and the most ____ microbes in water treatment processes.

• A. Rarest and least useful
• B. Abundant but harmful
• C. Dormant and inactive
• D. Prevalent and useful

Answer: (D)

Biological treatment relies on microbes to clean wastewater, and bacteria are the most prevalent and useful among them. They exist in large numbers in activated sludge and biofilms and carry out the bulk of organic matter breakdown and nutrient removal, making them the backbone of the process. While some bacteria can be harmful in other contexts, in a properly operated treatment system the active, beneficial bacteria drive performance. The idea that bacteria are rare or mostly dormant doesn’t fit real operation, since treating wastewater depends on active microbial metabolism. So the best description is that bacteria are prevalent and useful.