Activated sludge typically has what percentage of microbes in the sludge?

• A. 90%
• B. 95%
• C. 80%
• D. 100%

Answer: (B)

Activated sludge is a dense, biologically active mixture where the work of breaking down organics is done by microorganisms. Because of that, the solids in the sludge are predominantly microbial biomass. In typical activated-sludge systems, about 95% of the solids are microbes, with the remainder being inorganic material, dead cells, and other non-biological substances. That’s why 95% is the best answer. The other values aren’t representative because there is always some non-biological material present, so the microbial share isn’t effectively 100%, and it’s not as low as 80% or 90% in standard practice.