At 180 days DT, what removals are typical?

• A. 85-95% BOD removal; 99% fecal coliform removal
• B. 70-80% BOD; 90% coliform
• C. 50% BOD; 50% fecal coliform
• D. 0% BOD; 0% fecal coliform

Answer: (A)

When detention time is very long, the treatment system has more time for the biological and physical processes that remove contaminants to work. Microorganisms have more opportunity to consume organic matter, so BOD removal climbs into the high eighties to mid-nineties percent range. At the same time, fecal coliforms are reduced through settling, adsorption to solids, natural die-off, and disinfection effects that accumulate over time, often reaching about 99% removal. So the scenario that matches 180 days of detention is roughly 85-95% BOD removal with about 99% fecal coliform removal. The other options show lower removals that wouldn’t be typical after such a long detention time, or values that suggest virtually no treatment at all.