At a detention time of 60 days, what removal efficiencies are typical?

• A. 85-95% BOD; 99% fecal coliform
• B. 90-95% coliform bacteria; 70-80% BOD
• C. 70-80% BOD; 60% coliform
• D. 50% BOD; 50% coliform

Answer: (B)

Long detention times allow more complete biological digestion of organic matter and more time for natural die-off and inactivation of bacteria. With about 60 days in a stabilization system, organic matter has ample opportunity to be degraded, so BOD removal tends to be around 70-80%. At the same time, indicator bacteria like fecal coliforms are substantially reduced through extended biological activity, sedimentation, and sunlight exposure, giving a high removal range near 90-95%.

Other options don’t fit this combination for a 60-day residence: they either show too high BOD removal or too low coliform removal, which wouldn’t reflect the strong bacterial die-off expected with this detention time.