During the growth cycle, the population can double approximately every how long?

• A. Every 2 hours
• B. Every 20 minutes
• C. Every 24 hours
• D. Every 5 minutes

Answer: (B)

In the exponential (growth) phase, microbial numbers rise at a constant rate, so the population doubles after a characteristic interval called the doubling time. Under ideal conditions in wastewater treatment—plenty of nutrients and oxygen—many microbes have a doubling time of about 20 minutes, so the population can double roughly every 20 minutes during this phase.

Doubling every 2 hours would indicate much slower growth than typically observed in nutrient-rich wastewater environments. Doubling every 24 hours is far too slow for exponential growth in these conditions, and doubling every 5 minutes is unrealistically fast for most bacteria in standard wastewater settings.