Which parameter most directly determines the specific growth rate of bacteria in a wastewater treatment plant?

• A. temperature
• B. pH
• C. nutrient concentration
• D. dissolved oxygen concentration

Answer: (C)

Availability of nutrients controls the specific growth rate. In bacterial growth, the rate at which cells divide depends on how much usable substrate (carbon and essential nutrients) is available. This relationship is captured by Monod-type dynamics: when substrate is plentiful, the growth rate approaches a maximum; when substrate is scarce, the rate slows down in direct proportion to nutrient concentration. In wastewater treatment, nutrients often limit growth, so providing more nutrients directly speeds up growth until other factors become limiting. Temperature, pH, and dissolved oxygen shape and constrain growth, but they don’t set the rate as directly as substrate availability does. That makes nutrient concentration the best explanation for the observed specific growth rate.