Just like any population, the microorganisms in a treatment system follow a predictable “boom and bust” life cycle. This growth pattern is critical for engineers to understand and manage.

Here are the key phases of the microbial growth pattern:

1. Lag Phase: When microbes are introduced to a new environment (like wastewater entering a tank), there is an initial adjustment period. During this phase, the population doesn’t grow as the microbes adapt to the new conditions.
2. Log Growth Phase (Boom Time): With an abundance of food, the population enters a period of rapid, exponential growth. The microbes are metabolizing waste and reproducing at their maximum rate.
3. Declining Growth Phase: As the population grows, the food source begins to run out. The growth rate slows down because there is no longer enough substrate to support the rapid expansion.
4. Stationary Phase: The population size stabilizes. The rate of new cell growth equals the rate of cell death, resulting in no net change in the number of viable microorganisms.
5. Endogenous Phase (The Bust/Famine): With the food supply depleted, the death rate surpasses the growth rate. The population declines as microbes die off or enter the endogenous respiration state, consuming themselves to survive.

Understanding this natural cycle allows engineers to control the process and achieve specific treatment goals.