The central concept of biological treatment is simple: microorganisms, collectively known as biomass, consume organic pollutants from wastewater as their food source. This metabolic process allows them to grow and reproduce, effectively converting harmful waste into harmless byproducts and more biomass. This entire process can be understood through three key metabolic phases:

1. Oxidation (Breathing) Approximately one-third of the organic matter consumed is converted into carbon dioxide and water. This is an energy-generating reaction, much like breathing, that powers the microorganisms’ life functions.
2. Synthesis (Growth) The remaining two-thirds of the organic matter is used to build new cell material. This is the growth phase, where the microbial population increases, creating more workers to treat the wastewater.
3. Endogenous Respiration (Survival) When the food supply in the wastewater becomes scarce, microorganisms don’t simply stop working. Instead, they begin to consume their own cellular material for energy. This is a self-destruction process that occurs under starvation conditions.

Understanding this fundamental cycle of microbial metabolism is the key to seeing why we need to build carefully engineered systems—or reactors—to create the ideal conditions for this natural process to occur efficiently and on a massive scale.