Biological treatment is the most widely applied method for the removal and stabilization of biodegradable substances found in wastewater, which can be present in suspended, colloidal, or dissolved forms. The process harnesses natural biological phenomena, primarily employing bacteria, to accelerate the degradation of organic matter.

1.1. Key Microorganisms and Biological Components

The biological system responsible for treatment is a complex ecosystem. While bacteria are the primary organisms, other microorganisms play significant roles. The system involves:

• Bacteria (Primary Feeders): Consume organic substrates, oxygen, and growth factors to produce new cell biomass, energy, CO₂, and H₂O.
• Protozoa (Secondary Feeders): Consume bacteria and other products.
• Other Organisms: The full biomass often includes a collection of bacteria, yeast, molds, protozoa, rotifers, worms, and insect larvae, which aggregate into a gelatinous mass known as a biological floc.

1.2. The Microbial Growth Curve

When microorganisms are introduced into a wastewater environment, their population follows a distinct growth pattern over time, which includes several phases:

• Lag Phase: Initial period of adaptation.
• Log Growth Phase: Exponential increase in microbial population, where growth is non-rate limited.
• Declining Growth Phase: Growth slows due to the exhaustion of nutrients or the accumulation of toxic by-products.
• Stationary Phase: The rate of new cell growth equals the rate of cell death.
• Death/Endogenous Phase: The population declines as cells undergo auto-digestion or self-destruction (endogenous respiration) due to a lack of external food sources.

1.3. Governing Environmental Factors

The population dynamics and metabolic efficiency of microorganisms are highly dependent on environmental conditions, including:

• pH and Temperature: Reaction rates increase with temperature up to an optimum level.
• Substrate: Type and concentration of the food source (organic matter).
• Nutrients: Availability of essential elements like nitrogen, phosphorus, and sulfur.
• Hydrogen Acceptor: Presence of oxygen (aerobic) or its absence (anaerobic).
• Toxicity: Presence of inhibitory substances in the media or as by-products.
• Mixing: Degree of contact between microorganisms and the substrate.