A solid grasp of the fundamental biological principles is the bedrock of effective process control and optimisation. To manipulate a biological treatment system for peak performance, one must first understand the living components at its core and the environmental conditions that dictate their behaviour. This section reviews the core concepts governing the microbial conversion of wastewater contaminants into manageable byproducts, providing the essential foundation for the advanced strategies discussed in subsequent sections.

Biological treatment is the engineered use of microorganisms to stabilize biologically degradable substances present in wastewaters. These processes employ bacteria as the primary organisms to degrade and stabilize organic matter, using it as a food source to produce protoplasm for new cells.

To effectively design and operate these systems, the characteristics of the incoming wastewater must be clearly defined. The primary metrics used for this characterisation include:

• Chemical Oxygen Demand (COD): The total amount of oxygen required to chemically oxidize the organic and inorganic compounds in the water.
• Biochemical Oxygen Demand (BOD): The amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample.
• Total Organic Carbon (TOC): A direct measure of the total amount of carbon bound in organic compounds.
• Volatile Suspended Solids (VSS): The volatile fraction of total suspended solids, often used as a surrogate measure for the mass of active microorganisms in a reactor.

The growth of the microbial population responsible for treatment follows a distinct pattern. Understanding these phases is crucial for process control:

• Lag Phase: An initial period of adaptation where microorganisms acclimate to the new environment before significant growth begins.
• Log Growth Phase: A period of exponential growth where the rate of metabolism is at its maximum, driven by an excess of available food (substrate). While this phase represents the fastest substrate removal, operating a system here is undesirable as it produces a dispersed, poorly settling biomass.
• Declining Growth Phase: Growth slows as the substrate becomes limited or as inhibitory byproducts accumulate.
• Stationary Phase: The rate of new cell growth equals the rate of cell death, resulting in no net increase in microbial mass.
• Endogenous/Death Phase: With the substrate largely depleted, microorganisms begin to consume their own cellular material for energy (endogenous respiration), leading to a net decrease in the microbial population. Conversely, operating a system in this phase is often the goal, as it promotes the formation of a dense, well-settling floc, leading to a clear effluent.

The population dynamics within a treatment reactor are governed by several key environmental factors. Diligent control over these conditions is essential for maintaining a healthy and efficient microbial community.

• pH and Temperature: Each microbial species has an optimal range for growth; deviations can significantly slow metabolic rates.
• Substrate Concentration: The availability of “food” (organic matter) directly influences the rate of microbial growth and respiration.
• Nutrients: Essential nutrients like nitrogen and phosphorus must be available in sufficient quantities to support cell synthesis.
• Toxicity: The presence of certain substances can inhibit or halt metabolic activity, leading to process failure.

With these high-level principles of microbial population dynamics established, we can now examine the specific biochemical reactions and kinetic models that drive the treatment process.