Maintaining process stability and efficiency is critically dependent on managing the input of toxic or inhibitory substances. Even robust biological systems can be disrupted or completely disabled by chemical shocks. This section defines toxicity in the context of wastewater treatment, describes the microbial response to inhibitory compounds, and introduces the kinetic models used to predict system behavior under these challenging conditions.

Toxicity is defined as the property of a substance to deter or inhibit the metabolic processes of microbial cells. At high concentrations, toxic substances can kill the microorganisms, while at sublethal concentrations, they reduce metabolic activity, leading to a decline in treatment performance.

In some cases, microorganisms can develop a tolerance to an inhibitory substance through a process known as acclimation. This adaptive response may occur through several mechanisms, such as the biological neutralization of the toxic material or the selective growth of a more resistant microbial population within the culture.

The standard Monod kinetic model does not account for inhibition. To describe the kinetics of a system where high concentrations of the substrate itself become inhibitory, the Haldane equation is used:

μ = (μ_max * S) / (S + K + (S² / K_i))          (Eq. 18)

This equation is a modification of the Monod model that includes an inhibition constant (Ki). While the Monod model predicts that the growth rate will plateau at high substrate concentrations, the Haldane model shows that after reaching a peak, the specific growth rate will decrease as the substrate concentration becomes inhibitory. A smaller K_i value indicates a greater degree of inhibition.

Anaerobic processes are particularly sensitive to certain inorganic and organic compounds. Cations that are essential at low concentrations can become strongly inhibitory at higher levels.

Inhibitory Concentrations of Cations for Anaerobic Processes

Other key compounds are also highly toxic to anaerobic systems:

• Soluble Sulfides: Concentrations exceeding 200 mg/L will strongly inhibit the metabolic activity of the methanogenic population.
• Free Ammonia: This form of ammonia is more toxic than the ammonium ion. Concentrations of free ammonia exceeding 150 mg/L can cause severe toxicity.

Managing general toxicity is a prerequisite for reliable treatment, but some processes are specifically designed to remove substances that, while not always toxic, are considered pollutants requiring removal, such as nitrogen and phosphorus.