Toxicity is the property of a substance to deter or inhibit the metabolic processes of cells. Its effect depends on the substance’s nature, concentration, exposure time, and environmental conditions.

4.1. Impact of Toxicity and Acclimation

Toxic substances can interfere with the osmotic balance or enzyme systems of microorganisms. At high concentrations, they can kill microbes, while at sublethal levels, they reduce their activity. However, microorganisms can often acclimate or adapt to inhibitory substances through:

• Neutralization of the toxic material.
• Selective growth of a tolerant culture.
• Using the toxic substance itself as a substrate (e.g., cyanide, phenol).

4.2. Toxicant Interactions

When multiple toxicants are present, their combined effect can vary, as classified by Warren:

• Supra-additive: The combined toxicity is greater than the sum of individual toxicities.
• Strictly additive: The combined toxicity is equal to the sum.
• Infra-additive: The combined toxicity is less than the sum.
• No Interaction: The substances act independently.
• Antagonism: One substance reduces the toxicity of the other.

4.3. Treatment of Toxic Wastes

Biological treatment is increasingly used for toxic organic and inorganic wastes due to its economic advantages and the robustness of microbial systems. Fixed-film systems are considered more stable and less sensitive to toxicity than suspended growth systems because the biofilm structure provides a buffer against shock loads of toxins.