Anaerobic treatment stabilizes organic matter in the absence of free oxygen. It is widely used for high-strength wastes and sludges.

7.1. The Two-Stage Anaerobic Process

1. Acid Formation: Complex organic materials (proteins, fats, carbohydrates) are converted into simple organic acids by acid-forming bacteria.
2. Methane Formation: Methane-producing bacteria convert the organic acids into methane (CH₄) and carbon dioxide (CO₂), stabilizing the waste.

7.2. Anaerobic Process Designs

• Conventional Process: A flow-through system with no sludge recirculation, typically used for concentrated wastes like sludges. Hydraulic retention time (HRT) equals solids retention time (SRT).
• Anaerobic Contact Process: Includes solids recycle, allowing for a shorter HRT than SRT. This makes it suitable for more dilute wastes.
• Anaerobic Filter: A fixed-film process where wastewater passes through a contact media, retaining a high concentration of biomass.

7.3. Inhibitory Substances and Toxicity

Anaerobic processes are more sensitive to environmental conditions than aerobic ones. Methane-producing bacteria are particularly susceptible to inhibition.

Inhibitory Concentrations of Light Metal Cations | Cation | Stimulatory Conc. (mg/L) | Strong Inhibitory Conc. (mg/L) | | :— | :— | :— | | Sodium | 100–200 | 8,000 | | Potassium | 200–400 | 12,000 | | Calcium | 100–200 | 8,000 | | Magnesium | 75–150 | 3,000 |

Other inhibitors include:

• Soluble Sulfides: Strongly inhibitory above 200 mg/L.
• Ammonia: Free ammonia is highly toxic, with severe effects above 150 mg/L.