4.1 Introduction to Anaerobic Treatment

Anaerobic treatment is a biological process where microorganisms break down organic matter in the absence of free oxygen. It has long been the standard for stabilizing high-concentration organic wastes, such as sludges, and its application has expanded to high-strength soluble industrial wastewaters, driven by its unique metabolic pathway and potential for energy recovery.

4.2 Process Mechanism and Advantages

The anaerobic degradation of organic matter is a sequential, two-stage process:

1. Acid Formation: A group of “acid-forming” bacteria converts complex organics (proteins, fats, carbohydrates) into simpler organic acids, such as acetic and propionic acid.
2. Methane Formation: A distinct group of slow-growing, “methane-producing” bacteria consumes the organic acids and converts them into methane (CH₄) and carbon dioxide (CO₂).

The primary advantage of this process is that “a high percentage conversion of organic matter to gases and liquid and a low percentage conversion to biological cells” occurs. This directly translates to significantly lower sludge production compared to aerobic methods, reducing the costs associated with biosolids handling and disposal.

4.3 Operational Considerations and Sensitivities

Anaerobic processes are generally “more sensitive to operating parameters and their environments as compared to aerobic processes.” Careful control is required to maintain a balanced microbial population.

The single most critical control parameter is the biological solids retention time (SRT) or sludge age (θx). Due to the slow growth kinetics of methanogens, the SRT must be kept long enough to prevent the “washout” of these critical methane-producing bacteria from the system. Anaerobic systems are also highly sensitive to chemical inhibition, including:

• Light Metal Cations: While stimulatory at low concentrations, cations become inhibitory at higher levels.
  • Sodium (Na): Stimulatory at 100–200 mg/L; strongly inhibitory at 8,000 mg/L.
  • Potassium (K): Stimulatory at 200–400 mg/L; strongly inhibitory at 12,000 mg/L.
  • Calcium (Ca): Stimulatory at 100–200 mg/L; strongly inhibitory at 8,000 mg/L.
  • Magnesium (Mg): Stimulatory at 75–150 mg/L; strongly inhibitory at 3,000 mg/L.
• Sulfides: Soluble sulfide concentrations exceeding 200 mg/L will strongly inhibit the methanogenic population.
• Ammonia: Free ammonia is more toxic than the ammonium ion, with severe toxicity resulting if free ammonia concentrations exceed 150 mg/L.

4.4 Conventional Anaerobic Digester Parameters

The typical operating parameters for conventional anaerobic sludge digesters are summarized below.

4.5 Transition to Comparative Analysis

Understanding these operational requirements and sensitivities is crucial before proceeding with a direct, factor-by-factor comparison between the two major system types.