Anaerobic treatment offers a strategic advantage for the stabilization of high-strength organic wastes, such as municipal and industrial sludges. Unlike aerobic processes, anaerobic digestion operates in the absence of free oxygen. Its key advantage is the high conversion rate of organic matter into biogas (primarily methane and carbon dioxide), coupled with a very low production of new biological cells. This significantly reduces the volume of sludge requiring final disposal.

The process of anaerobic treatment occurs via a two-stage mechanism:

1. Acid Formation: In the first stage, a group of organisms known as acid-forming bacteria convert complex organic materials (proteins, fats, carbohydrates) into simpler, volatile organic acids like acetic and propionic acid.
2. Methane Formation: In the second stage, a distinct group of slow-growing, methane-forming bacteria (methanogens) convert these organic acids into the final end products: methane (CH₄) and carbon dioxide (CO₂).

The physical configuration of anaerobic systems can be tailored to the characteristics of the waste being treated. The three basic process designs are:

• Conventional Process: A simple, flow-through reactor where the hydraulic retention time (HRT) is equal to the solids retention time (SRT). This design is most suitable for concentrated wastes like sludges, where a long HRT is economically feasible.
• Anaerobic Activated Sludge Process: This design incorporates solids recycle, separating the digested solids from the liquid effluent and returning them to the reactor. This allows the SRT to be uncoupled from the HRT, making it possible to maintain a long solids age (necessary for slow-growing methanogens) while treating more dilute wastes at a shorter hydraulic residence time.
• Anaerobic Filter Process: This is a fixed-film configuration that uses contact media (similar to a trickling filter) to retain the active biomass within the reactor. Wastewater flows through the media, allowing for efficient treatment of soluble wastes.

Anaerobic processes are significantly more sensitive to operational changes and environmental conditions than their aerobic counterparts. The most critical operational factor is maintaining a Solids Retention Time (SRT) well above the minimum required for the methane-producing bacteria to reproduce. Because these organisms are very slow-growing, an SRT that is too short will cause them to be “washed out” of the system faster than they can grow, leading to complete process failure. In practice, design and operational SRTs are typically 2 to 10 times this minimum value to ensure process stability and prevent washout.

The sensitivity of anaerobic processes extends to their susceptibility to certain inhibitory substances, a critical management challenge that will be detailed in the next section.