Anaerobic Digestion for Biosolids: Process and Benefits

Anaerobic digestion is the controlled biological decomposition of organic solids in the absence of oxygen, producing stabilized biosolids and energy-rich biogas. For wastewater treatment facilities, anaerobic digestion wastewater solids management represents both the primary path to Class B biosolids and a significant opportunity for biogas recovery and energy offset.

Process Overview

Anaerobic digestion occurs through four sequential biochemical stages, each driven by different microbial communities:

1. Hydrolysis: Complex organic polymers (proteins, carbohydrates, fats) are broken down into soluble monomers by extracellular enzymes. This is often the rate-limiting step, particularly for waste activated sludge.

2. Acidogenesis: Soluble organics are fermented by acidogenic bacteria into volatile fatty acids (VFAs), alcohols, hydrogen, and carbon dioxide.

3. Acetogenesis: VFAs and alcohols are converted to acetic acid, hydrogen, and carbon dioxide by acetogenic bacteria.

4. Methanogenesis: Methanogenic archaea convert acetate and hydrogen/carbon dioxide into methane (CH₄) and carbon dioxide (CO₂). This final step produces the biogas.

The entire process operates at controlled temperature and pH, with typical solids retention times of 15–20 days for mesophilic and 12–15 days for thermophilic systems.

Mesophilic vs. Thermophilic Digestion

The two primary temperature regimes offer distinct advantages:

Most North American facilities operate mesophilic digesters due to their greater process stability and lower heating requirements. Thermophilic digestion is gaining interest where Class A biosolids are required or where higher volatile solids destruction improves downstream economics.

Digester Configurations

Conventional single-stage: The most common configuration. A single mixed and heated vessel receives raw sludge and produces biogas. Mixing is accomplished by gas recirculation, mechanical mixers, or draft tube systems.

Two-stage (acid/gas): The first stage operates at shorter retention time and lower pH to optimize hydrolysis and acidogenesis. The second stage is maintained at optimal pH (6.8–7.2) for methanogenesis. This configuration can increase gas production by 10–20% but adds operational complexity.

Egg-shaped digesters: Common in European facilities and increasingly adopted in North America. The geometry minimizes dead zones, improves mixing efficiency, and reduces scum accumulation. Higher capital cost but lower maintenance.

Temperature-phased anaerobic digestion (TPAD): Combines a short thermophilic first stage (3–5 days) with a longer mesophilic second stage (12–15 days). Achieves Class A biosolids while maintaining the stability advantages of mesophilic operation.

Representative digester mixing and covered-lagoon systems:

• Landia — GasMix external digester mixing (all moving parts outside the tank)
• EFI (Environmental Fabrics International) — covered-lagoon anaerobic digester (CLD) systems and geomembrane covers

Biogas Production and Utilization

Biogas from anaerobic digestion is typically 55–65% methane and 35–45% carbon dioxide, with trace amounts of hydrogen sulfide, siloxanes, and moisture. A well-operated digester produces approximately 12–18 cubic feet of biogas per pound of volatile solids destroyed.

Biogas utilization options include:

• Combined heat and power (CHP): Internal combustion engines or microturbines generate electricity and recover waste heat for digester heating. CHP systems can offset 30–80% of a facility's electricity purchases.
• Boiler fuel: Direct combustion in boilers for digester and building heating. The simplest and lowest-cost utilization option.
• Renewable natural gas (RNG): Biogas is cleaned and upgraded to pipeline-quality methane (>97% CH₄). Increasing interest due to renewable energy credits and low-carbon fuel standard incentives.
• Fuel cells: Emerging technology offering higher electrical efficiency (40–50%) than reciprocating engines (30–38%).

Pretreatment and Enhancement

Several technologies improve digester performance by making organic matter more accessible to microbial degradation:

• Thermal hydrolysis (THP): High-temperature, high-pressure pretreatment (165°C, 6 bar) disrupts cell walls, increasing VS destruction by 15–25% and improving dewaterability. Cambi, Veolia, and Haarslev are major THP suppliers.
• Ultrasonic disintegration: Cavitation disrupts floc structure and cell walls. Lower capital cost than THP but less effective for waste activated sludge.
• Ozone pretreatment: Oxidative cell lysis improves hydrolysis rates. Used at some European facilities.
• Co-digestion: Adding high-strength organic waste (FOG, food waste) increases biogas production. Many facilities accept hauled waste as a revenue source while boosting energy recovery.

Operational Parameters

Successful digester operation requires monitoring of:

• Volatile acids/alkalinity ratio: Should remain below 0.3. Rising VA/ALK indicates process stress.
• pH: Optimal range 6.8–7.4. Below 6.5 inhibits methanogens.
• Temperature: Maintain within ±1°C of setpoint.
• Gas production and composition: Declining methane percentage indicates process imbalance.
• Ammonia: Above 1,500–3,000 mg/L can inhibit methanogenesis, depending on acclimation.

Benefits of Anaerobic Digestion

For facilities generating sufficient solids to justify the capital investment (typically >5 MGD), anaerobic digestion provides volume reduction (30–50% of feed solids), pathogen reduction to Class B or Class A standards, energy recovery through biogas, reduced odor potential compared to raw sludge, and potential revenue from co-digestion tipping fees and RNG sales.

When Anaerobic Digestion Makes Sense

Anaerobic digestion is most cost-effective at facilities processing more than 40–50 dry tons per day of combined primary and waste activated sludge. Smaller facilities may find aerobic digestion more practical due to lower capital requirements. The economics improve significantl