Hydrolysis Acidification Tank vs Anaerobic Tank: Key Differences Explained

In wastewater treatment systems, both the hydrolysis acidification tank and the anaerobic tank in activated sludge processes are used to improve wastewater biodegradability.
Although they operate at similar biological stages, their functions and application goals are different.

Understanding these differences helps engineers choose the right process and optimize system performance.

Shared Functions of Both Tanks

Both tanks belong to the hydrolysis and acidification stage of anaerobic treatment.
Their main roles include:

• Breaking down large, hard-to-degrade organic matter such as proteins and cellulose
• Converting complex organics into small, easily biodegradable substances, mainly fatty acids
• Increasing the B/C ratio, which supports subsequent biological treatment processes

By improving biodegradability, both tanks help reduce the load on downstream treatment units.

Key Differences Between the Two Tanks

Despite similar biological stages, the operating conditions and treatment purposes differ significantly.

Oxygen Conditions

• Hydrolysis Acidification Tank
  Operates with facultative or obligate anaerobes.
  Dissolved oxygen (DO) is usually below 0.5 mg/L.
• Anaerobic Tank (Activated Sludge)
  Requires strict anaerobic conditions.
  DO is typically below 0.2 mg/L.

pH Operating Range

• Hydrolysis Acidification Tank
  Has a wider pH tolerance, generally 4–8.
• Anaerobic Tank
  Operates in a more stable pH range, usually 6.5–8.0, to support specific functional bacteria.

Main Treatment Function

• Hydrolysis Acidification Tank
  Focuses only on hydrolysis and acidification to improve biodegradability.
• Anaerobic Tank
  Performs hydrolysis and acidification, and also enables phosphorus release, which is essential for biological phosphorus removal.

Microorganisms Involved

• Hydrolysis Acidification Tank
  Dominated by acid-producing bacteria.
• Anaerobic Tank
  Contains various functional microorganisms, including phosphorus-accumulating organisms (PAOs).

Conclusion

The hydrolysis acidification tank is mainly used as a pretreatment unit to improve wastewater biodegradability before further biological treatment.

The anaerobic tank in activated sludge systems not only promotes hydrolysis and acidification but also enables phosphorus release, making it a key unit in biological phosphorus removal processes.

Choosing the correct tank configuration is essential for achieving stable operation and meeting discharge standards.