The notation "A2/O" is used in wastewater treatment to refer to a specific type of activated sludge process called the Anaerobic/Anoxic/Aerobic (A2/O) process.

Here's a breakdown of what each stage represents:

• Anaerobic (A): This stage involves a reactor where bacteria break down organic matter in the absence of oxygen. This process produces volatile fatty acids (VFAs) and biogas.
• Anoxic (O): This stage follows the anaerobic stage and utilizes bacteria that can use nitrate or nitrite as an electron acceptor instead of oxygen. This is essential for nitrogen removal as the bacteria convert nitrate into nitrogen gas.
• Aerobic (A): This final stage provides oxygen to allow aerobic bacteria to further break down organic matter, ultimately producing carbon dioxide and water.

Key benefits of the A2/O process:

• High efficiency in removing organic matter (BOD), typically exceeding 90%.
• Effective in removing nitrogen through the anoxic stage.
• Reduces sludge production compared to other activated sludge processes.

The A2/O process is a highly effective and versatile wastewater treatment method, widely used in municipal and industrial wastewater treatment plants.