Activated sludge process with extended aeration is carried out during biological treatment.

Inside the aeration tank, a fine bubble diffused air system is installed to maintain set levels of dissolved oxygen to sustain the growth of the micro-organisms required for  biological treatment. There are 3 zones inside the tank:

• contact zone
• anoxic zone
• aerobic zone

The preliminary treated sewage enters into the aeration tank without primary sedimentation process. It first flows through the air contact zone for complete mixing. After that, it reaches the anoxic denitrification zone. Finally, the sewage flows to the aerobic nitrification zone for nitrogen removal. 

The biologically treated sewage then flows to the final sedimentation tank along the aerated interconnecting channel, which prevents the settlement of sludge.