A city in south east asia has a sewage treatment plant based on cascade activated sludge process. Originally the plant was designed only for BOD removal at a capacity of 58,000 m3/day. The plant is having eight trains of cascades having individual capacity of 7250 m3/day. Each train is designed with four cascades of 10 m x 10 m x 5.7 m size having 570 m3 active volume for suspended growth based treatment. The plant is designed to cater the cold winter conditions of the region for BOD removal.

Upgradation of activated sludge
One typical Cascade with separate clarifier for the treatment


The plant is receiving TKN to the tune of 70 ppm and due to new discharge standards for municipal wastewater treatment, the Ammonia in the outlet should be brought down to < 5 ppm and TN < 30 ppm. Due to island country, the authorities operating the plant have severe space restriction and thus a solution which helps them to achieve desired level of nitrification within the same infrastructure without additional civil construction and land requirement is highly sought after.

  • Existing BOD removal plant
  • Having high TKN load of 70 ppm
  • Requires outlet Ammonia < 5 ppm and TN < 30 ppm

THE SOLUTION - Upgradation of activated sludge

Based on our extensive experience with cold weather nitrification using Levapor IFAS process, a solution was proposed to utilize two of the first reactor of the cascade as suspended growth reactors while the last two reactors to be converted in to Levapor carriers based IFAS reactors with 11% media filling in it.

Proposed Solution:

  • Use two first reactors of the cascade as suspended growth reactor
  • Two last reactor to be upgraded to Levapor IFAS reactor by adding carriers at 11% filling.

The above solution was tested in a pilot process configuration and the results were confirmed at site for its efficacy to achieve desired level of nitrification as per client’s requirement.

Upgradation of activated sludge
Levapor IFAS process pilot plant at site
Full scale plant modification with two IFAS reactors.

After successful trials, the project was implemented at site with minimal modifications of:

  • Retention screen placement at the IFAS reactors
  • Rearrangement of diffuser grid to meet air requirement for nitrification and IFAS reactor mixing requirement.


The plant is achieving consistently nitrification to Ammonia < 5 ppm level and COD < 30 ppm at the outlet with much lower down time for the process start up and upgradation.

Summary of results achieved:


  • Minimal changes to existing infrastructure
  • Quicker implementation and process start up after upgradation
  • Achieving consistent level of Nitrification even with less than 6 hours HRT for cold climate
  • Quick start up of the process and stabilization