How to Reduce Total Nitrogen Levels

AnoxKaldnes Hybas™ Helps Meet New Limits For Field's Point Wastewater Treatment Facility

The Client's Needs

The Narragansett Bay Commission's service area encompasses the metropolitan Providence and Blackstone Valley areas. Originally constructed in 1901, the Field's Point Wastewater Treatment Facility (FPWWTF) provides secondary treatment for maximum monthly flows of up to 77 MGD and primary treatment and disinfection for an additional 123 MGD of wet weather flows. Total treatment capacity at the Field's Point WWTF is 200 MGD.

After receiving an NPDES operating permit that required the plant to remove total nitrogen (TN) to 5 mg/L from May through October the Narragansett Bay Commission chose AnoxKaldnes Hybas (Hybrid Biofilm Activated Sludge), an integrated fixed film activated sludge (IFAS) process, as their solution.

The Solution

In 2008, AnoxKaldnes Hybas™ designs were completed by Veolia for the upgrade of the Field's Point Wastewater Treatment Facility to meet new seasonal effluent limits for TN and TIN.

The AnoxKaldnes Hybas™ process has the advantage of accepting a significant combined sewer overflow (CSO) while maintaining a nitrifying biomass and eliminating the need to construct new aeration basins on a constrained site.

The IFAS conversion was accomplished without the need to add secondary clarifier capacity. FPWWTF began operation as the largest IFAS system in the world.

Process Description

The Field's Point Wastewater Treatment project involved process design, equipment supply, training, in-field services, performance testing and guarantees related to the AnoxKaldnes Hybas™ process. The biological nutrient removal process included the following unit processes:

  • Pre-Anoxic Stage with two reactors per train for denitrification using the influent BOD as a carbon source and return mixed liquor from the aerobic stage for the source of nitrate.
  • Aerobic Nitrification Stage with IFAS and post- IFAS oxic zones for BOD and Nitrification.
  • Post-Anoxic Stage with two reactors per train for additional denitrification using an external carbon source (if needed).
  • Re-Aeration Stage for removal of any residual carbon source and nitrogen gas stripping.

The Client

The Narragansett Bay Commission owns and operates the Field's Point Wastewater Treatment Facility (FPWWTF) in Providence, Rhode Island.


  • Retrofit of the existing infrastructure allows the plant to meet stricter nutrient limits without new basins
  • Total nitrogen removal in existing activated sludge basin volumes
  • Reduced oxic volume required for nitrification allowing for primary and secondary anoxic zones