PAR 942 North Secondary Complex Improvements Project

Metro Wastewater Reclamation District, 
Colorado

PAR 942 North Secondary Complex Improvements Project

Awards: 
2012 Rocky Mountain States Best Civil/Infrastructure Project - ENR
Service Type: 
Project Highlights: 
  • Design of the largest capital improvements project in the history of the 160-mgd Robert W. Hite Treatment Facility.
  • Construction of a novel sidestream treatment process to remove centrate ammonia.
  • Use of computational fluid dynamic modeling coupled with full scale testing of the secondary clarifiers to confirm capacity improvement through inlet modifications
  • Detailed construction sequencing and operational strategies developed to allow continued compliance throughout construction.
MWRD Aerial
Four CaRRBs (on right) provided sufficient capacity to avoid construction of two additional aeration basins and secondary clarifiers.

The Metro Wastewater Reclamation District (MWRD) selected Carollo in 2004 to perform its North Secondary Improvements Project. The project consisted of capacity and operational improvements to MWRD’s 160 MGD Robert W. Hite Treatment Facility (RWHTF). The RWHTF site is home to two separate treatment plants each rated from 60 to 90 mgd. The north secondary complex air activated sludge process is operated in nitrification-denitrification mode, and the south secondary complex high-purity oxygen activated sludge process is operated for carbonaceous BOD removal only. Both complexes share common solids handling facilities. All of the nitrogen-rich centrate from the solids dewatering process is returned to the North Secondary Complex. The effluent criteria for discharge, which are applied on the combined RWHTF final effluent, include monthly limits for ammonia and a weekly maximum limit for NOx (nitrates plus nitrites). The facility achieves discharge requirements for the South Platte River through blending of the two plant effluents.

In 2004, Metro hired Carollo to initiate an expansion project to increase capacity at the North Secondary Complex. The project was to add two new aeration basins and secondary clarifiers to the existing complex of 12 aeration basins and secondary clarifiers. As an alternative approach, Carollo proposed the concept of centrate treatment with return activated sludge (RAS) in a re-aeration basin. The concept envisioned the construction of a common centrate and RAS re-aeration basin (CaRRB) in lieu of two new aeration basins and secondary clarifiers. This increased capacity came with a reduction in cost of approximately 50 percent. Construction of CaRRB was completed in June 2009, and the process was started up shortly thereafter. CaRRB performance has exceeded expectations and provided strong confirmation of the significant benefits offered by this process.

The existing twelve aeration basins were also modified to improve treatment and enhance nitrogen removal through addition of a new swing zone for oxic or anoxic operation and increased mixed liquor return. Another major component of the project was the upgrade of the 12 secondary clarifiers at the North Secondary Complex. The clarifiers exhibited inferior performance due to a variety of process and physical configuration limitations. Carollo evaluated both sludge settleability and clarifier hydraulic limitations. Hydraulic limitations were evaluated using hydrodynamic computer modeling supported by field testing to provide a “shopping list” of potential modifications, estimates of anticipated performance improvement, and costs associated with each. Full-scale testing of the recommended clarifier modification confirmed capacity improvements.

In addition to improved nitrification and centrate management, and increased treatment capacity, the design incorporated detailed construction sequencing and operational strategies to allow continued compliance throughout construction while nearby deep excavation and tie-in to existing operating channels and basins was completed.