Secondary Clarifier Upgrades in San Francisco

Client: San Francisco Public Utilities Commission
Location: San Francisco, CA

As the sixteen secondary clarifiers at the San Francisco Public Utilities Commission (SFPUC) Wastewater Enterprise Southeast Plant (SEP) were reaching the end of their useful life, SFPUC retained Hazen and Sawyer to assist in planning and designing the clarifier upgrades.

Upgrades included a larger center well for flocculation, energy dissipating inlets and the removal of target baffles.

Hazen, as an owner’s representative throughout the procurement and construction phase, worked with the construction manager, contractor and the clarifier manufacturer to build consensus on clarifier features and a testing plan.

Project Outcomes and Benefits

  • Full process evaluation of the existing clarifiers that included clarifier stress testing, sludge zone settling tests, flocculation testing and dispersed solids testing.
  • CFD modeling revealed several opportunities for clarifier performance improvement that were incorporated into the design of the clarifier upgrades.
  • Upgrades included a larger center well for flocculation, energy dissipating inlets, and the removal of target baffles.
  • Startup results showed significant performance improvement in retrofitted clarifiers compared to existing clarifiers.

Planning began with a full process evaluation of the existing clarifiers that included clarifier stress testing, sludge zone settling tests, flocculation testing and dispersed solids testing, among others. The extensive field testing, executed in conjunction with SFPUC staff, was used to develop a computational fluid dynamics (CFD) model of the clarifiers. The model revealed several opportunities for clarifier performance improvement that were incorporated into the design of the clarifier upgrades. These upgrades included a larger center well for flocculation, energy dissipating inlets and the removal of target baffles.

With the assistance of Hazen, SFPUC recommended features in the performance based specification for the clarifier upgrades. Hazen, as an owner’s representative throughout the procurement and construction phase, worked with the construction manager, contractor and the clarifier manufacturer to build consensus on clarifier features and a testing plan.

With two of the sixteen clarifiers retrofitted, Hazen completed startup testing of the retrofitted clarifiers to prove the equal or better performance of the retrofitted clarifiers. The startup results showed significant performance improvement in retrofitted clarifiers compared to existing clarifiers. The testing allowed the contractor and SFPUC to concur that the contractor satisfactorily met the performance specification.

For more information on this project, or to discuss a similar project in your area, contact Irene Chu at ichu@hazenandsawyer.com