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Keeping the Water Flowing at Full Capacity from Hetch Hetchy, One Underdrain at a Time

Underdrains Upgrade at Harry Tracy Water Treatment Plant
  • Steve Kech
Demolishing the plastic underdrains at Harry Tracy Water Treatment Plant.

The SFPUC’s Harry Tracy Water Treatment Plant has just completed upgrading their filtration systems, which was part of the Hetch Hetchy Water System Improvement Program. According to Resident Engineer, Joseph Liu with the Water Enterprise, the necessary maintenance being done ensures the Mountain Tunnel plant, the Harry Tracy plant and other local reservoirs, will be functioning at full capacity to intake water from Hetch Hetchy. 

“Those plastic drains were installed in 2009. A couple of them ran into some issues, and it’s been about three years since the second one failed in 2020,” Liu said.   

The project to demolish the deteriorating plastic underdrains and replace them with modern stainless-steel models began last fall. Six of the fifteen filter boxes at the Harry Tracy plant needed to be modernized.  “It’s neat seeing these rebuilt. This isn’t something you get to see done too often,” Liu added.

Replacing the anthracite coal filter media in one of the underdrains at Harry Tracy Water Treatment Plant.

How do these filters work? Imagine six giant Brita pitcher water filters, each the size of a volleyball court. The filtration method used by that kitchen counter model is the solution to purification practiced at Harry Tracy, but only bigger. These filtration devices work by using anthracite coal and sand filter media to latch onto unwanted particulates in the water. Then, the underdrains allow the water to be filtered thoroughly by the treatment plant to create clean water that you use daily, like tap water.  

However, the filters need to be cleaned regularly too – generally after 40-70 hours of constant filtration of the water. Cleaning involves backwashing the underdrains to dislodge captured particles as well as blowing air through to wash out filtration ingredients. At the Harry Tracy site, three of the filters have been disinfected and will be put into service any day now. The other three will be finished soon.  

"Support from the plant staff has been helpful on the completion of the project," Liu said.