Project Overview

This background image shows the water intake screen at the entrance of Big Cottonwood Water Treatment Plant, where water appears between a concrete barrier and a silver rail, behind which people can stand on a platform to perform maintenance on the intake screen. A green, u-shaped tube opens toward the water and trees grow in the area above the concrete wall, dividing the water from the roadway.

The Big Cottonwood Water Treatment Plant (BCWTP) is located in Big Cottonwood Canyon, in the city of Cottonwood Heights. Designed in 1956 and brought online in 1959, the 38-million gallon per day (mgd) water treatment plant is owned and operated by Salt Lake City Department of Public Utilities (SLCDPU). The BCWTP is a critical component of SLCDPU’s drinking water system and supplies 40% of the drinking water in the department's regional service area.

BCWTP needs to be rebuilt to replace deteriorating structures and equipment, increase plant and regional resiliency, and meet current code requirements (e.g., seismic, electrical). This will improve plant safety and continue to protect public health by providing safe drinking water to the communities served by BCWTP. The project will rebuild the plant within the existing fence line.

A history of Big Cottonwood Water Treatment Plant reads as follows in a blue ombre rectangle with gray text over a mostly white background: 1957: Plant constructed. Early 1980s: Capacity upgrade. Early 1990s: New clarifier and modifications to the receiving basin. 1998: Plant upgrades (seismic, chemical, mechanical, and electrical). 2012: installed on-site generation for chlorine disinfectant. 2015: Intake screen installation. 2016–2020: Completed plant condition assessment and Facility master Plan. Recommendation was made to rebuild the treatment plant within the next 5 – 10 years. 2018: Improvements made to the electrical and control systems based on immediate needs. 2021: Consensus to rebuild in place. 2022: Maintenance repairs to flocculation basin addressing immediate needs.

Project Schedule

Alternative analysis and preliminary design have been completed on this project, however, due to budgetary constraints the overall project is on hold. The Salt Lake City Department of Public Utilities is looking into grants and other funding options for an accelerated project schedule. Once funding has been secured it is assumed that construction will take place over the course of several years.

What is a Pilot Study?

The Big Cottonwood Water Treatment Plant (BCWTP) provides safe, delicious drinking water to a LARGE number of Salt Lake City Public Utilities customers and is HUGELY important in protecting public health from water borne diseases. The plant’s upcoming “BIG Rebuild” will replace its aging equipment and structures.

BCWTP’s impact & importance are VAST, so before its complete rebuild, our project team had to start with something a little smaller…

Pilot studies are commonly used by engineers and water treatment operators to replicate different water treatment methods and conditions at a much smaller scale before designing and constructing a much larger facility.

In this case, our pilot testing was done in a small shipping container setup just outside the BCWTP. From April 2022 through September of the same year, this nondescript container held a wealth of information gathering processes and technologies.

Inside the pilot unit, engineers and plant staff were able to chart a drop of water from its “raw” state in the natural world through key phases of water treatment: flocculation, sedimentation, and filtration.

A photograph of a light blue shipping container and a smaller, more vibrant blue container to its right. Both containers appear in a parking lot outside of the BCWTP. Mountains can be seen in the background with green trees, shrubs, and gray rock pictured.

During the flocculation stage, the coagulant (typically a chemical salt) combines with natural organic material in water to form small particles called floc.

In the next stage, sedimentation, floc and heavier particles (like sand) are allowed to settle out of the water.

With these heavier pieces out of the water, anthracite and sand filters remove the remaining smaller particles from the water (filtration).

A computer-illustrated image of the filtration columns that were used in the pilot study. Four slim columns are surrounded by wiring, several different panels that take measurements of the water’s chemical makeup, and gray structural equipment weaves in and around the main columns.

A photograph from inside the pilot study (shipping container pictured above) shows a man looking down into the filtration columns. He is labeled as an environmental engineer. The filter columns are similarly labeled, along with sampling taps and gauges that extend about six inches in front of the columns themselves and allow water to be collected for further testing from the simulated water conditions created in the pilot study’s testing structure.

In the pilot study, the partially treated drop of water was then returned to the beginning of the BCWTP for complete treatment at full-scale where pathogens are destroyed using chemical addition (chlorine) during the disinfection stage, and fluoride is added per Salt Lake County, Rule #33. Once the disinfection stage is complete, the water is conveyed into the drinking water distribution system and to communities around the valley.

This pilot study allowed our water treatment experts to replicate changes in the amount of turbidity* in the water entering the plant and measure the ability to remove that turbidity at different operating conditions (e.g., chemical dose, loading rates*).

The study allowed the team to adjust elements of their overall design to ensure that the new plant could continue producing high quality water even at higher turbidity levels and loading rates. The results of the pilot testing confirmed the use of a design approach that will pack just as much treatment capabilities of the existing BCWTP into a smaller footprint, proving that good things come in small packages!

* Turbidity is the measure of water’s clarity, often due to suspended particles like clay. The lower the turbidity, the cleaner our drinking water!

* Loading rate is the amount and rate of water applied over a surface area. It is often measured in gallons per minute per square foot.