DDW-2024-014866

December 30, 2024
Annalee Munsey
Metropolitan Water District
3430 Danish Road
Cottonwood Heights, Utah 84093
Subject: Treatability Study Concurrence, LCWTP (TP002) Treatability of Big Cottonwood Creek; Metropolitan Water District, System #18016, File #18521
Dear Annalee Munsey:
On September 21, 2024, the Division of Drinking Water (the Division) received a request to review a proposed study of Little Cottonwood WTP treatability, “Study of Big Cottonwood Creek”.
The updates on the studies and construction on pipeline studies for execution was received by Division staff on December 18, 2024.
Our understanding of the study project is that SLCDPU retained Hazen and Sawyer (Hazen) to perform a desktop study of the feasibility of treating Big Cottonwood Creek (BCCr) water at
the Metropolitan Water District of Salt Lake and Sandy’s (MWDSLS) Little Cottonwood Water Treatment Plant (LCWTP) (Sys 18016, TP002) during a temporary Big Cottonwood Water Treatment
Plant (BCWTP) (Sys 18026, TP001) shutdown as part of a regional strategy to continue to treat BCCr while BCWTP is rebuilt.
Hazen completed following desktop study:Evaluated and compared raw water parameters and seasonal water quality changes for BCCr, Little Cottonwood Creek (LCCr), and Salt Lake Aqueduct
(SLA) water sources,Partitioned historic water quality and flow data provided by SLCDPU and MWDSLS into four separate seasons for further water quality blending analysis,Compared the
predicted average seasonal BCCr, LCCr, and SLA blended water alkalinity and pH with the historical LCCr and SLA blended water alkalinity and pH currently treated at the LCWTP.
The multistep approach to use RTW modeling to verify the feasibility of treating BCCr water at the LCWTP during challenging water quality scenarios and predicting impacts to finished
water is as follows:Identify ten blended BCCr, LCCr, and SLA raw water treatment scenarios for RTW modeling. Treatment scenarios should include periods of historical average raw water
quality and flow as well as periods with more challenging raw water quality and flow that may be encountered while treating BCCr at LCWTP.Perform additional statistical analysis of the
provided BCCr, LCCr, and SLA historical water quality and flow data as needed to determine model input parameters for RTW modeling of the ten identified treatment scenarios.Perform RTW
modeling for each treatment scenario to estimate the required chemical dosages needed to treat the blended water and meet LCWTP’s established finished water pH and alkalinity goals of
7.8 to 8.0 and greater than 49 mg/L as CaCO3 and identify predicted corrosion indices values.
Treating LCCr at LCWTP required careful raw water quality monitoring, blending, and finished water oversight to produce high quality drinking water. The predicted water quality from
each treatment scenario modeled met LCWTP’s established finished water pH and alkalinity goals of 7.8 to 8.0 for HAA control and greater than 49 mg/L as CaCO3, respectively. In addition,
predicted LSI and CCPP values for all the treatment scenarios are all slightly greater than zero with potential for calcium carbonate precipitation/scaling. Predicted CSMR values for
treatment scenarios 1 through 6 were the upper threshold of 2.0 for the increased potential to cause corrosion. LCWTP staff will likely continue to use SLA water as required in situations
with water quality similar to treatment scenarios 7 through 10 to keep the LCWTP finished water CSMR below the upper threshold of 2.0.
This will be the most difficult to treat and have the highest potential to result in corrosive finished water. These worst-case scenarios were modeled to confirm LCWTP’s ability to treat
BCCr during worst case conditions. However, SLCDPU and MWDSLS will rely on the expertise of the LCWTP operators to operate within the limitations of the plant. BCCr water will be sent
to LCWTP on a case-by-case basis. For example, if BCWTP is shut down for a long period of time, it is likely that water quality similar to treatment scenarios 5 through 8 may need to
be treated. However, if BCWTP is only shut down temporarily, then it likely only makes sense to pump historical average quality BCCr to LCWTP (scenarios 1 – 4). Given that BCCr high
turbidity events typically have a short duration of a couple hours to a day it likely never makes sense to pump highly turbid BCCr water to LCWTP (scenarios 9 and 10).
In summary, RTW modeling confirmed that treating BCCr at LCWTP with water quality similar to treatment scenarios 1 through 10 should not result in corrosive finished water or negatively
impact the distribution system. With careful treatment management, LCWTP staff have the experience and tools available to successfully treat blended BCCr, LCCr, and SLA water at LCWTP.
We hereby concur with the methodology of the proposed study of Little Cottonwood WTP (TP002) treatability Study of Big Cottonwood Creek.
This concurrence is not Plan Approval to construct any facilities or renovation. Prior to installation of any facilities, the water system must submit plans and specifications, and a
complete Project Notification Form to the Division for approval.
If you have any questions regarding this concurrence letter, please contact Nagendra Dev, P.E., of this office, at [phone], or me, at (385) 515‑1464.
Sincerely,
Michael Newberry, P.E.
Permitting and Engineering Support Manager
ND/mrn/mdbcc: Matthias Neville, Salt Lake County Health Department, [e-mail] Williams, Hazen and Sawyer, [e-mail] Barry, Salt Lake City
Public Utilities, [e-mail] Gray, Salt Lake City Public Utilities, [e-mail] Charlebois, Salt Lake City Public Utilities, [e-mail]
Sorensen, Metropolitan Water District, [e-mail] Shalvoy, Hazen and Sawyer, [e-mail]
Annalee Munsey, Metropolitan Water District of SL and Sandy, [e-mail]
Sarah Page, Ph.D., Division of Drinking Water, [e-mail]
Dylan Martinez, Division of Drinking Water, [e-mail]
Nagendra Dev, P.E., Division of Drinking Water, [e-mail] 18016 18521 Treatability Study Concurrence LCWTP