The Seattle Public Utility’s new 180 mgd Cedar Treatment facility represents the largest application of UV technology for municipal drinking water in the world. This project is unique given that the Cedar River supply is unfiltered, and the project is being delivered using a Design Build Operate (DBO) approach.
The Seattle Public Utilities (SPU) supplies water to approximately 1.3 million people in Seattle and 26 wholesale cities and water districts. This regional system is supplied by two large surface water sources—the filtered South Fork Tolt River and the unfiltered Cedar River. The Cedar Treatment facility is located on the site of Lake Youngs Reservoir.
The Cedar River is a high quality source of drinking water. This is primarily due to exceptional source water protection, but also due to the combination of groundwater inflow to the Cedar River and water quality improvement afforded by the equalization that occurs in the large (11 billion gallon) Lake Youngs Reservoir. Treatment for the Cedar supply did include chlorine contact to achieve 3-log Giardia inactivation, fluoridation, and pH adjustment for corrosion control. The chlorine contact was provided in the transmission pipelines upstream of Lake Youngs Reservoir, which is classified as finished water storage.
Treatment and regulatory compliance
The need for additional treatment was spurred by a Raw Water Quality violation. SPU studied various options and alternatives to address the regulatory requirements associated with this violation ranging from additional source water protection to filtration. They then pursued a compliance strategy based on enhanced disinfection using a multi–barrier approach incorporating ozone and UV while keeping their unfiltered status of the Cedar supply. The passage of the alternative to filtration provisions of the Surface Water Treatment Rule helped to continue to evolve the strategy.
In April of 2001, SPU and CH2MHILL entered into a contract to design, build and operate new treatment facilities. The facilities included a new intake and raw water pump station to withdraw water from Lake Youngs, ozonation for taste and odor control and primary disinfection, UV treatment for primary disinfection, chlorine for residual disinfection, 20 million gallon of finished water storage and corrosion control treatment using hydrated lime.
The disinfection systems were designed and operated to provide 3-log Cryptosporidium, 4-log Giardia, and 5-log virus inactivation in order to meet the agreement with the EPA/ Washington State Department of Health (WSDOH). Flexibility is allowed to meet the requirements with multiple disinfectants, e.g. 4-log Giardia disinfection with a combination of ozone (3-log) and UV (1-log). There would be a Treatment Technique violation if treatment drops below 2-log Cryptosporidium, 3-log Giardia, and 4-log virus inactivation.
Implementing UV technology
Due to performance uncertainty at the time, inability or extra cost to change plans at a later date, uncertainty of obtaining regulatory approvals and a host of other factors, SPU had a difficult time deciding to proceed with the use of innovative technology. The decision to build with a DBO approach helped mitigate some of these factors. SPU decided to solicit qualification statements from interested parties so they could gain more knowledge on the emerging technologies and determine the willingness of the parties to assume some risk.
Through this process they learned about new information such as UV’s ability to inactivate pathogens such as Giardia and Cryptosporidium and the effectiveness of ozone for Cryptosporidium inactivation in cold waters. This information also suggested that a much higher combination of dose and contact time would be needed for ozone to achieve 3-log Cryptosporidium inactivation than was previously thought.
Equipment selection
The process for equipment selection was very competitive and initially driven by the DBO selection process. The contractual provisions required the contractor to provide a facility that met very strict performance criteria. The UV criteria required demonstrated delivery of a dose of 40 mJ/cm2, which was deemed in the contract to be equivalent to a 3-log Cryptosporidium inactivation.
In the final analysis, Trojan Technologies, Inc., became the selected vendor for the Cedar project.
“Based on our evaluation process which considered many technical and non-technical factors, TrojanUVSwift provided the best overall solution for the unique needs of this project,” said Scott Trusler, design manager for CH2MHILL.
One of the key decision factors was the availability of performance data for the 24-in. TrojanUVSwift reactor. As a part of equipment development, Trojan had tested the 24-in. reactor under a wide range of operating conditions. The testing protocol incorporated available information from sources such as the NWRI requirements, and all of the work was overseen and certified by an independent third party. Given the contractual performance requirements and related risks, these test results provided additional confidence that UV could be successfully implemented using the 40 mJ/cm2 equipment.
Equipment validation testing
As with most innovative technologies, and particularly so with the UV systems, it was necessary to demonstrate the system performance through full scale testing. In the case of Cedar, the primary goals of testing were twofold: obtain regulatory approval for system operation and demonstrate that contractual obligations had been satisfied. The testing occurred in late November 2002 at the Lake Huron Water Treatment Plant in Grand Bend, Ont.
With the testing a success, the final results of the validation were certified by an independent third party, and SPU and the WSDOH approved documentation for the overall validation in April 2004.
Implementation of innovative technologies is always a challenge. This is especially true in the absence of regulations and other information, as was the case for implementation of UV for SPU’s new Cedar Treatment Facility. The implementation process included a detailed assessment that resulted in the selection of an equipment vendor to create a process that allowed for open dialogue on creative solutions. Contingency planning was also an important consideration when implementing innovative technology, and the plans needed to balance risks and costs so the benefits of the proposed technology could be obtained. Lastly, it was always important to ensure the fundamental goal of providing high quality water, and public health protection remain a priority throughout the process.
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