City of Glasgow Water Treatment Plant Rehabilitation and Retrofit
The City of Glasgow recently completed a $7 million upgrade to their water treatment plant (WTP). Originally a 1960s groundwater softening plant, and converted to surface water treatment in 1987, the upgrade rehabilitation and retrofit project installed modern technology to treat the low turbidity source water. All components of the upgrade were constructed within the existing building footprint.
Older technology replaced with package units for specific treatment goals
The existing plant was built as a groundwater softening facility in 1966. The plant was converted to Missouri River water in 1987, which remains the current, high-quality and low-turbidity water source, diverted downstream of Fort Peck dam. The original solids contact clarifier in the plant used for lime softening was still in use but was eventually converted into a detention basin. The clarifier did not provide adequate pre-treatment, so the upgrade project included replacing the single clarifier with four contact adsorption clarifiers (CACs) housed in steel tanks—a proprietary process specifically used for low turbidity water. These units were pilot-tested to ensure performance, and the design included negotiating a price with the supplier to procure the equipment.
New Filters, New Treatment Tools
Another design element involved completely replacing the single media filtration system. The existing clarifier was removed, and the basin was repurposed with concrete filter basins, each with plastic block underdrains and dual media consisting of silica sand and anthracite. The old filters used a gravity backwash process with a low wash rate without filter-to-waste capabilities. The new system provides a pumped backwash system to achieve higher wash rates and more efficient backwash and a filter-to-waste process to enhance operations.
Hydraulics ‘Flipped’ to Keep Costs Down
An important aspect of the project for the city was to avoid costly building expansions on the limited site footprint. To meet this need, our team worked with the city to develop design-bid-build contract documents that included a detailed construction sequencing process. This step ensured seamless operation of the plant, while also allowing the contractor flexibility for a cost-effective project
For example, during the Phase 1 construction, the old clarifier was demolished in order to construct the new concrete filter basins, filter piping gallery, and backwash pump system. When the new filters were commissioned, Phase 2 included demolishing the existing filters, installing a concrete deck over the old filters, and converting the area to house the CAC tanks, piping, and valves — all which flipped the plant hydraulics completely around. The existing raw water pumps were able to accommodate this slight change in hydraulics. The old filter basins were converted to provide additional volume for backwash water supply and chlorine contact time.
This phasing concept saved significant costs, allowed the design team to incorporate the improvements without building additions, and minimized construction interference with the city operations staff.
Regulatory Compliance Plan to Meet Construction Phasing Requirements
With the detailed construction sequencing plan, the design team worked with Montana DEQ to develop a regulatory compliance plan. The plant was classified as a conventional filtration plant with chemical addition, flocculation, sedimentation (the old clarifier), and filtration. The construction phasing plan required the clarifier to be demolished in Phase 1, and the existing filters were operated in a direct filtration mode (no clarifier), which requires additional operator attention to chlorine dose and detention time. When the new filters were commissioned and placed on-line in Phase 2, the plant continued to operate in direct filtration mode.
Finally, at the completion of the project, the plant was reclassified back to a conventional filtration system. This approach took planning with DEQ staff during preliminary design and careful coordination during the project to keep DEQ fully informed of the classification status.