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    Alberta water treatment plant maintained production during UV installation

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    Ultraviolet disinfection system.

    The City of Lethbridge, Alberta operates a 150 million litre per day water treatment plant, which supplies 77,000 residents, as well as commercial, industrial, institutional and agricultural sectors. To address growing concerns regarding drinking water quality, Alberta Environment established more stringent standards, including 4 log (99.99%) inactivation of Giardia, protists common in raw water supplies. The City was faced with significant and expensive chemical disinfection upgrades to meet this standard, and sought a more economical solution.

    Ultraviolet (UV) disinfection for waterborne cysts such as Giardia and Cryptosporidium offered a solution. Not only would UV allow the City to defer an intermediate 3 log inactivation requirement, saving $500,000, but the City would also be able to reduce the projected capital budget for 4 log inactivation from $20 million to $5 million.

    The stringent regulatory deadline challenged the design team of Associated Engineering, CH2M HILL, and the City stakeholders to meet an aggressive implementation schedule. To meet the deadline, the City decided to manage construction, thus expediting implementation. This approach allowed the City to start construction two months earlier than a conventional design-bid-build project. Long delivery items were pre-ordered, including the UV reactors, large diameter butterfly valves, and magnetic flow meters.

    The UV disinfection facility was designed for 150 million litres per day to match the plant capacity. Five, 600 millimetre diameter Trojan UVSwift reactors with a capacity of 50 million litres per day each were installed to meet the capacity, plus a minimum 50% redundancy requirement. Chemical injection points for chlorine were installed upstream and downstream of the reactors. Fluoride and ammonia injection points were installed downstream of the reactors for induction with a hydraulic mixer. As an added benefit, UV disinfection allowed the City to maintain chloramination in the distribution system, which was a consumer preference over free chlorination. Ammonia used for chloramination had to be kept downstream of the UV reactors due to potential interference with the UV transmittance.

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    To maintain drinking water production, the City needed to keep the water treatment plant clearwell on-line during the construction tie-in. The City and the design team explored a number of options for bypassing the clearwell during construction, but none of these was deemed feasible.

    Piping layout for ultraviolet reactors.

    An innovative tie-in detail was developed that allowed the plant to remain on-line during construction. The live tie-in involved pre-drilling 50 mm diameter pilot holes through the clearwell walls to guide divers from the inside. Divers then attached 900 mm diameter top hats over each pilot hole. These top hats were raised blind flanges, gasketted around the edge and bolted to the inside of the clearwell wall. Isolation valves on the pilot holes were then opened to relieve the pressure behind the top hats; the pressure from the water inside the clearwell effectively sealed the top hats to the wall.

    The top hats formed an annulus against the wall that allowed a coring machine to drill a 750 millimetre diameter hole from the outside into this annulus. Once the cores were completed, the outlet and inlet piping headers to the clearwell were installed, grouted in place, and a cast-in-place concrete thrust structure was poured to hold the header in position. The top hats were then removed from the inside of the clearwell.

    Construction was completed and the plant commissioned by December 2003, meeting the regulatory deadline. The project, originally estimated at $5 million, was completed $900,000 under budget.

    “The City of Lethbridge is very pleased with our new facility”, reports Doug Kaupp, the City’s Water Utility Manager. “It (the UV facility) has operated flawlessly for the first year and has served to reassure the community that our drinking water is of the highest quality.”

    Contact: Lianna Mah, P.Eng. who is with Associated Engineering Limited e-mail: aetoday@ae.ca