Top Hats Prove Effective

Nov. 1, 2005

About the author: Andrew Barr, P.E., is a senior project manager for Associated Engineering. He can be reached at 403/262-4500 or by e-mail at [email protected].

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The city of Lethbridge, located in Alberta, Canada, operates a 39 mgd water treatment plant, which supplies drinking water to 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, a protozoa 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.

The advent of ultraviolet (UV) disinfection for waterborne cysts such as Giardia and Cryptosporidium offered such a solution. Not only would UV allow the city to defer an intermediate 3 log inactivation requirement, saving them $500,000 (CAN), but the city also would be able to reduce the projected capital budget for 4 log inactivation from $20 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 flowmeters.

Disinfection facility

The UV disinfection facility was designed for 39 mgd to match the plant capacity. Five Trojan UVSwift reactors with a capacity of 13 mgd 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.

To maintain drinking water production, the city needed to keep the water treatment plant clearwell online 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.

The team developed an innovative tie-in detail that allowed the plant to remain online during construction. The live tie-in involved pre-drilling 2-in.-diameter pilot holes through the clearwell walls to guide divers from the inside. Divers then attached “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.

Top hats

The top hats formed an annulus against the wall that allowed a coring machine to drill a 30-in.-diameter hole from the outside into this annulus. Once the cores were completed, the outlet and inlet piping headers to the clearwell were installed and 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,” said 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.”

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