System required 3,600 UV bulbs and 6,500 cu yards of concrete
Six hundred and nine days in the making, KAI Design & Build recently completed a construction management assignment to build one of the largest ultraviolet (UV) wastewater disinfection systems in the country, to date.
To the outside world, the project appeared to be a typical construction project, but to the project team, the project was anything but typical, according to KAI Resident Project Representative Dave Tesson.
KAI Design & Build, partnering as a sub-consultant with global construction management consultant CH2M Hill, served as construction manager on the Metropolitan St. Louis Sewer District's (MSD) $21.2 million Lemay Wastewater Treatment Plant disinfection facility, which included construction of a UV wastewater disinfection system utilizing 3,600 individual UV bulbs and with a treatment capacity of 240 million gal of effluent per day.
UV irradiation systems disinfect by inactivating pathogenic microorganisms, such as viruses, bacteria and parasites, according to the UV system's manufacturer Ozonia. In the UV-C light spectrum (200-280 nm), the wavelength 254 nm has been proven to be the most efficient wavelength to inactivate microorganisms by damaging the nucleic acids (DNA and RNA), which disrupts the organisms' ability to replicate.
MSD's current boundaries cover 525 sq miles and encompass all of St. Louis City and approximately 80% of St. Louis County. MSD serves a population of approximately 1.3 million and has more than 425,000 single-family residential, multi-family residential and commercial/industrial accounts. MSD currently operates seven wastewater treatment facilities, treating a combined average of 370 million gal of wastewater per day.
Construction layout and dimensioning of the UV disinfection system's UV channels was a critical component of the project, said Tesson. Requiring approximately 6,500 cubic yards of concrete, an estimated 10,400 wire terminations to connect the UV power and controls, and 560 tons of reinforcement, the system's UV channel widths had to be constructed to an accuracy of +1/2 in., minus 0 in.
The project also required two, 144-in. pre-stressed concrete cylinder pipe connections to existing structures carrying the entire plant effluent flow. The existing bulkhead gates in both structures leaked excessively, preventing the removal of the bulkhead pipe caps intended to allow future expansion. The leaks could not be fixed through conventional methods.
"Divers were brought in and were able to seal the leaks from the inside at one of the structures. The other structure required a more extreme approach. Using experience gained from a previous project, the team coordinated an effort to retain wastewater in the collection system and the plant for later treatment. This allowed crews to get inside the structure and work on it dry. Additional wastewater storage was also required to complete modifications of a weir wall within the existing plant piping system," Tesson said.
A final, flow stoppage and storage for the entire plant was required to complete upgrades to the existing incoming plant electrical switch gear. This work required additional coordination with the electric utility to shutdown both utility power feeds to the plant substation.
"The successful completion of this project demonstrates that it is possible for the owner’s project team including the contractor, engineer and construction manager to collectively work together to resolve the many challenges that develop during the construction of a complex project," he said.
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