WWTP servicing growing Utah cities utilizes ozone disinfection to improve energy efficiency
A $250 million capital program by the Metropolitan Water District of Salt Lake & Sandy (MWDSLS) has equipped the wholesale water utility to produce improved water quality and to meet projected demand through and beyond 2025.
By 2025, Salt Lake City expects to gain additional 100,000 residents, and the City of Sandy expects to gain 30,000. The accelerating growth of the MWDSLS customer agencies, along with the need for system reliability and redundancy, spurred the proactive capital improvements.
The Little Cottonwood Water Treatment Plant (LCWTP) was the cornerstone of the MWDSLS’ capital improvement program. The facility draws raw water from Little Cottonwood Creek and Deer Creek Reservoir. LCWTP operated as a conventional 100-mgd facility employing gravity sedimentation, granular media filtration and chlorine disinfection. Powdered activated carbon was added to suppress seasonal taste and odor (T&O) issues, but it had limitations.
Taking these factors into account, the MWDSLS’ initiative involved producing a 30-mgd treatment increase at LCWTP and replacing chlorine with environmentally friendly Wedeco ozone disinfection.
By using ozone with side-stream injection, the site was able to eliminate construction of deep, fine-bubble diffusion contact chambers in favor of converting the plant’s two 470-ft-by-13-ft-by-14-ft open aeration basins into parallel ozone contact chambers. The retrofit required seismic upgrades and the addition of an 8-in.-thick segmental precast concrete lid to enclose them. The water receives the gas via a reconstructed, 84-in.-diameter raw water line using four axial dispersion tubes. A new calcium thiosulfate chemical feed system quenches any residual ozone. To establish a decay curve, dosage, appropriate ozone concentration and contact time, five sampling points were incorporated along the lengths.
The two Wedeco PDO 7000 ozone generators, sized at 3,750 ppd, offer significantly more capacity than initially needed. The system is designed to dose up to 3.5 ppm ozone during T&O events, but the utility typically doses 1 ppm ozone during normal operations.
The design consultant applied computational fluid dynamic modeling in developing a solution for the LCWTP retrofit. The methodology eliminated some new construction at the site, and estimates are that these design initiatives utilizing Wedeco high-performance technologies saved the MWDSLS more than $5 million. This would not have been possible without the efficient operation offered by the Wedeco equipment with up to 20% less power being used and 30 times less oxygen.
The plant uses ozone to achieve the required additional 0.5 log removal/inactivation of Giardia. In 2011, this goal was met 85% of the time. Chlorine is used downstream to meet any additional removal/inactivation requirement. The inclusion of ozone in the plant retrofit has also lowered finished water turbidities, boosted filter run times and provided a high degree of flexibility when selecting gas suppliers.