Selkirk Waste Water Treatment Plant Upgrade Project
The Project • Old Rexnord bar screen removed and replaced with new Headworks bar screen. • Screenings compactor installed • Chlorine Disinfection system removed and replaced with UV disinfection system
Bar Screen and Screenings Compactor Screenings removed by rake and dropped into screenings compactor. Screenings are then compacted and dropped into a waste bucket, which is periodically emptied by a private contractor.
Bar Screen • A screen with uniformly spaced bars meant prevent debris from getting into the process • A rake is used to clear the bar screen of built up materials. • A wiping mechanism cleans the rake of material. Bar screen removes solids from influent flow
Original bar screen • Rexnord Cable Operated Bar Screen • Bar dimensions: 2” x 3/8” x 3’10” • Bar spacing: ¾” • Overflow weir upstream bypassing bar screen • Wet weather design capacity of 8.5 million gallons per day before overflow when half plugged • Wet screenings fall directly into waste bucket
Reason for Upgrade • Not effectively screening out smaller debris • Too much solids getting through • Aging (40+ years old) Old bar screen rake
New Bar Screen • HeadworksMahr continuous rake bar screen • Bar Dimensions: 0.8cm x 0.4cm x 100cm • Bar Spacing: 3/8” • 10 degree incline (from vertical) increases surface area exposed to flow The continuous rake travels to the bottom of the channel and sweeps up, clearing the screen
New Bar Screen Cont’d • Bars are easily replaceable, installed without welds or permanent fixtures • Heavy duty roller chains can sustain a minimum average exertion of 137.5 kN of force applied by the rake(if required too) • Screenings discharged into screenings compactor Screenings discharge from screenings compactor
Screenings Compactor • Wet screenings pile into the top of the screenings compactor • Once screenings compactor fills to a certain level, a hydraulic ram pushes the screenings through the compactor, and into a waste bucket • Hydraulic ram presses out water held in screenings, significantly reducing volume and weight • If the bar screen runs continuously for a set period of time, the screenings compactor will run continuously until the bar screen stops
Effluent Disinfection System Treated effluent from UV modules is discharged from the plant Flow can be directed through either UV module bank, or overflow
Chlorine disinfection system • 2 Wallace and Tiernan V-notch Chlorinators • Injected chlorine on a flow proportional basis • Chlorine contact chamber water retention time 15 minutes • Ideal residual chlorine in effluent: .5 ppm
Reasons for Upgrade • Upgraded to beat new legislation banning residual chlorine in wastewater effluent • UV system is lower maintenance than chlorine system
UV Disinfection system • Aquaray 40 VLS HO UV disinfection system • Produces Ultra-Violet light at the 253.7 nm wavelength. • Disinfects by altering Bacteria DNA to prevent reproduction • Effectiveness dependent on water quality (Clarity, Suspended Solids, etc.) Two channels with a bank of 3 UV modules each are online. Generally only two modules of one bank are needed to effectively disinfect, based on the flow rate
UV Disinfection Safety and Maintenance • UV light can severely damage skin and eyes if overexposed. UV protective goggles and clothing required for prolonged exposure • Cleaned approximately every 2 months • CLR, Scotch Bright pads, and high pressure water are used to clean light jackets Cleaning pit and bubbler provided proved only marginally effective. Modules are cleaned by hand and rinsed with a pressure washer. Modules are easy to lift out of the banks with an overhead hoist
UV Maintenance Cont’d • UV light bulb change quick and simple, reducing down time • Apart from cleaning, UV modules require very little human interaction. Monitoring is done to ensure UV modules are still disinfecting Lights are simple to unplug and replace, minimizing the time a single module has to be offline for a light bulb change. Other modules can be turned on to compensate for offline module.
Conclusion • Upgrades improve ease of operation of treatment plant • Reduced maintenance time and costs • Discharge more environmentally friendly • Beat legislation and costly upgrades due to failing equipment