By Joshua Ranger
The Sudbury Wastewater Treatment Plant (WWTP) in Ontario has evolved tremendously over its 45 years of existence. This can be attributed to a number of factors, such as changing environmental requirements, increased sewage flows, health and safety concerns, power failures, or outdated equipment that reached the end of its useful life.
The plant was constructed in 1971 and was designed as a typical extended aeration facility, consisting of a raw sewage pump station (constructed in 1962), headhouse/degritting building, control building, four aeration tanks, four clarifiers, and a chlorine contact chamber.
Effluent is discharged to Junction Creek, located north and east of the plant. For many years the plant had operated well, meeting the needs of the City of Greater Sudbury. In the late 1980s, it was determined that the plant needed to be expanded to handle future flows.
In 1989, R.V. Anderson Associates Limited (RVA) was retained by the City to complete an Environmental Study Report (ESR) which paved the way for the future expansion of the plant. The ESR recommended a staged implementation of various upgrades, increasing the average daily capacity from 68,250 m3/day to 102,375 m3/day.
Sewage collection system
The sewage collection system for the City is unique since all flow is ultimately transported to the WWTP by an underground rock tunnel, measuring roughly 1.5 m wide by 2.1 m high. This was constructed in 1962 to transport sanitary sewage from the downtown core of the City to the WWTP. Over the years, the rock tunnel network has been extended, with the most recent extension, the South End Rock Tunnel, being completed in 2010.
The rock tunnel is extremely important to the Sudbury WWTP as it allows for additional operational flexibility. If required, the WWTP can be isolated from the rock tunnel, which has the capacity to store average sewage flows for as long as 12 hours. This allows plant operators to temporarily shut down the entire plant if required for maintenance. In addition, the tunnel can be utilized to buffer the plant against high peak instantaneous flows, helping to reduce the likelihood of bypass events.
Since the rock tunnel is located more than 25 m below ground, a pump station had to be constructed to pump wastewater out of the tunnel. In 1962, the Sudbury WWTP pump station was constructed. It consisted of a pump station control building over an 18 m deep vertical shaft with access elevator, a pump drywell, a sluice gate screening room and two wet wells.
In 1996, a major failure occurred at the pump station which flooded it. This prompted the City to complete various upgrades and modifications to the facility between 1997 and 2011. In total, six new submersible pumps were installed to handle peak flows of 409,500 m3/day. To design and install these pumps, several factors had to be considered:
- Physical pump size (could the pumps fit through the front door?). The only access to the drywell/wetwell was by an existing elevator shaft.
- Increased total dynamic head (TDH) requirements. As a component of headhouse upgrades requirements, the hydraulic gradeline between the pump station discharge and aeration tanks was raised 2.55 m.
- Performance of the proposed pumping configuration. The large increase in capacity raised concerns as to the performance of the proposed pumping configuration. As such, computational fluid dynamic (CFD) modeling was completed, which helped to better understand the characteristics of the existing inlet and discharge conditions, and how they would affect the proposed pumping upgrades.
Additional upgrades at the lift station include installation of two mechanical bar screens and two cast-iron sluice gates. The sluice gates are used to isolate the rock tunnel from the lift station, and control flow to two separate pump wetwells.
The headworks facility, commonly referred to as the headhouse, was constructed in 1972. It was comprised of manually raked bar screens and detritors equipped with grit classifiers. In recent years, a number of operational, maintenance and safety concerns in the headhouse were raised, leading to the construction of an upgraded headworks facility.
Upon further review by the City and RVA, it was recommended that the existing screening and degritting facility be upgraded to take advantage of the existing building, plant layout and recent upgrades completed to the pump station. As a result, two separate treatment trains were designed and constructed, each capable of handling the maximum day flow of 204,750 m3/day. The upgrades included:
- Two automated fine screening systems – complete with washing and dewatering compacting system.
- Two raised channels to facilitate the removal of the intermediate pumps located between the headhouse and aeration tanks.
- Two grit vortex units. Grit removal is necessary to remove fine solids that can impact plant processes and equipment. Grit vortex units utilize a natural vortex flow pattern coupled with gravity to effectively separate grit from sewage influent. The grit is removed from the units using a pump that discharges into a classifier which separates the grit from the liquid matrix.
- Three new ferric sulfate storage tanks and injection system. Ferric sulfate is added to facilitate the removal of phosphorus from the wastewater.
Like most construction projects at a WWTP, the upgrades to the headhouse had to be completed while maintaining plant operation. As such, this project had a number of construction constraints and risks associated with it. These included constructing the new elevated raw sewage channel and bypass over the old Parshall flume and channel which had to remain operational until upgrades were complete. Upgrades to the headhouse were completed in January 2015.
Odour control system
Over the years, the Sudbury WWTP has been subject to odour complaints due to its proximity to adjacent residential neighborhoods. As a result, the City decided to install an odour control system to treat the foul smelling air from the headhouse and raw sewage lift station. The City and RVA evaluated three treatment options: carbon filter absorption, biofiltration and photoionization.
It was recommended that the City install a photoionization system, which utilizes ultraviolet light and carbon filtration to decompose odorous compounds.
Among the options evaluated, photoionization had the lowest life cycle cost, smallest footprint and was a modular design. This was important as there were space limitations adjacent to the headhouse and pump station, requiring a compact system. This type of technology was the first of its kind in Ontario and was completed in April 2016.
Control building upgrades
The control building was constructed in 1972 and is located between the headhouse and aeration tanks. It contains control panels, city central control/SCADA system, administrative offices, aeration blowers, and distribution piping for the aeration tanks.
Until March 2014, all flow from the headhouse was pumped to a raw sewage conduit located in the basement of the control building, using intermediate lift pumps. Upon completion of the headhouse upgrades, the intermediate lift pumps were decommissioned and all sewage now flows by gravity to the aeration tanks. This helped to reduce power consumption, maintenance requirements and operating costs for the plant.
In addition, a new turbo blower was installed in December 2015, further reducing the energy consumption of the plant by an estimated 490 MWh per year.
The clarifiers at the Sudbury WWTP are circular and have conventional scraper mechanisms and scum skimmers. The tanks are centre-feed with revolving mechanisms to transport and remove the sludge from the bottom of the clarifier. The first four clarifiers were constructed in 1972. In 1994, the City constructed two additional clarifiers to increase plant capacity to 79,625 m3/day. The original four clarifiers were also replaced in the 2000s.
Several things had to be considered when designing and constructing the clarifiers:
- Planning for the final two clarifiers. To facilitate future construction, a portion of the final clarifiers was built.
- Connection to the mixed liquor conduit. Since it was not practical to connect to the existing mixed liquor conduit, a separate conduit was constructed from the aeration tanks to the two new clarifiers. This has also allowed for additional plant operational flexibility.
- Connection of clarifier effluent to the chlorine contact chamber required additional in-ground chambers.
- Dewatering and rock excavation for the clarifier foundation resulted in additional challenges during construction.
Chlorination system/dechlorination system
Prior to 1994, the Sudbury WWTP utilized chlorine injection and a chlorine contact tank to disinfect the effluent before discharging into Junction Creek. The new chlorine contact tank and facility was designed to treat a peak day flow of 409,500 m3/day. The chlorine system was also designed to allow for the treatment of bypassed plant flows before being discharged into Junction Creek.
In 2004, Environment Canada issued requirements for allowable levels of inorganic chloramines in chlorinated wastewater effluents. As such, the City prepared and implemented a Pollution Prevention (P2) Plan in accordance with the P2 Planning Notice published by Environment Canada. Ultimately, dechlorination using sodium bisulfite was installed at the plant.
Prior to 2015, all waste activated sludge was hauled to the City’s transfer station and pumped to Vale’s tailing ponds for disposal. This was common practice for the City for more than 30 years, but operational issues would arise on occasion. This resulted in odour problems, complaints from area residents, and issues relating to plastics and debris being washed up on the tailing pond beaches.
The odour problems were exacerbated in 2005 and 2007 due to a combination of changes that altered the tailing pond dynamics. As a result, the City undertook a comprehensive Biosolids Management Master Plan, following the Class EA process.
The Class EA recommended the construction of a biosolids treatment facility at the Sudbury WWTP, with an end goal to produce a Class A soil-type product. This facility would also be capable of receiving sludge from the City’s other wastewater facilities. In the end, a solution involving an alkaline stabilization process was chosen by the City. The new biosolids treatment facility was constructed through a Public Private Partnership, and was completed in May 2015.
Every wastewater treatment plant will experience new challenges with respect to process and capacity upgrades. The Sudbury WWTP is not immune from such challenges, as a number of future projects are expected, including:
- Expanding the aeration system and possible conversion to a moving bed bioreactor.
- Constructing the final two secondary clarifiers.
- Constructing a tertiary treatment facility.
- Constructing primary clarifiers/storm tanks.
- Expanding the standby power system.
- Various aesthetic and non-process related upgrades (parking lot, RV dumping station, works garage, walking trail adjacent to the plant, increased security improvements).
These projects will continue to change and evolve, based on the needs of the community, and changing flow and regulatory requirements. They will require a great deal of planning between the City, engineers and contractors to ensure that the projects meet or exceed the City’s requirements. It will be interesting to see the evolution of the Sudbury Wastewater Treatment Plant over the next 45 years and beyond!
Joshua Ranger is with R.V. Anderson Associates Limited (Sudbury). This article appears in ES&E Magazine’s October 2017 issue.