David Street Water Treatment Plant
January 1, 2006
By Canadian Consulting Engineer
In order to meet the stringent demands of Ontario's Drinking water protection regulations passed in 2000, the city of Greater Sudbury had to upgrade its David Street Water Treatment Plant. The origina...
In order to meet the stringent demands of Ontario’s Drinking water protection regulations passed in 2000, the city of Greater Sudbury had to upgrade its David Street Water Treatment Plant. The original section of the pumping plant was built in 1895 and the city wanted to keep the building’s historical character. The plant is now surrounded by a residential community.
The plant draws raw water from nearby Ramsey Lake, which is subject to urban run-off and other potential sources of contamination. As well, there had been problems with manganese and substances such as iron, which at times had given the treated water a black tinge.
Building a new plant on the alternative site 30 kilometres outside the city would have incurred higher pumping energy costs. Instead it was decided to upgrade the existing plant. The plant also has to use water very efficiently. Ontario’s Ministry of the Environment will permit only 27,300 m3/day on a monthly average to be taken from the lake, and 40,000 m3/day in any 24 hour period.
Faced with the above parameters, CH2M Hill designed a 40,000 m3/day plant using ultrafiltration membranes and UV disinfection. The original 1895 pump house remains, but a later addition to the plant was demolished and replaced with structures designed in the architectural style of the historic structure. There is also new green space providing a link with public walkways around the lake.
The upgraded plant’s treatment process first involves pre-oxidizing the supply water to remove manganese. The water then passes through the first-stage ultrafiltration which involves four process trains, each with four cassettes, of unsupported hollow fibre membranes with a nominal pore size of 0.02 microns. The water that passes this first stage filtration, called “permeate,” flows by gravity siphon into a surge-backpulse tank located below. Meanwhile, the backwash reject water is passed through the second stage filtration. This stage has two process trains, each with four cassettes of reinforced, vertically oriented membranes with a nominal pore size of 0.04 microns. These membranes are designed to filter out higher solids concentrations. Water permeating through the second stage also flows into the surge-backpulse tank by gravity siphon. The water in the surge-backpulse tank is treated by contact with chlorine and by ultraviolet radiation to destroy pathogens. The small amount of final reject water is discharged into the city’s sewer system by gravity. The two-stage ultrafiltration membrane system makes this one of the most sophisticated plants in North America in terms of achieving 99% water efficiency with only 1% wastewater.
The plant’s use of a hydraulic process and gravity siphon is also an innovative application of the plant’s unit processes. The gravity siphon was specially designed by CH2M Hill. In order to generate permeate flow through the submerged membrane system, a vacuum is required to pull water into the fibres. At the David Street plant, a gravity siphon creates the vacuum, combined with a varying water level in the surge tank downstream. Competing uses for the surge tank required balancing and controlling the process through all stages.
Traditional engineering techniques would use large storage reservoirs for balancing the flow, which would have required constructing tanks and excavating rock on a site that is already compact and confined. Eliminating the need for large reservoirs kept costs down and accelerated the construction schedule. Also, using a gravity siphon to generate the vacuum meant the elimination of six variable frequency drive motors and pumps, thus reducing capital, operating and maintenance costs.
CH2M Hill required the membrane supplier to include automatic testing to monitor the condition of the membranes.
The construction costs were under $18 million, within 5% of the budgeted amount. The plant has been operating since October 2004, and now supplies about 40% of Greater Sudbury’s daily water needs.
Client: City of Greater Sudbury
Prime consultant: CH2M Hill (Ken Mains, P. Eng, Toby Brodkorb, P. Eng., Val Woloshyn EIT, Norm Huggins, P. Eng.)
General engineering subconsultant: Northland Engineering
Geotechnical: Golder Associates
Architect: Nicholls Yallowega Belanger Architects
Construction: North America Construction (1993)