V-Box channel in combined sewer overflow system, St. Thomas, Ontario

WASTEWATER

In 1996, the City of St. Thomas in Ontario identified the need for an in-line combined sewer overflow (CSO) facility, upstream of its water pollution control plant. The CSO would reduce the frequency of overflows into Mill Creek and the adverse effects of the discharge of combined sewage on aquatic life, recreational uses and water supplies within Kettle Creek and Lake Erie. In addition, the facility would capture 90% of wet weather flows, allow a substantial reduction of the total by-pass events, and eliminate secondary by-pass events within the St. Thomas plant.

A report recommended that 4,000 m3 of storage was required. The conceptual design for the facility included two parallel (3000-mm diameter) sewers with a length of 280 metres.

The city retained R.V. Anderson Associates to conduct an environmental assessment and prepare the detailed design of the CSO facility.

During the design phase, R.V. Anderson staff found that the proposed twin sewer pipe system was not a simple solution. The control system for the twin sewers would be complex since dry weather flows should be conveyed through only one of the two pipelines to maintain self-cleansing. To accomplish this, the control system would require four automatic sluice gates to fill and drain the sewers in sequence (i.e. at each end of both sewers). In addition, the inverts of the 3000-mm pipes would have to be benched to maintain self-cleansing velocities during low flow periods.

Each of the twin sewers would have at least one local sewer connected to it as local sewers had to be picked up on both sides of the CSO facility, making self-cleansing difficult. Furthermore, since the storage facility would be located within the floodplain of Mill Creek, the proposed earth cover over the pipes was limited, and would not be sufficient to counter the buoyant force on the pipes if the creek flooded the valley when the sewers were not flowing full. Encasing the pipes in concrete collars, or anchoring them to a concrete base would be necessary, and the cost would be significant.

The consulting engineer proposed an alternative system that would solve the problems inherent in the twin pipe configuration. The alternative design called for a cast-in-place concrete base channel with precast CON/SPAN units to form the walls and roof. The inlet and outlet chambers would be significantly smaller than the chambers required by a twinned pipe.

The design allowed the normal dry weather flow to pass unimpeded at a velocity that is adequate to maintain self-cleansing conditions. The side slopes on the concrete channel are 1.5 horizontal to 1 vertical, which is steep enough to minimize the accumulation of solids. Provisions were made to access the structure for inspection and maintenance. Outflow is throttled to the rated capacity of the plant (45.5 million L/day) using two motorized sluice gates. It is expected that this facility will be filled, on average, four times each year. One overflow weir to Mill Creek is housed in the outlet chamber.

The city invited tenders for the construction of both alternatives. The Con/Span structure recommended by R.V. Anderson was $376,000 less than the low bid for the twin pipe alternative.

With support from Con/Span Canada and Con Cast Pipe, Elgin Construction developed the 6100-mm x 1500-mm units for the cast-in-place V-channel base. The project was a unique application of the precast units, and one of the largest of its kind produced at Con Cast Pipe. (Con Cast Pipe is one of five licensees of Con/Span Canada.)

Once site construction got under way in June 1999, production of the 114 pieces commenced. In total the plant produced 109 standard units (each weighed 13,910 kg.), two short length units, two maintenance hole tees, and one field closure unit. The cast-in-place channel was ready by September and units were installed at up to one every 20 minutes. The new facility was commissioned in January, at a cost of $2.2 million.

By Dave Metcalfe and Enrico Stradiotto, Con Cast Pipe. (Information provided by Hans Vierhuis, P.Eng., and Ken Collicott, P.Eng., R.V. Anderson Associates and John Dewancker, P.Eng.,City of St. Thomas.)