By Tom Davey, Publisher
Environmental déjà vu? Almost two decades ago Toronto beaches were closed to swimmers as temperatures soared in the nineties. I responded with an article for The Toronto Sun at the request of editor Barbara Amiel, now the wife of Conrad Black. I wrote: “The hottest summer in 30 years had resulted in ideal conditions for bacterial growth along the various shorelines, where pockets of stagnant water have accumulated.”
In response, The Toronto Sun has produced a masterly review of the beach pollution which has closed the majority of the City’s beaches. This comprehensive coverage must have surprised those who think The Sun is not a serious paper. Unlike much newspaper coverage of environmental issues, The Sun’s July 4 spread contained some serious technical data on E. coli, a refreshing change from sensational headlines, which often lack scientific substance. Sun writers included a review of Chicago’s environmental progress, which described TARP, the Windy City’s famed Tunnel And Reservoir Plan. TARP has 160 km of deep underground tunnels and three massive reservoirs, which can contain as much as 40 billion gallons without having to discharge into a river or lake. Chicago’s beaches, needless to say, remained open July 4, when America celebrated its national holiday, while many Torontonians had to celebrate their Canada Day three days earlier on signposted beaches.
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But the Mega City technical staff have not been idle. A huge Combined Sewer Overflow (CSO) system which should help clean up Toronto’s beaches is well underway.
The concept of storing sewage flows for subsequent treatment is well proven and has been utilized by many other cities in Europe and North America. Earlier, storage facilities (detention tanks) which were constructed in 1989 and 1994 at Toronto’s Eastern Beaches, did result in reasonable improvements in water quality.
The system under construction runs along the Toronto lakefront from Parkside east to Strachan Avenue. The entire system will ultimately have an overall capacity of 85,000 cubic metres for storage which allows for the retention of stormwater and combined sewer overflows up to the two-year storm level. Hydraulically, the system will be able to handle flows up to a five-year storm event but the increased flow would be released to the lake at three locations instead of the original ten. These discharges will be at the location of the large shafts, two of which are east of the beach area. The smaller existing sewers are directed to discharge into the tunnel through a series of drop shafts at each intercept location. Existing outfalls at these locations will normally be closed off. They can be re-opened and flows isolated from the tunnel if maintenance of the tunnel is required.
Pumping Systems
All flows which enter the system will be collected within the tunnel and large shafts up to the two-year storm as noted previously. Following storm events, the quiescence within the system will allow sediments to settle, separating suspended solids from clearer liquid. Then, following a period of approximately 8 – 10 hours, pumping from the system will start.
As the solids are expected to collect at the lower extremities of the Strachan shaft, these will be pumped first through a forcemain which follows a box culvert from Strachan and Lakeshore to Strachan and King Street. At this location, the forcemain will discharge into the mid-Toronto interceptor (MTI) sewer from where gravity flow exists to the Main Treatment Plant (MTP) at Ashbridges Bay. Because of limitations on the capacity of the MTP influent works, the rate of flow for discharge to the plant will be restricted to 3001/s. The underground pumping station will use two submersible 250 hp pumps supplied by KSB Pumps. The variable speed pumps are controlled to maintain specific flows.
As pumping continues, online sensors (turbidity and suspended-solids meters) on the pump discharge piping will determine that clearer water is being pumped and this will initiate a change in the pump discharge location. Clearer liquid can be discharged back to Lake Ontario as long as it meets specific criteria and as long as it is disinfected during the summer bathing period from May to September. To provide disinfection, the pumping station has a UV disinfection chamber. The proposed supplier of the system is UltraGuard from British Columbia. During non-bathing periods, treated discharges can go to the lake without disinfection. During periods of lake discharge, the maximum flow rate will be increased to 500 1/s.
Complexities of the design of the pumping system relate to the substantial range in possible heads which each pump will see during the emptying process. Assuming that the system is full, the pumps will initially start and discharge through the forcemain to the MTI. Total system head at this time will be less than 10 metres, made up of approximately 5 metres of static lift and 5 metres of friction head. If the system is full, and discharge is made to the lake, total head drops to less than 5 metres and, even at minimum speeds, there would be concern over pump run-out.
As the Certificate of Approval (C of A) is very stringent on maximum discharge rates to either the lake or the MTI, the control of flow with head using variable speed is the predominant control requirement. Even with one pump running, there is difficulty in controlling the discharge to the MTI, although it is now estimated that one pump can be restricted to an output of 240 1/s as compared to the maximum of 300 1/s.
As levels drop in the shaft, which reflects diminishing levels in the three large shafts, the first duty pump speed will be regulated so that the discharge does not exceed 300 1/s. By the time the system has dropped 10 metres and assuming that discharge is still to the MTI, the second pump can start. Limited information suggests that, by the time the level is lowered even a small amount, the liquid will clarify to a point where lake discharge is possible.
As discharge valves close to the forcemain and open for lake discharge in response to turbidity measurement on the discharge, the control system will adjust the rate of flow and pump speed. It is now expected that the shaft system will be drained substantially in the lake discharge mode until the levels get down to close to the tunnel level, at which point, solids may migrate along the tunnel requiring changes back and forth be tween the two discharge points.
As the system is emptied, assuming solids are being pumped to the MTI, the maximum head on the pumps will have increased to 65 metres, of which close to 50 metres are within the shafts as static lift. Again at the bottom, it may not be possible to sustain the maximum rate of flow.
Treatment System
Effluent discharges during the bathing season will be disinfected with ultraviolet light. With regard to the design parameters of the UV, considerable discussion occurred. Limited sampling revealed that existing outfalls had limited solids levels and there was not a substantial amount of organic matter included in the waste. Even after 15 days, a sample had not gone septic. Sampling also showed that the solids settled quickly and the resultant transmissivity of UV light through the liquid was relatively high, i.e., between 30 and 50.
To be conservative, the UV system was selected to meet a maximum TSS level of 50 mgl while providing for an effluent E. coli level of <1000/100 ml. The design is based on transmissivity levels of 30 but the selected system will allow for reduced UV intensity when transmission of light is determined by sensors to be better than 30.
Until 1998, the Mega City was called Metropolitan Toronto and comprised six cities and boroughs. There were many predictions that amalgamation would be disastrous but Mel Eastman, it is generally conceded, has done a great job as the first Mega City Mayor. Technical engineering staff, too, have generally adapted well to the complex challenges posed by amalgamation such as the huge CSO project.
| By The Numbers Toronto’s system of managing wastewater involves some mind-boggling statistics: |
|
|---|---|
| 475 billion litres | treated annually |
| 9.5 billion litres | stormwater untreated |
| 4.5 billion litres | partially treated |
| 4 | treatment plants |
| 45 | pumping stations |
| 5 | detention tunnels/tanks |
| 36 | stormwater ponds |
| 463,300 | service connections |
| 120,000 | maintenance fioles |
| 4,143 km | sanitary sewer lines |
| 4,533 km | storm sewer lines |
| 1,301 km | combined sewers |
| 2,632 | storm sewer outfalls |
| 79 | combined sewer outfalls |
| 371 km | watercourses |
| $151 million | 1999 operating costs |
| $182 million | 1999 capital costs |
| 1,071 | full-time employees |
| Source: Toronto Sun | |