when it pours; dealing with combined sewer overflows
Looking at rainwater as a resource rather than an impediment may seem like the commonest kind of sense in this age of environmental awareness. But it hasn't been seen that way by most municipalities. ...
Looking at rainwater as a resource rather than an impediment may seem like the commonest kind of sense in this age of environmental awareness. But it hasn’t been seen that way by most municipalities. As we render more and more of the earth’s surface impervious to precipitation, cities find that run-off carrying oil, pesticides, metals, animal droppings and other pollutants is a serious problem. From the point of view of engineers trying to deal with this pollution, as well as the erosion and flooding caused by stormwater and combined sewer overflow, rain as anything other than the enemy is indeed a revolutionary idea.
“We’re trying to get residents to deal with their own water,” says Michael D’Andrea, P.Eng., manager, infrastructure asset management of the City of Toronto’s water and wastewater services division. Offering to disconnect downspouts that feed rainwater directly from eavestroughs into combined storm and sanitary sewer pipes, and providing rain barrels as an alternative, is one of the ways the city is attempting to mitigate excess flows at their source. But such “at source” solutions, while preferred by environmentalists, don’t help much unless they really catch on. D’Andrea cites a recent poll indicating that very few Torontonians understand that stormwater is a major contributor to poor water quality in Lake Ontario.
Although pipes carrying a combination of sewage and stormwater were recognized as leading to pollution problems in the 1950s, and separate sanitary sewers have been built since then, the problem remains in older urban centres still served by combined sewers such as the old City of Toronto. In 1997, the former Metropolitan Toronto Works Department initiated the Wet Weather Flow Management Master Plan to address combined sewer overflow (CSO). Dillon Consultants were in charge of the first phase of this project. The plan’s vision statement recognizes rainwater as “a potential resource to be utilized to improve the health of Toronto’s watercourses and the near shore zones of Lake Ontario; and enhance the natural environment of Toronto’s watersheds.” And its hierarchical approach mandates that stormwater should first be dealt with where it falls, then as it is combined with sewage and transported across the city and finally, before it is being discharged.
With the plan in the development phase, a steering committee is determining how best to prevent, control and reduce pollution from wet weather flows. The public has recently been invited to a number of workshops (ironically, foul weather tends to result in low attendance at such gatherings), and D’Andrea says an innovative new web site has been launched that will allow “e-consultation.”
With only about seven per cent participation in the rain barrel program after years of effort by the city, it won’t have much of an impact on the water quality at the city’s beaches any time soon. And opportunities for natural remediation measures at the other end of the pipe — developing wetlands or retention ponds at outfalls, for instance — are limited by existing development along the lakeshore.
As a result, structural “end-of-pipe” solutions, although they are seen as a last resort in the context of the city’s wet weather flow plan, offer the quickest fixes. “They’re the only way to realize a measurable achievement in a short space of time,” says Ken Collicott, P.Eng., a self-described “interested observer” who is with R.V. Anderson. “[But] A lot of people prefer source controls… It has to be a combination of these efforts.”
Already the two storage tanks ($4 million, 2000 m3 completed in 1990; $10 million, 8000 m3 commissioned in 1995) under the lake at the eastern beaches have reduced the number of beach swimming closures from 15 or more per season to just two last summer. CH2M Gore + Storrie and MacViro Consultants were the prime consulting engineers. Over 60 overflows a year were common before the tanks were built.
In the western beaches, 90% of the combined sewer overflow will be handled by four kilometres of new storage capacity under construction by a design-build partnership that includes engineers Acres & Associates. Scheduled for commission this spring, this concrete-lined tunnel is three metres in diameter and has three 45-metre deep storage shafts (a total capacity of 85,000 m3). The shafts (an alternative to building a larger-diameter tunnel) and the use of an ultraviolet (UV) light system to disinfect the water as it is pumped out into the lake are among the most innovative aspects of the $50-million project according to Ralph Davidson, P.Eng. of CH2M Gore & Storrie. That firm, along with MacViro, carried out the pre-design. “Ultraviolet disinfection of CSO is leading edge,” he says.
He adds that other ways to minimize run-off and manage peak volumes, such as roof storage and improved infiltration (by routing run-off to underground pipes designed to allow percolation, for instance) are also being considered under the Wet Weather Flow Management Plan. “The volumes of combined sewer overflow requiring end-of-pipe treatment should be reduced as these other strategies are implemented.”
A common denominator for most Canadian municipalities — budget constraints — led Edmonton to improve its existing system before constructing new facilities. Last year, the city completed its five-year, five-phase plan to control overflow from its 900 kilometres of combined sewers. The only city in Alberta with combined sewers, it was required by the province’s environment ministry to come up with measures that would result in an equivalent or better level of environmental protection than would be achieved by complete separation.
Unlike in the U.S. where budgets have been high, it was necessary for Edmonton to be more innovative. This, according to Marcel LeBlanc, P.Eng., senior project engineer with UMA Engineering, the prime consultant for developing Edmonton’s plan. “We have to use the existing system to capacity before adding to it,” he says. While Phase B (which is to be reassessed upon completion of Phase A in 2020) involves constructing new containment for overflow, Phase A includes measures such as relatively simple real-time control (having gates and dams react to what is actually happening downstream) to ensure that the most cost-effective options like in-line (in-pipe) storage are used first. Phase A also includes separating combined sewers when the opportunity arises as a result of neighbourhood improvement programs. Plus there are plans to upgrade the Gold Bar Wastewater Treatment Plant by enhancing and increasing the capacity for primary treatment, providing bypass disinfection, adding pipes at two river crossings; and modifying weirs to reduce overflow.
Real-time controls in Quebec
In Quebec City, the Quebec Urban Community has taken optimization using real-time controls several steps further with its combined sewer overflow control plan. The plan was developed by BPR-CSO (a subsidiary of the BPR Group specializing in wet weather flow management) in Montreal. By improving the existing system, the engineers have come up with a $150-million plan, and, according to Hubert Colas, ing., at BPR-CSO, are continuing the process in an effort to lower costs still further.
The optimal global and predictive (OGP) real time control (RTC) system implemented as part of the plan has become something of a showpiece. Colas says, “It’s the only application [of OGP-RTC] in the world.” A paper co-authored by engineers from the Quebec Urban Community and BPR lists the benefits of the system. It allows, for example, operators to take advantage of the storage capacity in two tunnels, which could otherwise only be used for conveyance. It also has the ability to access Environment Canada radar rainfall images via the web, which are updated every five minutes. These images are calibrated with data from rain gauges distributed across the municipality.
Considering rain patterns and run-off predictions over the next two hours, along with flow data, regulator status and the state of the sewer network, the system’s “MED”
software computes the optimal and co-ordinated set points of all flow regulators every five minutes. The effect is to reduce combined sewer overflows, maximize the retention capacity in both tunnels and wastewater treatment plants, and preclude flooding and sewer surcharges that cause back-ups into basements and onto the land. Operators follow the development of the software’s decision-making through a user-friendly supervisory system and can interact with and direct the system at all times. Data and decisions are stored in a database management system for analysis and reporting purposes.
Although it is only being used in the western catchment area (where 66 kilometres of sewer pipe serves 65% of the municipality’s geographical area and almost 50% of its population), Colas says the system has reduced combined sewer overflows by 50% in terms of volume and 60% in terms of the number of occurrences. The implementation cost was about $4.2 million, not including preliminary design and software costs that were incurred in an earlier phase of the project. Before the OGP-RTC system was operating, overflows from the western network’s four outfalls to the St. Charles River and five to the St. Lawrence River totalled on average two million cubic metres during the summer season (May to September).
With space for big containment or treatment structures at a premium in the older cities that have the worst combined sewer overflow problems, making the most out of existing systems may be the best, or at least, the first route to cleaner water at the outfalls. Despite the success of the Quebec City system, Colas isn’t sure this is the way his colleagues are moving: “A lot of engineers are still thinking of (constructing) civil works instead of optimizing what we have.”
Sophie Kneisel is the former editor of Canadian Consulting Engineer, and now a freelance writer in Baltimore, Ontario.