Real Time Control: Two Cities, Two Stories

BPR CSO, which undertook both projects, is a consulting company that works mainly in North America but also has interests in Europe. Experience suggests that older cities, in North America as well as Europe, are facing challenges in meeting the increasingly strict requirements of environmental legislation, but have to work within a restricted budget to deal with combined sewer overflows and capacity issues within their wastewater systems.

Why Real Time Control? RTC is a way of adding artificial intelligence into a wastewater system, to utilize the advances in hydraulic modeling of the past ten years as well as technical advances in flow monitoring, rainfall forecasts, computer technology, telecommunication, instrumentation and automated control.

Model simulations are running ever faster, so these technologies can be combined and applied to traditional sewer management, with the aim of continuing to maximize the capacity of the existing infrastructure or, if upsizing is needed, to maximize the use of the tunnels and storage to leverage the capital investment involved.

BPR CSO has developed a software tool called CSoft, a model-based decision support system specifically designed to operate real-time control systems on-line, to optimize and balance flows in a wastewater collection system. Integrating seamlessly with an existing SCADA system through which it extracts information from the field, it reaches an optimized decision and then sends out set points to control sites to efficiently manage collection system flow using control gates, valves, pumps and inflatable dams to distribute wastewater flow the way traffic lights manage the flow of traffic during rush hour.

The primary reason for the interest in RTC in North America is the cost saving that can be achieved. Real-time control maximizes the use of existing infrastructure, so construction of expensive new facilities to meet regulatory requirements is significantly reduced. In the two case studies outlined below, the expected cost saving using RTC (rather than investing in major capital works without optimization) was USD65 million in Ottawa (a 67% expected saving) and EUR62 million in Bordeaux (a 63% expected saving) while meeting the same environmental control objectives.
By investing in RTC at an early stage within the existing infrastructure, the clients can also benefit from obtaining positive results over the short term rather than waiting for the five or ten years that a tunnel design and construction project would require.

There are many major advantages to RTC, particularly in the day-to-day life of wastewater system operation where there are many conflicting objectives that can occur within a very short time of each other. Improving operational efficiency requires the system to be flexible and adaptable to any unforeseen situations, unlike a static system that is designed based on a chosen synthetic or real rainfall event. With proper conditions being monitored, acknowledged and controlled, a sophisticated RTC operational strategy takes into account the distribution of flow and the status of facilities in the entire system, both in current conditions and in the future. The inflow to the system can be controlled and transferred between sites according to the available capacity in the entire system.

Ottawa

Ottawa, the capital of Canada, with a population of 350,000, has a traditional combined sewer system that covers 728.4ha of the city’s total 4047ha. While most of the combined systems have been separated into sanitary and stormwater systems, 15% of the downtown area of the city remains combined.

The impetus for improvement was a requirement from the Ontario Ministry of Environment to meet the F-5-5 procedure with the main objective to capture 90% wet weather flow from April to November annually. The city also required the wastewater system to operate better and be protected against flooding. It looked at RTC because the first long-term control study recommended constructing a tunnel, which would have been a considerable expense.

BPR-CSO undertook a feasibility study to ascertain if it was possible to use RTC to bring more wastewater to treatment, to ensure that the interceptor was full before it began to overflow, and to explore possible in-line storage in the system.

The conclusion was that local reactive control would be sufficient to meet the regulatory requirements, and that the city did not need the proposed major tunnel project. Simply by retrofitting the existing regulators, which was going to happen in any case because most were over 50 years old, a great deal of major capital investment could be avoided.

The project identified seven control sites. The first task was to integrate pump station operation at the treatment plant. Using RTC provides a holistic system-wide approach to manage both the collection system and treatment operations, even at the simplest level of local reactive control. The project will cost around $25 million in total over a period of two years, including the new capital elements and an outfall monitoring program.

Once the project had progressed to pre-design stage, InfoWorks CS was chosen to fine tune the preferred RTC approach to meet the pre-design requirements, evaluate annual performance and undertake stress tests of failure scenarios and critical flooding events.

InfoWorks CS proved able to undertake this complex task. The modeling results show that more wastewater could be sent to the treatment plant by maximizing the use of existing capacity in the network. The annual wet weather flows (WWF) capture rate increased from a 74% baseline condition to 91%, meeting the control objectives. InfoWorks CS was also useful for the “stress” tests, in which it was used to assess critical situations such as pump failures, a city wide power failure, and multiple gate failures, proven that an RTC system with a proper, reliable failsafe design does not have a negative impact on the system under critical events, and is sufficient to protect the interceptor even with gate or power failures.

Case study: Louis Fargue Basin
The Louis Fargue basin of Bordeaux, France has a population of 287,000, covers some 6200ha and has around 90% combined sewers. The area had a pollution control objective to reduce spills into the Garonne River and to optimize inflows into the treatment plant. The system had many storage basins that were used for flood control, so the area’s flood control objectives included the fast evacuation of these storage facilities and downstream networks, and restricting the hydraulic grade line under critical levels to prevent street flooding.

BPR-CSO also undertook a phased feasibility studies to see if it could apply more intelligence to the system to meet its control objectives and improve operational efficiency, while providing system-wide optimized control coupled with rainfall and flow prediction.

The area also had a tunnel project planned for its downtown areas, which would have been costly because of the city’s historical significance and the need to work with great care. The existing system was under operator-supervised control, but in reality the client was only able to check operating conditions in the storage basins.

An InfoWorks CS model was coupled with Csoft (for optimisation) and used to determine whether the capacity within the downstream part of the system near the treatment plant could be maximized. Better use of the upstream capacity was then investigated, to integrate the use of the controls for the various storage basins.

The Médoc Grenouillère complex had a number of particular modeling challenges including generating backwater flows from Quai 14 to the complex while staying within hydraulic constraints, and achieving optimal control of the Quai 14 and Grenouillère pumping stations.

The existing storage basin was already under local control. A gate was installed at Quai 14 so water from the collector sewer backed up at that point into the existing storage basin, which was not being filled to its full capacity because it was only receiving flows from one branch of the sewer. Using the in-line storage and capacity in the network, the consultancy was able to fill the storage basin to capacity, thus maximizing the capital investment.

Conclusions

As is evident from the case studies, Real Time Control is a very cost-effective way to undertake both overflow and flood control and adapt to real-life day-to-day operation. More advanced global optimal predictive RTC is proven technology as is shown in Quebec City where the system has been implemented and has now been operating successfully for ten years.

With the choice of the right equipment, sensors and good engineering design to ensure that all of the necessary failsafe systems are in place; such systems can run very smoothly and be an invaluable tool for the operator.

The consultancy found in these two projects that InfoWorks CS works very well as a tool to validate a design choice and then examine the different scenarios to reassure clients that their chosen option will work as planned. When the solution is coupled with CSoft, modeling in the control room becomes a reality - this has been achieved in Ottawa, Quebec, Montreal in Canada and in Louisville, Kentucky in the USA, and a system of this type is currently being designed and implemented for Wilmington, Delaware.