EPA/600/JA-01027
2001
HED: Get Real! D
DECK: Implementing real-time control schemes offers combinedD
sewer overflow control for complex urban collection systemsD
Richard Field, Elise Villeneuve, Mary K. Stinson, NathalieD
Jolicoeur, Martin Pleau, and Pierre LavalleeD
Combined sewer overflow (CSO) is a significant source
of pollution in receiving waters. However, implementing a
real-time control scheme operates automatic regulators more
efficiently to maximize a collection system's storage,
treatment, and transport capacities, reducing the volume and
number of CSOs. Real-time control schemes are being used to
manage complex urban collection systems around the world,
including a demonstration study in Canada for the Quebec
Urban Community (QUC) collection system. Funded by the U.S.
Environmental Protection Agency (under a contract to the
Office of Research and Development) to assess the use of
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real-time control schemes, the QUC study evaluated the
effectiveness of three schemes in managing CSOs.
THE QUEBEC URBAN COMMUNITY TERRITORY DEMONSTRATION SITE
Located on the North shore of the St-Lawrence River,
the QUC territory covers 200 mi2 (500 km2), has a population
of 500 000, and is composed of an Eastern and Western
catchment. The QUC study team's evaluation of real-time
control schemes focuses on the Western catchment.
The Western catchment covers 65% of the QUC territory,
with close to 50% of the total population (230 000).
Wastewater is conveyed through 41 mi (66 km) of interceptor
pipes to a 82-mgd (310 000-m3/d) wastewater treatment plant
(see Figure 1, p. xx). The collection system has three main
interceptor branches and two tunnels that together provide
approximately 3.4 MG (13 000 m3) of inline storage. The
overflows of the western collection system represent 528 MG
(2 million m3).
Nine of the 22 regulators have significant overflows
that empty into the St-Charles and St-Lawrence rivers. The
Dijon, Jones, and Suete CSO structures and the Affluent, and
Versant-Sud tunnel regulators overflow into the St-Lawrence
River and the Hopital, Lessard-Durand, Talus, and Myrand CSO
structures overflow into the St-Charles River.
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The QUC's long-term CSO control plan is to maximize the
Western catchment's intercepted flow and existing
facilities, two inline storage tunnels, and the treatment
plant. The long-term plan includes implementing an Optimal
Global Predictive (OGP) real-time control scheme in the
entire system and constructing offline storage facilities
and is projected to control more than 85% of CSOs and cost a
total of $107 million, 37% less than before implementing the
OGP scheme.
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Real-time Control Scheme Simulations
The QUC study team evaluated three real-time control
schemes using 32 real rainfall events ranging from very
small events to a large once-in-5-year event, as well as
back-to-back rainfalls between July 1 and August 28, 1988.
Five raingauges collected data to represent, in part, the
Western territory's rainfall heterogeneity. The data then
were translated into combined collection system flow rates,
which were fed to a custom-built, nonlinear hydraulic model
Using the model and simulation software, a total of 128
simulations were carried out to observe the performance of
the three different control schemes. The control schemes
were evaluated for CSO volumes, number of CSO events,
surcharge occurrence, treatment plant utilization, and
inline storage capacity.
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Real-time Control Schemes
The study team investigated (1) Local Reactive Control
(LRC Type 1) that operates collection system gates at fixed
flow set points on the intercepted flow, requiring local
site control; (2) Local Reactive Control (LRC Type 2), which
works similar to Type 1, except it operates the gates at
both fixed and variable set points in respect to flow
capacities located at some specific pipes; and (3) OGP that
operates the gates at optimal variable set points proactive
to actual rainfall conditions, which predicts flow 2 hours
in advance using rainguages and flow and rainfall prediction
models.
Implementing the LRC Type 1 scheme can be as simple as
employing a mechanical device to open or close a system gate
while the Type 2 scheme is more complex, similar to the OGP
scheme. Both the LRC Type 2 scheme and the OGP scheme
require more instrumentation and equipment; however, the OGP
scheme differs by using a central decision-making system,
prediction models, and other more sophisticated programs and
equipment.
Selecting a real-time control scheme depends on the
architecture of the collection system and the environmental
objectives pursued. Collection systems with small storage
capacities, few flow control devices, and restrictive flow
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constraints can be properly managed with relatively simple
control schemes but more complex systems require a global
control approach. Selecting a real-time control scheme not
only depends on performance, it also depends on criteria
such as implementation and process control, capital cost,
and operations and maintenance costs.
Control Objectives
Within the Western catchment's existing collection
system (currently without offline retention tanks), the
selected real-time control scheme must:
• Reduce CSO frequencies and volume as much as
possible during operational season activities (from May 15
to September 15) to meet water quality levels for contact
with the St-Charles and St-Lawrence rivers;
• Eliminate surcharge flow caused by flooding from
private connections along the inceptor at a setting of 95%
of its total capacity;
• Allow variable flow set points to maximize the
Western treatment plant's capacity, which fluctuates with
the St-Lawrence River tide; and
• Use the Western system's two major inline storage
tunnels to maximum capacity and ensure no premature
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overflows occur while residual storage capacity is
available.
Overall Performance
In comparing the three control schemes with one
another, the OGP scheme has the lowest CSO volumes and
number of CSO events (see Figures 2 and 3, p. xx). Because
it can constantly readjust its control set points according
to updated field information, this control scheme is the
most efficient to control and minimizes the surcharge flow
in the system (see Figure 4, p. xx). The OGP scheme also
permits programming more sensitive overflow sites as
priorities and allows the system to constantly adapt to
protect these sites. In fact, the more complex the
collection system — number of flow paths and storage
options — the better the OGP scheme performs.
The difference in total CSO volume between the OGP
scheme and LRC Type 2 is relatively small compared to the
total CSO volume recorded with the other two control
methods. The LRC Type 2 scheme did not eliminate surcharges
and is not flexible enough to properly manage future offline
storage facilities. The LRC Type 2 scheme is more suitable
for controlling relatively simple systems that accept a
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certain amount of surcharge. At this time, without offline
storage tanks, the OGP scheme behaves as a flow management
scheme that conveys the maximum amount of water to the
Western treatment plant, within the flow constraints.
Using an August 27, 1988 rain event as an example, the
inflows to the Western treatment plant without a real-time
control scheme and under OGP control show that no overflow
occurs at the plant with the OGP scheme (see Figure 5, p.
xx). However, without a real-time control scheme, a 0.45-MG
(1700-m3) overflow occurred. The OGP scheme also conveyed 78
MG (295 000 m3) of combined wastewater to the plant while
operating the system with no real-time control scheme only
conveyed 72 MG (271 000 m3) and allowed a 6-Mg (22 700 m3)
overflow at the plant. Furthermore, without a real-time
control scheme, the Versant-Sud tunnel was used only as a
conveyance system, whereas under the OGP scheme, the tunnel
also was used for storage [up to 2-MG (8000-m3) ] .
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Implementation and Process Control
Implementing either LRC scheme poses more operations
and maintenance concerns, depending on the quality and
quantity of measurement and control devices installed. To
maintain a prescribed flow set point, flow routines must be
programmed and calibrated, and controllers, such as
Proportional Integrative Derivatives (PIDs) — mathematical
equations used to adjust the position of the system gates —
need to be implemented and properly tuned. Downgraded
management modes must be defined and implemented at the
local control stations to address equipment breakdowns or
other system anomalies and should include predefined flow
and gate opening set points for every kind of foreseeable
failure or breakdown. Finally, a telecommunication system
and a central supervisory control station are recommended to
monitor the performance of the control scheme.
The difficulty of implementing the telecommunication
system varies with the topography of the territory covered
by the collection system. For the Western network, the land
is relatively flat, ideal for using a radio
telecommunication system. Moreover, the fewer local control
stations, the less data traffic to interfere with
telecommunications.
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Implementing the OGP control scheme requires a more
sophisticated level of process control. The difficulties
encountered are similar to those described for both LRC
schemes. However, design parameters require determining
variable measurements and accuracy of the hydraulic model,
transmission distortion of control signals, meteorological
predictions, and flow set points using optimization (the
equivalent to an "intelligent" decision-making machine) and
filtering algorithms (such as averaging or exponential
computation) and nonlinear programming. In addition, the
implementation of a central control station is more complex.
An optimal control problem has to be setup and solved in
real-time using an optimization algorithm. A meteorological
forecasting model, calibrated with raingauge measurements,
may be needed to guarantee good performance. If the
forecasting algorithm relies on radar images, the
availability of these images in real-time must be
considered.
Capital Cost
The capital cost of implementing a real-time control
scheme depends on the quality and quantity of control and
measurement devices required for a successful
implementation, as well as the models and the algorithms
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needed to compute the flow set points. A preliminary study
of the Western collection system shows that implementing the
OGP scheme costs approximately $4 million (less than 4% the
total cost of QUC's long-term CSO plan), the LRC Type 2
costs approximately $2.5 million, and the LRC Type 1 costs
approximately $1.5 million.
Operation and Maintenance Costs
For the Western network, the real-time control schemes
are in operation only during the regulated period — May 15
through September 15, meaning there are no maintenance costs
for a significant period of the year.
Operation and maintenance costs depend on the
sophistication of the implemented control scheme (the number
of control and measurement devices, as well as the
geographical characteristics of the collection system).
Implementing any one of the three real-time control schemes
can be a relatively inexpensive solution compared to
conventional alternatives. In fact, in the QUC study each
scheme represents less then 4% of the total cost for
complying with long term CSO control regulations. Operating
the mobile actuators, telecommunication systems, and
supervisory systems generate electricity costs and certain
models require regular purchases, such as radar images if
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using a meteorological forecasting model. However, the OGP
scheme allows for additional control objectives to minimize
electricity costs for pumping and treatment.
Operations and maintenance considerations for
implementing any real-time control scheme includes weekly
cleaning of sensors, monthly testing of programmable logic
controllers and personal computers in downgraded mode, and
regular mechanical maintenance of gates and actuators. For
implementing the OGP scheme, additional operations and
maintenance considerations include calibrating and
validating meteorological forecasting model every 3 months.
Quality control must be performed on the database processing
archives monthly and after each rainfall event. Quality
control also must be performed on the collection system
configuration every 3 months and after any modifications.
The hydraulic models must be calibrated yearly, and
statistics and reports on performance and default conditions
must be compiled monthly and after each rainfall event. The
decision-making system, control objectives, and global and
local priorities also must be verified and adjusted monthly
and after each rainfall event. Constraints included in the
non-linear programming algorithm must be verified and
adjusted monthly.
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Real Results
The QUC example demonstrates the potential of real-time
control schemes in maximizing the capacity of collection
systems and reducing CSOs. Even with a relatively simple
system with no offline storage to manage, the real-time
control schemes evaluated in the QUC reduced CSO volumes by
24% to 47%, representative of potential performance in most
collection systems. However, real-time control schemes
should be selected depending on a collection system's
configuration and the control and operational objectives
specified by the utility authority.
Richard Field is the project leader and a senior
environmental engineer and Mary K. Stinson is a physical
scientist for the U.S. Environmental Protection Agency, Wet-
weather Flow Research Program,. Urban Watershed Management
branch, Water Supply Water Resources division, National Risk
Management Research Laboratory (Edison, NJ). Elise
Villeneuve is a project director at BPR Consultants
(Montreal, P.Q., Canada). Nathalie Jolicoeur and Martin
Pleau are project engineers and Pierre Lavallee is the
senior project director and executive vice president of BPR
Consultants (Quebec, P.Q., Canada).
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