GROUP ON REAL-TIME CONTROL OF URBAN DRAINAGE SYSTEMS (RTCUDS)
	REAL TIME CONTROL CONCEPTS

Imagine an automobile factory representing an investment of $100 million. Peak daily production at the plant is 1000 cars but normally it produces only 50 cars per day. Sometimes, bottlenecks and accidents occur due to the non-synchronised assembly lines, loosing 20% of all production. Sometimes parts of the production line are inundated with supplies and on the contrary other parts cannot produce due to shortages. However, no one knows exactly what is happening in the plant since it runs without supervision.

Consider a wastewater system representing an equal investment of $100 million. The system is designed to carry a 5-year peak flow, but the normal dry weather flow is only 5% of the design flow. The treatment plant is sized to handle twice the normal dry weather flow. During even small rain events, raw sewage often overflows, resulting in untreated discharge of 20% of the annual pollution load to the receiving waters. Occasionally, parts of the wastewater system experience flooding while other parts are below capacity and upstream detention tank are not full.

The automobile factory would soon close because of inefficiency and lack of competitiveness, but the inefficient wastewater system, having an equal investment of capital, is still considered standard engineering practice in large parts of the world.

During the last three decades several investigations have been carried out dealing with the implementation of real time control (RTC) in the wastewater systems and the assessment of its potential benefits in terms of improved performances and efficiency.

A wastewater system is controlled in real time if process data (such as water level, flow, pollutant concentration, etc.) are concurrently monitored in the system and used to operate flow regulators during the actual flow.

Typically, this task involves activating a number of pumps, sluice gates, weirs, etc. to allow the occurrence of adverse effects (e.g. flooding, combined sewer overflow CSO, unnecessary decrease in treatment plant effluent quality and in receiving water quality) only if the system is at capacity and only at those locations where the least damage is caused. In traditional static systems this can only be achieved in the rare case when the wastewater system is receiving its design load. If, for example, the outflow of a detention pond is controlled by an orifice, the maximum outflow rate is reached when the pond is full. During other periods the outflow rate is smaller than the maximum and, consequently, the emptying time is longer. To activate excess storage in a large sewer, a (static) high-side weir overflow regulator can be used. The overflow opening has to be large enough to allow passage of the design overflow rate. Thus, much of the available storage cannot be used in most situations.

It is obvious that some of the deficiencies in static systems may be overcome by introducing moveable regulators to maintain a pre-set flow or water level, respectively. Many of these regulators use process measurements taken directly at the regulator site (e.g. by a float, counterweight, etc.). Such a system is called a local control system. Under local control, regulators are not remotely manipulated from a control centre, even if operational data are centrally acquired. Local control is a good solution if the system has only one regulator (e.g. inflow equalisation tank at a treatment plant).

If the system is more complex or if all regulators need to be operated in a co-ordinated manner, global control is applied. Here, all regulators are operated with respect to process measurements throughout the entire system.

RTC was demonstrated as practical in the mid-1970s with successful USEPA demonstration projects. This technology has proven to be feasible and efficient, making the best use of the public capital investment in wastewater conveyance infrastructure.

Thirty years later, there are still few sewerage agencies, and even fewer municipalities which practice RTC of their wastewater systems.

In the United States, only nine real time control projects have been found in literature; most of them were initiated in the 1970s and implemented in the 1980s: Minneapolis-Saint Paul, Mi; Seattle, Wa; Rochester, NY; Cleveland, Oh; Detroit, Mi; Chicago, Il; Milwaukee, Wi; San Francisco, Ca; Lima, Oh. All these projects have implemented supervisory systems and, except for Seattle, all their control systems operate gates, inflatable dams, and pump stations with local reactive control logic. Seattle is the only with an automatic central control but it has been taken out of operation in 1995. Reasons for not going forward with central control include: unreliable communication (Cleveland); hardware failures (Detroit); operators resistance (Rochester, Detroit); the control system never worked as planned (Minneapolis-Saint-Paul); abandoned (San Francisco).

In Canada, the Montreal Urban Community also realised local control stations for the wastewater system very early in the 1980s and it is now looking for implementing centralised control whereas a state-of-the-art global optimal predictive RTC systemhas been operating since 1999.

In Europe, RTC projects lagged behind the North American experiences. It is not before the 1980s that such projects emerged, although from the few experiments of the 1980s, such as in Hamburg (Germany) and Seine-Saint-Denis (France), the impetus has never vanished, and has even gained momentum in recent years.

In France there are actually 8 RTC working systems in Bordeaux, Hauts-de-Seine, Marseille, Metz, Nancy, Seine-Saint-Denis, Val-de-Marne and Paris agglomeration. In Germany, there are 4 RTC projects (Bremen, Hamburg, Munich and Stuttgart), in Switzerland there are also 4 RTC systems in operation (Berne, Fribourg, Geneva and Lausanne), in Sweden just one (Göteborg), and in Denmark two (Copenhagen, Aalborg). The structures of these systems are quite similar to those in the U.S.: all of them have central supervisory systems, and except for 4 applications, all have implemented local reactive control rather than centralised control. Other testing or planning process of RTC are actually in progress in Flensburg (implementation and testing), in Bamberg (planning, hardware installed), Krefeld (planning), Dresden (planning), and Vienna (planning).

Since the development of the first RTC systems, the computer and information technology has taken a giant leap in processing capacity, in user-friendliness, in accessibility, robustness and in reliability, then technology no longer limits the spreading of RTC in wastewater systems. There still exists resistance from operators, there still remains competition and lack of coordination between Departments, Engineering versus Operations, Collection System versus Wastewater Treatment Plant. Nevertheless, these old barriers are weakening due to competitiveness and greater awareness and pressures from the public. This let us foresee a brighter future for RTC in the years ahead.

Inefficiencies are bound to fade away!

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