ISM: What ist real time control?

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.
(Schilling, Smart Sewer Systems: Improved Performance by Real Time Control, European Water Pollution Control, Vol 4, No. 5, 1994)

A review of the current state of the art of real time control of urban wastewater systems has been presented by Schütze, Campisano, Colas, Schilling and Vanrolleghem in "Real-time control of urban wastewater systems - where do we stand today?" (9th International Conference on Urban Drainage, Portland, USA, 2002).
Control options not only for the sewer system, but also for the wastewater treatment plant and of receiving water bodies are considered. The paper provides concise definitions of terms frequently used in the literature. Some fundamental concepts of the integrated approach are outlined.

Erbe has given an overview of realized RTC projects in Germany in "Kanalnetzsteuerung - Überblick über umgesetzte Projekte und Erfahrungen aus der Praxis" (Innovationen in der Abwasserableitung und Abwassersteuerung, Dresdner Berichte Band 19, Dresden, 2002).

"Planungshilfe Abflusssteuerung (PASST)" (Planning aids for RTC) of the German ATV-DVWK workgroup ES 2.4 illustrates the subject RTC in an easy, vivid and interactive way. The objective is to activate planners and especially operators and representatives of supervisory and authorising agencies, so that rtc will be increasingly taken into account at processing wastewater oriented problems in the future.