WaterSubject

Combined sewer overflows (CSO) are part of common combined drainage systems. Due to the improvement achieved in the efficiency of wastewater treatment plants, the discharge of such CSO became the focus of discussion regarding the quality of the natural water bodies. Thus operation of combined sewer systems and especially of existing detention facilities aims at an extensive avoidance of combined sewer discharge. To fulfil this task, real-time control may be one of the possible solutions. In Germany many communities already made the first step towards real-time control, i.e. data logging and supervision of the sewerage condition. The experience in application of real-time control and the achieved results are described regarding to case studies of existing real-time control systems.We live in a time of great public and institutional concern about the environment. Protection of natural water resources is one of the main challenges of civilisation and a bare necessity for the supply of drinking water. Environmental awareness encompasses both a responsible use of water and an appropriate treatment after use. However, in spite of the fact that the European countries have grown together over the last years, there are still some discrepancies with regard to water pollution control, storm water runoff management, and wastewater treatment. It is part of the remits of governments and administrations to judge the present varying regulations and standardise future requirements.

In Germany, wastewater treatment reached a high standard during the Eighties. In 1983 already, in West-Germany more than 90% of all inhabitants were connected to a sewer system, and the sewage of more than 75% underwent biological treatment. Due to the continual improvement in sewage treatment achieved since the early seventies, combined sewer overflow (CSO) has become increasingly an important cause of water pollution. To respond to this, water quality management had to focus not only on wastewater but increasingly also on storm water treatment, which resulted in the setting up of new storm water management programmes aimed at CSO reduction. In most cases, storm water detention tanks were installed within the combined sewer systems to comply with legislative and regulatory effluent requirements. In recent years, alternative routes––such as, for example, reduction of runoff or various methods of CSO discharge treatment––have been considered as well.

In Germany, storm water treatment plants are generally recognised as essential components of sewerage systems which becomes evident when judging their ‘standing’ on the disposal route by the steadily growing financial expenditure. Furthermore, public authorities increasingly focus attention on these facilities which means more stringent requirements for operational care and monitoring.

The use of state-of-the-art instrumentation and control technique has become indispensable to reliably manage this considerable number of storm water treatment plants and to establish and maintain optimum operating conditions. So more and more German operators are presently on the point of implementing central monitoring and control systems for the most important storm water treatment plants in a catchment area as well as a central evaluation of storage and discharge behaviour. These efforts combined with a better use of the possibilities of real-time control will contribute significantly to an enhanced runoff scenario within sewerage networks.

Application of storm water management by real-time control seems to be a suitable tool to solve current and future storm water problems. The benefits of its use have meanwhile been proved in practice in several sewer systems. However, its implementation also means for the operators to cope with the demands and constraints of constructional and operational changes, concerning, for example, the structure of the sewer network, the location of control stations, the types of available control devices, and so on.

Only the quantitative parameters have so far been used by the real-time control systems in operation in Germany. This may be a question of availability of appropriate devices. It is expected that in future real-time control will focus on pollution loads or even on water quality in the receiving water rather than on volumes of storm water. Other trends, which are already discernible today, move towards a combined operation of sewerage network and WWTP (which means, for example a variable inflow rate as a function of the available capacity) and towards an integrated control of sewerage network, WWTP, and receiving water (which means an interconnected control of network and WWTP to maintain sufficient oxygen concentrations in the bodies of water) .

Yet in spite of these promising prospects, operators should not close their eyes to the already feasible, which is to make an efficient use of network control. Efforts in this direction will prove profitable if operators are well aware of all technical and administrative boundary conditions and really identify themselves with the new operation mode of ‘real-time control’. Like in many other areas, it might be advisable to make a step-by-step approach with clearly defined and easily comprehensible targets. With this, it will be possible to benefit from the economic advantages offered by real-time control.