THE USE OF MODELS IN ADVANCED WWTP CONTROL – NOT ONLY MODEL-BASED PREDICTIVE CONTROL

In the contribution a (non-exhaustive) overview is given on the use of mathematical models in control systems of wastewater treatment plants. As a starting point for the overview, the four building blocks of any controlled process are used, 1) the process itself, 2) the actuators that allow manipulation of the process, 3) the control algorithm that calculates a proper action for disturbance rejection or setpoint tracking and 4) the measurement devices that provide information on the important output variables and disturbances. The use of models in controller design and tuning is reviewed. Besides traditional feedback control via PID, optimal control schemes and MIMO control systems are covered briefly. Attention is drawn to the usefulness of feedforward control strategies to compensate for the effect of predictable disturbances. The impact of the time-varying and nonlinear nature of bioprocesses on the choice and design of controllers is discussed subsequently. Adaptive and nonlinear control approaches that deal with these characteristics are explained. Because measurements of disturbances and output variables form an integral part of a control loop, and a lack of adequate instrumentation is still felt as the main bottleneck for the introduction of control systems in wastewater treatment processes, the potential solution provided by incorporation of mathematical models in monitoring systems is presented. As a first application of models in monitoring systems, the concept of software sensors is described and illustrated with practical applications. First, the construction of state observers on the basis of process models will be introduced. Next, it will be shown how on-line estimates of model parameters can be obtained. As software sensors aim at extracting the utmost information from the scarce data that are typically available at WWTPs, the problem of the richness of information in the data is introduced and some possible solutions will be proposed. It is illustrated with actual data obtained at pilotand full-scale WWTPs how information rich data can be obtained (i) by “excitation” of the process variables using advanced control schemes (that allow some dynamic variations of the monitored variables) or (ii) by performing so-called "InSensor-Experiments". A second use of mathematical models in providing data to the WWTP control algorithm is the prediction of disturbances to the treatment plant and prediction of the allowable discharge to the receiving water. For instance, the effect of rain events, the impact of return liquors from sludge treatment or the optimal batch scheduling of industrial production facilities may be summarised in a model that can be used to predict future influent flows and composition over a certain time horizon. Similarly, incorporating the current and future receiving water quality dynamics in the control strategy receives increasing attention. It is illustrated using a test case of a large city how mathematical models can be useful to determine the minimum effluent quality that is required during the forthcoming period to guarantee the receiving water's quality.