H.A. Preisig

Indicators for the sustainability assessment of wastewater treatment systems

Abstract Sustainability challenges us to reflect on wastewater treatment differently. Instead of focussing on end-of-pipe-treatment for emission prevention, attention shifts towards optimal resource utilisation, favouring the development of decentralised systems. But are these systems more sustainable than centralised wastewater treatment systems? What aspects determine sustainability? In an extensive literature review we give an overview of sustainability assessment methods and currently used indicators. Based on this we propose a general assessment methodology that builds on multi-objective optimisation and a complete set of sustainability indicators, yielding insight into the trade-offs made when selecting sustainable wastewater treatment systems.

A mathematical approach to discrete-event dynamic modelling of hybrid systems

Abstract Presently, design of supervisory control systems is based on ad hoc design methods. This project attempts to approach the design of supervisory controllers using the structure of continuous and sampled systems. The approach builds on process models, incorporates performance specifications and constraints imposed by the plant and otherwise and synthesises the controller by partial inversion of the procese model. This paper describes a modelling technology derived for hybrid systems. The hybrid system consists of a continuous plant forced by commands from the supervisory control system and observed through event detectors. The event detectors generate the “measurements” used as inputs to the supervisory control system. The modelling technology quantises the state space systematically and leads to discrete-event dynamic process models that depend on the discrete inputs and the effects of the environments of the plant. Modelling and control of a simple plant is discussed. The resulting controlled systems are analysed for undesirable effects.

Concept and design of Modeller, a computer-aided modelling tool

Abstract This paper presents the main ideas and design concepts behind Modeller , a computer-aided modelling tool which aims to effectively assist in the development of process models and to facilitate hierarchical modelling through a user friendly, graphical interface. The tool generates first principles based, mathematical models of (bio-)chemical processes in a quick and easy manner, based on a modelling methodology designed for such processes. Modeller is intended to be a component in a larger framework of modelling-assistant software tools and simulation packages. Its output can be transformed and interfaced with (or serve as input for) existing modelling languages and/or simulation packages (such as gProms, ASCEND, Omola, Modelica). Modeller has a graphical user interface, supports model reuse and is the result of an object oriented design.

Parameter identifiablity in the IAWQ Model No. 1 for modelling activated sludge plants for enhanced nitrogen removal

The IA WQ Model No. 1 is well accepted and used for dynamic modelling of activated sludge plants. The model contains many parameters. Some of the model parameters depend on the wastewater and/or plant. In some applications, the model has to describe a real plant quite accurately and some of the parameters have to be determined. As a consequence, model tuning or calibration has become an active area of research during the last few years. This contribution briefly reviews recent literature on calibration strategies and methods for assessing parameter identifiability of the Model. Some identifiability results for full scale plants are presented obtained by a combined analysis of the parametric sensitivity and the Fisher information matrix.

Computer-aided modelling — Two paradigms on control

Modelling plays a central role in any process engineering project. Mechanistic models are being used increasingly not only for simulation but also for design, operation and control related activities. The integration of these activities is highly desirable as it would not only build a link between the various activity groups, but would also increase the efficiency of project activities. The key to improving efficiency is an increased use of common process information and effective exchange of data, models and information between the applications.

Modelling of a pneumatic dryer for potato starch

A dynamic model of an industrial pneumatic dryer is constructed for the purpose of modelling the variations in product quality as a consequence of the variations in the feed. Rigorous mechanistic modelling is augmented with empirical assumptions to result in a nonlinear system of ordinary differential equations. A new short-cut method for the computation of the evaporation rate is introduced. Simulation results are compared with actual measurements and show that our model follows quite well the behaviour of the real plant in spite of the relative simplicity of the model.

Control of quantized systems based on discrete event models

This paper presents controller design methods for dynamical systems that are observed by discrete sensors. These sensors induce a partitioning of the state space and only this quantized information is available for the controller. The so-called quantized system is modelled by a discrete-event model that serves as a basis for the controller design methods. However, instead of using solely the classical control methodologies for discrete-event systems as found in the literature, improvements are proposed by including additional information provided by the fact that the underlying plant is continuous by nature, such as continuity of the state trajectories and information on derivatives that holds for parts of the state space. The concept of discretely controlled invariant sets will play a crucial role in the development of control strategies and necessary and sufficient conditions for controlled invariance are presented. Also algorithms are included to compute the smallest and largest discretely controlled invariant sets with respect to a given set. All the techniques and computations are explicitly described by Boolean matrices and vectors and are ready for application. This is demonstrated for a benchmark problem of a two tank system.

A model structure for product quality in processing agro-material for process control purposes

In this paper, a model structure is presented that captures product behaviour with respect to quality properties. Modelling product quality properties involves nominal (bulk) dynamic behaviour and the variation of these properties. Nominal behaviour is modelled using a limited number of basic reactions. To deal with the variation the presented model structure is extended with a three-step approximation procedure using discretised intervals. The model structure is suitable for control purposes and will contribute to closing the gap between product specialists and the system and control community. The applicability of the model structure and the possibility to describe quality properties is shown with existing models from the literature that show a good fit with the described model structure and by an industrial case study on potato storage.

Automaton-based fault detection and isolation

Abstract We present a fault detection and isolation method which is based on the observation of events. Several propositions are given that can be used directly for a fault detection and isolation scheme, and that can serve as a tool to analyze fault detection and isolation properties of the system. The results are illustrated by means of an example.

Model-based product quality control for a potato storage facility

The design and implementation of a control structure for a potato storage facility is presented that is directly geared at the optimal preservation of product quality. The optimal storage conditions are determined based on a model of product quality that takes into account the effect of the storage conditions. The controller is based on a model predictive scheme and optimises a cost function including product quality and cost of storage. A weather module ensures an efficient use of external ventilation. The scheme is currently under implementation in a real storage facility and results are promising for an improved performance with respect to conventional structures.