Ghislain Vansteenkiste

Sludge storage for countering nitrogen shock loads and toxicity incidents

Spare nitrification capacity is usually needed for a nitrifying activated sludge plant to counter nitrogen shock loads and/or toxicity incidents. The traditional way to provide this capacity is to apply a sludge retention time which is much longer than what needed to obtain a stable nitrification, resulting in over designed plants. Another approach, which is investigated in this paper, is to store the spare biomass in a separate sludge storage tank and return it to the main stream process when a shock nitrogen load or a toxicity incident occurs. Model based analysis reveals the unique feature of the scheme: different particulate components in the sludge have a different retention time, and more specifically, active biomass stays longer in the plant than inert solids. This results in that a plant with a storage tank can have the same amount of active biomass as a traditional plant but less sludge, opening the potential of reducing the volume of the plant. Analysis shows that a plant with a sludge storage tank can be about twenty percent smaller than a traditional plant that has the same treatment capability. Analysis is verified by simulation studies.

Improving the observer-based FDI design for efficient fault isolation

A bank of observers is usually needed to detect and isolate faults (FDI) with robust observer-based approaches. In order to improve the real-time property of the FDI system, the total order of the observers should be designed as small as possible. In this paper, two means that contribute to the reduction of the observer orders are studied. First of all, an approach to the design of minimal order robust FDI observers for given fault sensitivity conditions is developed. The minimal observer order is revealed as a function of a minimal row index of a matrix pencil deduced from known system matrices. Then, the possibility and approach of realizing multiple fault sensitivity conditions with a single robust FDI observer is addressed. A sufficient condition is obtained. The decision logic design is also studied in this paper. A sequential decision logic is proposed, which can be applied to time critical situations where running all the FDI observers in real time is not possible. By revisiting an example taken from literature, the advantages of the presented techniques are illustrated.

Modeling error identification of activated sludge models

Information about the location of modeling errors is crucial for the efficient improvement of an invalid model. This article discusses how to pinpoint modeling errors through comparison of experimental data with data obtained through simulation of the invalid model. An observer-based approach is presented. By designing a dedicated observer for the system using the invalid model, a signal vector is generated, on which each modeling error imposes an easily identifiable feature. An algorithm to analyze the featured signal is then presented. With this algorithm, the features of each of the modeling errors are extracted. The approach is illustrated for a denitrification reactor model in which errors in the dimension of the state vector, in the structure of the biokinetic relationship and in the values of the parameters could be identified.

An Environment for Ill-Defined Systems Research

Ill-defined systems like in environmental, biomedical and biotechnological studies pose several challenging problems when studied through simulation. The impact they have on simulators are touched. Methodology is stretched as well as computer hard-and software needs come into play. A proposed simulator environment currently under investigation at the University of Ghent is described in detail.

Computer-Control of Fermentation Processes

Already since the 18th century one can witness a steady trend towards the use of machines to replace the muscle power of animals and man. The rise of the digital computer in the fifties has speeded up this process. In the beginning, the computer was used for lower level mental processes such as information retrieval and tedious calculations. Nowadays the digital machine can replace higher level mental processes such as the ability to recognize patterns, to identify systems, to make decisions and so on.

Building an On-Line Sensor for Biomass through Modelling

By using partial elemental balances a chemical balance representation of the growth process could be implemented on a process computer for monitoring biomass. Due to the fact that the elementary compositions of biomass, substrate and in some cases of products, are not always known, one has to introduce inductive information in the proposed model. A sensitivity analysis will show the impact of this information on the ultimate “net-effect” sensor. Finally, it is hoped that the paper will bring some insights in the use of model activities in sensor technology for computer- coupled fermentation.

Implementation of the modal technique to diffusion processes

It has always been a tremendous task for numerical analyses to solve non-linear partial differential equations. The modal technique presented in this paper, approaches the problem on his own specific way transforming the partial differential equation into two infinite sets of ordinary differential equations, very well suited to be solved on a hybrid computer. Since this technique gives rise to only differential equations, it can be classified as a continuous time continuous space method. Application of this technique to the moisture-flow equation shows that this method gives extremely good results, which are in close agreement with experimental data.

General Concepts of Bioprocess Modeling.

Every abstract model represents an attempt to logically rationalize the behavior and perhaps even the mechanism of one or other .system under study. The successes in modeling are very much dependent on the nature of the system, probably not because some systems are too complex but rather because our mental constructs are too meager. Just as the equations relatingmass, force and velocity deal with primitives in the primitives in engineering, a mathematics that deals with the fundamental primitives of the system under study is needed. The concept of a model as a database has not yet been exploited its to its full potential.

An approach to validation of stochastic dynamic models with initial state uncertainty

It has been well realized that model validation plays an important role in a modelling and simulation development process. One important validation approach is to directly compare the experimental data with the data produced by the simulation. A problem with this approach is that the residual used in the analysis is usually nonstationary and contaminated by unknown initial conditions. A new approach is proposed in this paper, in which a Luenberger observer or a Kalman filter is used to generate the residual. In this way, the generated residual conveys the information of modelling errors, and meanwhile the effect of unknown initial conditions upon it is minimized. Furthermore, the residual is Gaussian and white when no modelling errors exist, a property which can be easily tested. The approach is illustrated with the validation of a biological model, which is typically hard to validate.

On weak invertibility of linear time-invariant systems

Systems are often uninvertible under the definition of invertibility in the strict sense. The possibility and method to get incomplete information of the input of uninvertible systems are studied. The concepts of Tu-invertibility, invertible space and Tu-inverse system are defined first. The algorithms to find the invertible spaces and to construct the Tu-inverse systems for linear, time-invariant systems are then developed. A numerical example illustrates the algorithms developed.<<ETX>>