Ilse Smets

Detection of Filamentous Bulking Problems: Developing an Image Analysis System for Sludge Composition Monitoring

This article describes a fully automatic image analysis procedure for fast and reliable characterization of the activated sludge composition, that is, the floc and filament features. The algorithms developed for each of the analysis steps, that is, segmentation, object recognition, and characterization, are described in detail. Although the application range of the recognition method is a priori expanded by introducing a number of control parameters, the procedure proves to be intrinsically robust as it produces satisfactory results for a fixed set of parameter values for a wide variety of image types.

Feedback Stabilization of Fed-Batch Bioreactors: Non-Monotonic Growth Kinetics

This paper deals with the design of a feedback controller for fed‐batch microbial conversion processes that forces the substrate concentration CS to a desired setpoint CS*, starting from an arbitrary (initial) substrate concentration when non‐monotonic growth kinetics apply. This problem is representative for a lot of industrial fermentation processes, with the bakers yeast fermentation as a well‐known example. It is assumed that the specific growth rate μ is function of the substrate concentration only. A first approach exploits the availability of on‐line measurements of both the substrate and biomass concentration. A second approach is merely based on on‐line measurements of the biomass concentration, which provide an estimate for the specific growth rate. After a reformulation of the substrate concentration setpoint into a specific growth rate setpoint, it is demonstrated that the fed‐batch process can still be stabilized around any desired operating point along the non‐monotonic kinetics.

Modeling the competition between floc-forming and filamentous bacteria in activated sludge waste water treatment systems-II. A prototype mathematical model based on kinetic selection and filamentous backbone theory

In this paper a prototype mathematical model is introduced which describes the growth of flocs and filaments in activated sludge waste water treatment systems. The model is based on both kinetic selection and filamentous backbone theory. A stability analysis of the model is performed and it is shown that this model, in contrast with mathematical models only based on kinetic selection theory, is able to describe coexistence of both flocs and filaments for a wide range of dilution rates. Afterwards the model is extended in order to describe a standard activated sludge waste water treatment system.

Optimal control theory: A generic tool for identification and control of (bio-)chemical reactors

Abstract In this review paper the potential of optimal control theory for optimization in the time as well as in the space domain is highlighted. Various case studies in the area of (bio-)chemical reactors are discussed ranging from the dual problem of performance optimization and accurate parameter identification (time domain) to plug flow reactor optimization (space domain). Furthermore, it is illustrated that application of the Minimum Principle of Pontryagin to distributed parameter systems leads to extremal control profile structures (in the space domain) which are very similar to those obtained during optimization (in the time domain) of well mixed bioreactors. The analogy is reflected at various levels during analytical optimal control computations.

Addition of polyaluminiumchloride (PACl) to waste activated sludge to mitigate the negative effects of its sticky phase in dewatering-drying operations

This paper presents a new application of polyaluminiumchloride (PACl) as a conditioner for waste activated sludge prior its dewatering and drying. It is demonstrated at lab scale with a shear test-based protocol that a dose ranging from 50 to 150 g PACl/kg MLSS (mixed liquor suspended solids) mitigates the stickiness of partially dried sludge with a dry solids content between 25 and 60 %DS (dry solids). E.g., at a solids dryness of 46% DS the shear stress required to have the pre-consolidated sludge slip over a steel surface is reduced with 35%. The salient feature of PACl is further supported by torque data from a full scale decanter centrifuge used to dewater waste sludge. The maximal torque developed by the screw conveyor inside the decanter centrifuge is substantially reduced with 20% in the case the sludge feed is conditioned with PACl. The beneficial effect of waste sludge conditioning with PACl is proposed to be the result of the bound water associated with the aluminium polymers in PACl solutions which act as a type of lubrication for the intrinsically sticky sludge solids during the course of drying. It can be anticipated that PACl addition to waste sludge will become a technically feasible and very effective method to avoid worldwide fouling problems in direct sludge dryers, and to reduce torque issues in indirect sludge dryers as well as in sludge decanter centrifuges.

Field performance assessment of onsite individual wastewater treatment systems

On-site individual wastewater treatment systems can provide a financially attractive alternative to a sewer connection in locations far from the existing sewer network. These systems are, however, relatively new, and therefore, shortcomings in the design or operation problems still occur frequently. A previously performed survey revealed that most system owners neither carry out routine operation and maintenance tasks nor have a maintenance agreement with the manufacturer. This suggests that in reality, systems are often improperly managed and do not provide the level of treatment necessary to adequately protect surface and ground waters. To substantiate this statement, the field performance of 23 currently installed individual wastewater treatment systems in Belgium has been assessed.The results of this study confirm that many installed individual systems do not perform well: 52% of them do not meet all the legal effluent standards (BOD, COD, SS). Activated sludge systems prove to be less efficient than biofilm-based systems and extensive systems perform better than compact systems. Maintenance problems and to a lesser extent improperly designed systems are indeed the main cause of unsatisfactory results through, e.g., wash out.

Optimal fed batch experiment design for estimation of monod kinetics of Azospirillum brasilense: from theory to practice.

In this paper the problem of reliable and accurate parameter estimation for unstructured models is considered. It is illustrated how a theoretically optimal design can be successfully translated into a practically feasible, robust, and informative experiment. The well‐known parameter estimation problem of Monod kinetic parameters is used as a vehicle to illustrate our approach. As known for a long time, noisy batch measurements do not allow for unique and accurate estimation of the kinetic parameters of the Monod model. Techniques of optimal experiment design are, therefore, exploited to design informative experiments and to improve the parameter estimation accuracy. During the design process, practical feasibility has to be kept in mind. The designed experiments are easy to implement in practice and do not require additional monitoring equipment. Both design and experimental validation of informative fed batch experiments are illustrated with a case study, namely, the growth of the nitrogen‐fixing bacteria Azospirillum brasilense.

Optimal control of (bio-)chemical reactors: generic properties of time and space dependent optimization

This paper deals with the optimal control (OC) in the space domain of (bio-)chemical reactors. As an example, the determination of optimal heat exchanger temperature profiles of exothermic tubular reactors under the assumption of steady-state and plug flow characteristics is investigated. To enable a trade-off between process performance and energy loss a weighted cost criterion is defined. Application of the Minimum Principle of Pontryagin leads to extremal control profile structures which are very similar to those obtained during optimization (in the time domain) of well mixed bioreactors. The latter problem has been analyzed in detail over the last two decades. The analogy is reflected at various levels during analytical optimal control computations.

Modeling the competition between floc-forming and filamentous bacteria in activated sludge waste water treatment systems—I. Evaluation of mathematical models based on kinetic selection theory

One of the main reasons for failing of the sedimentation process in activated sludge waste water treatment systems is the phenomenon of filamentous bulking. This occurs when filamentous bacteria overgrow floc-forming bacteria. In this paper mathematical models are discussed which describe the competition of flocs and filaments based on the kinetic selection theory. It is proven with a stability analysis for a continuous reactor, that, in order to control filamentous bulking, the dilution rate plays a crucial role. Moreover, it is shown that coexistence of both organisms is generically not possible. Afterwards the continuous reactor model is extended in order to describe a standard waste water treatment system. Due to the complexity of the extended model it is difficult to perform the stability analysis analytically. It is proven that the model can be reduced without loss of stability properties. For the reduced model it is proven with a stability analysis that the transport terms play a crucial role in the survival of one or the other organism. As for the continuous reactor model it is shown that coexistence of both organisms is generically not possible.

Assessment of activated sludge stability in lab-scale experiments.

The majority of activated sludge research is conducted in a laboratory environment with, most often, the start-up sludge being taken from a large-scale wastewater treatment plant. Inoculating this sludge in a lab-scale set-up induces a transient period, which, evidently, has a direct impact on the experimental results during this period of acclimatization. In the currently published literature, the acclimatization period is either neglected or fixed to two or three times the sludge age, without any guarantee that stable conditions are indeed reached. To develop a strategy that assesses the stability of activated sludge, three experiments were performed during which the activated sludge was extensively monitored through a series of physical, microscopic and biochemical analyses. It is demonstrated that it is possible to objectively quantify activated sludge stability through the monitoring of the total averaged filament length per image, the sludge volume index and the maximum specific oxygen uptake rate. Hereto, a moving window approach is adopted: within a 7 days interval the mean slope and the gap between the maximum and minimum value has to be smaller than a pre-specified threshold value. Once stability is reached, the true impact of test conditions can be studied without interference of adaptation phenomena.