Norbert Räbiger

Minimization of excess sludge production by increase of oxygen concentration in activated sludge flocs; experimental and theoretical approach

The disposal of excess sludge from waste water treatment plants represents a rising challenge in activated sludge processes. Hence, the minimization of excess sludge production was investigated by optimizing the process parameters. Laboratory scale experiments showed, that the excess sludge production was effected by the sludge loading and oxygen concentration. Therefore, a mathematical model that describes substrate removal, oxygen utilization, and excess sludge production within a microbial floc particle, surrounded by biodegradable substrate, will be presented. After calibration of the model parameters, the model validation has been carried out with experimental results. The model takes into account the mass transfer of substrate oxygen, the biological reaction within the floc particle, and the endogenous respiration process. So it allows a detailed calculation of the excess sludge production. The results indicate that the oxygen concentration in the bulk liquid has a significant effect on the amount of excess sludge produced.

Methane production from glycolate excreting algae as a new concept in the production of biofuels.

It is the aim of the present work to introduce a new concept for methane production by the interaction of a glycolate-excreting alga (Chlamydomonas reinhardtii) and methanogenic microbes operating in separate compartments within one photobioreactor. This approach requires a minimum number of metabolic steps to convert light energy to methane thereby reducing the energetic and financial costs of biomass formation, harvest and refinement. In this feasibility study it is shown that the physiological limitations for sustained glycolate production can be circumvented by the use of C. reinhardtii mutants whose carbon concentrating mechanisms or glycolate dehydrogenase are suppressed. The results also demonstrate that methanogenic microbes are able to thrive on glycolate as single carbon source for a long time period, delivering biogas composed of CO(2)/methane with only very minor contamination.

Experimental Analysis and Modeling of Micromixing in Microreactors

For the investigation of chemical reactions in micro- and minichannels the threedimensional velocity field and three dimensional concentration field of an inert and reactive tracer was measured in a T-shaped micromixer. For this purpose a measurement method for micro particle image velocimetry and confocal laserscanning microscopy has been developed. By solving the continuity equation the calculation of three dimensional streamlines, residence time distribution and local energy dissipation is possible. It becomes out that a regime transition to engulfment flow occurs at a certain Reynolds number dependent on reactor dimensions that can be predicted by the modified model of Soleymani. This transition leads to a different efficiency of micromixing measurable by the reactionproduct of a fast chemical reaction by confocal laserscanning microscopy. The influence of the flow regime on the yield and selectivity has been investigated by means of a parallel consecutive reaction system.

Simulation modeling of bottling line water demand levels using reference nets and stochastic models

In this paper simulation modeling of a brewery bottling line is described. Reference nets as an extended version of high level Petri nets are being used for the modeling environment and make use of external Java programming language based models. The study focuses on a bottling line used within a small-to-medium sized brewery. Machine data, flow measurements and the determination of the chemical oxygen demand from various effluent locations within the bottling line are used to build stochastic models, which are implemented into the reference net models. The resulting models are shown and a simulation experiment is compared to a real bottling process within the mentioned brewery.

Model fundamentals for the design of three-phase loop reactors

Loop reactors under multi phase operation are distinguished by low-maintenance and usually economically and ecologically advantageous operating conditions and thus have abroad spectrum of applications in chemical as well as biotechnological industries. Nevertheless, the design of these reactors is currently based on in accurate physical and mathematical models. The interactions found in three-phase flows, which can result, e.g., in both an increase and a decrease in gas-void fraction due to an increase in solids-void fraction, have been explained phenomeno logically in recent experiments and a reschematically shown in Fig. 1.

Improved model for the calculation of homogeneous gas-liquid flows

The research project presented below gives a detailed explanation of the phenomenon of an increased relative velocity between gas and liquid phase in bubbly flows. It points out the conditions under which classical models for the description of two-phase flows tend to predict higher gas hold-ups than those determined experimentally, and explains the underlying physical relations. The cause for the increased relative velocity is an interactive effect of the bubble wakes on the shapes and trajectories of other bubbles. The experimental facilities of this research project which allowed the simultaneous observation of integral and local effects under a broad range of operating conditions are described in detail. p]A model for the calculation of the relative velocity in homogeneous bubbly two-phase flows is presented. It overcomes deficiencies of classical models which do not consider the mutual effects of swarm turbulence, gashold-up,bubble shape and size on the relative velocity. Based on observations of the shape and trajectories of single bubbles under swarm conditions ane xtension to the model, which includes further parameters of the liquid phase, is proposed.