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| 2. |
- van der Wijngaart, Wouter, et al.
(författare)
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Valve-less diffuser fluid micropump
- 1999
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Ingår i: Gordon Research Conference 1999, Analytical Chemistry, New Hampshire, USA.
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Konferensbidrag (refereegranskat)
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| 3. |
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| 4. |
- Wik, Torsten, 1968, et al.
(författare)
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Transfer functions for series of continuously stirred biofilm reactors
- 1997
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- There is an increasing interest in modeling and applications of biofilm reactors. Commonly, biofilm reactors are modeled as a single continuously stirred biofilm reactor (CSBR), or as a series of such. The models can be used to extract information about the reactor, for design and to predict reactor effluent characteristics as a function of influent characteristics. A CSBR consists of a stirred tank, which the bulk water flows through, and from which substrates diffuse into a biofilm where they may be transformed into new substances by bacteria living in the biofilm. Here, standard assumptions are used to derive a general and flexible dynamic model of CSBR-systems, where the reaction kinetics are of zero or first order. An exact, and an approximate transfer function, which enables easy simulations, analysis, and implementation in real-time softwares, is derived. Particular focus is on pulse responses, which is an important experimental procedure in control and reactor design. Explicit equations for the pulse responses are presented, and parameter dependancy is discussed. Experimental data from a pilot plant nitrifying trickling filter are used to illustrate the use of transfer functions for identification of reactor and biofilm parameters.
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| 5. |
- Wik, Torsten, 1968
(författare)
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An investigation of the fast dynamics in a nitrifying trickling filter
- 1996
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The fast dynamics of a large pilot scale nitrifying trickling filter (NTF) using effluent waste water from Rya Waste Water Treatment Plant in Göteborg, Sweden, has been investigated experimentally and by simulations. The plant is 7.2 m high, have a diameter of 2.7 m, and was filled with a cross-flow media with a specific surface area of 226 m^2/m^3. Continuous ammonium meters connected to the NTF measured influent and effluent ammonium concentrations.Two different phenomena may affect the fast dynamics in this type of biofilm reactors: The mixing in the bulk flow and the dynamics within the biofilm, i.e. the mixing in the biofilm and the response time for the bacteria to changes in substrate concentrations.Pulse response experiments conducted at two different flows (7.3 l/s and 14.5 l/s), where dissolved LiCl was used as trace substance, showed that the flow through the NTF could not be characterized as laminar, but significant mixing occurs inside the plant. The residence time distributions are well approximated by four identical continuously stirred tanks in series, and from the mean residence time the liquid film thickness was estimated to be 0.5 mm.Two experiments, where the influent ammonium concentration was rapidly increased while the flow was held constant at 14 l/s, were also conducted as well as an experiment where the flow was stochastically varied around 12 l/s. Comparisons are made between the measured effluent concentrations in these experiments and simulations of a model of the plant, where the NTF is divided into a series of continuously stirred tank reactors (CSTRs) having the total volume estimated from the pulse response experiments. The nitrification rate in each CSTR is modelled as a first order dynamic system driven by a physically derived expression of the stationary nitrification rate. Simulations for different values of the time constant of this dynamic system, and comparisons with the experimental data, shows that the dynamics within the biofilm are much faster than the dynamics of the mixing in the bulk. Implicitly, this means that the response times for the nitrifying bacteria to changes in ammonium bulk concentration were less than a few minutes. Hence, the fast dynamics in the biofilm can in many cases be neglected, which greatly facilitates simulations of many nitrifying biofilm reactors when more complex biofilm models are used.
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| 6. |
- Wik, Torsten, 1968
(författare)
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Dynamic Modeling of Nitrifying Trickling Filters
- 1996
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Trickling filters, one kind of biofilm reactors, for removal of nitrogen in municipal wastewater are presently increasing in number in Sweden. For improved control and operation of wastewater treatment plants, where such reactors are used, dynamic models describing their behavior are necessary. In this thesis a physical dynamic model of cross-flow nitrifying trickling filters (NTFs), based on a general multi-species biofilm model, is presented. The model predicts effluent concentrations of ammonium, nitrite, nitrate, and alkalinity as functions of the corresponding influent concentrations, the water temperature, the flow, and the present state of the distribution of the nitrifying bacteria {\it Nitrosomonas} and {\it Nitrobacter} in the biofilm of the NTF. Efficient methods to solve the model equations, also in steady state, are presented. Experimental data achieved on a large pilot scale trickling filter are compared with model simulations, and model assumptions are experimentally validated. From the modeling and experiments it is concluded that the dynamics of the NTF can be divided into two modes: One fast mode that can be assumed to depend only on the mixing in the bulk, and one slow mode that depends on the bacterial growth and decay in the biofilm. The settling times of the two modes are separated by a factor of order 1000, which considerably simplifies model simulations. Comparisons between experimental data and a simplified version of the model show that it takes less than a few minutes for the nitrifying bacteria in the biofilm to change their substrate uptake rate after changes in substrate bulk concentrations.From pulse experiments and laminar flow theory analysis it is shown that the flow through the NTF is turbulent and, hence, significant mixing occurs inside the NTF. The residence time distribution can be approximated by a model of continuously stirred tanks in series.Comparisons between simulations and semi-stationary data show that the substrate flux into the biofilm is enhanced by an increase in flow, probably due to increased turbulence in the bulk. Analysis and simulation of steady-state multi-species biofilms in general indicate that bacterial coexistence is not only dependent on the bulk water substrate concentrations, but also on the biofilm thickness, which means that control of the biofilm thickness may be a way of controlling the bacterial composition in biofilms.
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| 7. |
- Wik, Torsten, 1968
(författare)
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Experiment utförda på en nitrifierande biobädd 1995
- 1996
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A number of experiments was conducted on a pilot scale nitrifying trickling filter during 1995. The experiments aimed for better knowledge of the flow-dependent fast dynamics, and the slow bacterial dynamics.The residence time distribution was investigated by a number of impulse response experiments where dissolved LiCl was added to the influent. The experiments showed that the amount of water in the trickling filter is almost independent of the flow through the plant, and corresponds to a liquid film thickness of approximately 0.5 mm. The residence time distribution can be approximated by a model with four or five identical and ideally continuously stirred tanks.Based on the results of the impulse response experiments the plant is modelled by four continuously stirred tank reactors in series, where the nitrification in each tank is described by a physically derived nonlinear expression.Data from a few step response experiments, where the ammonium concentration in the influent was raised from a low constant level to a high constant level at a constant flow through the plant, was compared to model simulations. The comparisons showed that the fast dynamics in the biofilm can be neglected in comparison to the dynamics caused by the mixing in the bulk and the residence time distribution. Implicitly, this means that the response time for the active nitrifying bacteria to changes in ammonium concentration is less than a few minutes, also when the ammonium load has been very low for a long time.An experiment, where the flow was stochastically varied around an operating point during one day, showed that the simple model derived sufficiently well describe the fast dynamics of nitrifying trickling filter also when the flow changes. When the ammonium concentration in the effluent is low, a model where the nitrification rate is assumed constant is not sufficient.The slow dynamics that depend on the growth and decay of the active nitrifying bacteria was investigated by a three months long step response experiment, where the ammonium concentration in the influent first was held at a high level (not full nitrification) for approximately one month and then at a low level (approximately 50% of the nitrifying capacity) for one month, and finally at the same high level as before for one more month. In spite of several practical problems, the experiment indicated that it takes one to two weeks for the concentration of active bacteria in the biofilm to increase to a new higher concentration after the raise in influent ammonium concentration. The corresponding increase in nitrification rate is approximately 20%.The two periods of the same high influent ammonium concentration was during periods with different water temperature. Comparisons of the nitrification rate between the two periods indicated a stronger dependency on the temperature than has earlier been observed. The standard temperature dependency of the maximum growth rate for nitrifying bacteria that are used for laboratory scale experiments may well apply also for this large scale process.During periods of the experiment the ammonium sensors were not working. Therefore the possibility to determine the influent ammonium concentration based on the flow into the plant was investigated. Both black box models and a physically based model was fitted to data. The investigation showed that with a good model of the influent flow to the plant it may be possible to predict the ammonium concentration with quite good accuracy.The trickling filter was flooded weekly for a couple of hours for predator control. An investigation of the nitrification rate before and after the floodings showed no short term effects of the flooding.When the pilot plant was taken out of operation at the end of the year the uppermost meter of the plant was investigated. It was observed that the biofilm thickness was approximately 0.5mm and no bare surfaces without biofilm could be observed.
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