A model to describe the performance of the UASB reactor

A dynamic model to describe the performance of the Upflow Anaerobic Sludge Blanket (UASB) reactor was developed. It includes dispersion, advection, and reaction terms, as well as the resistances through which the substrate passes before its biotransformation. The UASB reactor is viewed as several continuous stirred tank reactors connected in series. The good agreement between experimental and simulated results shows that the model is able to predict the performance of the UASB reactor (i.e. substrate concentration, biomass concentration, granule size, and height of the sludge bed).     

Biofilm reactors for industrial bioconversion processes: employing potential of enhanced reaction rates

This article describes the use of biofilm reactors for the production of various chemicals by fermentation and wastewater treatment. Biofilm formation is a natural process where microbial cells attach to the support (adsorbent) or form flocs/aggregates (also called granules) without use of chemicals and form thick layers of cells known as "biofilms." As a result of biofilm formation, cell densities in the reactor increase and cell concentrations as high as 74 gL^-1 can be achieved. The reactor configurations can be as simple as a batch reactor, continuous stirred tank reactor (CSTR), packed bed reactor (PBR), fluidized bed reactor (FBR), airlift reactor (ALR), upflow anaerobic sludge blanket (UASB) reactor, or any other suitable configuration. In UASB granular biofilm particles are used. This article demonstrates that reactor productivities in these reactors have been superior to any other reactor types. This article describes production of ethanol, butanol, lactic acid, acetic acid/vinegar, succinic acid, and fumaric acid in addition to wastewater treatment in the biofilm reactors. As the title suggests, biofilm reactors have high potential to be employed in biotechnology/bioconversion industry for viable economic reasons. In this article, various reactor types have been compared for the above bioconversion processes.     

From open ponds to vertical alveolar panels: the Italian experience in the development of reactors for the mass cultivation of phototrophic microorganisms

The need to develop new concepts in reactor design and the growing interest inSpirulina prompted our group to abandon open ponds in the seventies and to focus interest mainly on closed systems. Two substantially different closed photobioreactors have been developed and are at present under investigation in our Research Centre: the tubular photobioreactor (made of rigid or collapsible tubes) and the recently devised vertical alveolar panel (VAP) made of 1.6-cm-thick Plexiglas alveolar sheets.The technical characteristics of the two systems are described and discussed in relation to the main factors which regulate the growth of oxygenic photosynthetic microorganisms in closed reactors.This paper was presented at the Symposium on Applied Phycology at the Fourth International Phycological Congress, Duke University.     

Studies on the kinetics and mechanism of BOD exertion in dilute systems

The mechanisms and kinetic course of BOD exertion were compared in both open and closed systems. Two open reactors, a simulated stream device, and an open stirred reactor were employed, and the closed systems consisted of standard BOD bottles and 2.4-liter vessels. In the closed systems, both quiescent and stirred conditions of incubation were examined. Biological solids concentration, bacteria and protozoa concentration, substrate analysis, and chemical oxygen demand as well as biochemical oxygen utilization were employed to assess the performance of these systems. Oxygen uptake rate constants were observed to increase with increasing concentration o carbon source, thus militating against irect use of the usual dilution technique for predicting rate of deoxygenation in receiving streams. The relationship between specific O₂ uptake rate and substrate concentration approximated a hyperbolic function similar to the Mono relationship for specific growth rate and substrate concentration. A technique using an open stirred reactor than the standard BOD bottle dilution technique is recommended.     

Hydrodynamic study of an internal airlift reactor for microalgae culture

Internal airlift reactors are closed systems considered today for microalgae cultivation. Several works have studied their hydrodynamics but based on important solid concentrations, not with biomass concentrations usually found in microalgae cultures. In this study, an internal airlift reactor has been built and tested in order to clarify the hydrodynamics of this system, based on microalgae typical concentrations. A model is proposed taking into account the variation of air bubble velocity according to volumetric air flow rate injected into the system. A relationship between riser and downcomer gas holdups is established, which varied slightly with solids concentrations. The repartition of solids along the reactor resulted to be homogenous for the range of concentrations and volumetric air flow rate studied here. Liquid velocities increase with volumetric air flow rate, and they vary slightly when solids are added to the system. Finally, liquid circulation time found in each section of the reactor is in concordance with those employed in microalgae culture.     

The performance of immobilized glucose isomerase supported by shrimp chitin in various types of reactors

Five different types of reactors were employed for glucose isomerization using shrimp shell as the support on which to immobilize the glucose isomerase. The Michaelis-Menten constants and effective diffusivity of glucose in the immobilized enzyme bed were experimentally determined and used in a theoretical analysis of the radial-flow reactor. The fractional conversions of the radial-flow, fluidizedbed, and packed-bed reactors with the same -residence time were found experimentally to be almost the same. This result reveals that the use of radial-flow and fluidized-bed reactors for this immobilized enzyme system is highly feasible.     

Fluid flow pattern in upflow reactors for anaerobic treatment of beet sugar factory wastewater

Residence-time-distribution experiments for the fluid in a 30-m(3) pilot plant and a 200-m(3) prototype upflow reactor were performed by means of continuous injection of an LiCl solution as a tracer in the influent of the reactor and measurement of the response of this stimulus on several location in the reactor and in the effluent. In a similar way as described in an article published earlier, models have been developed by use of the measured data of the fluid flow pattern which consisted of region of ideal mixing, plug flow, dead space, and short circuiting. It appeared that the fluid flow patterns in the two reactors were to a large extent analogous. For the pilot plant, three-mixer models appeared to be appropriate while for the prototype reactor two-mixer models have been found. This differences was a result of the difference in the heights of the sludge beds in the reactors: 2-3 m in the pilot plant and only 0.4 m in the prototype reactor, a result of too small an amount of sludge. Another differences was that, due to large amount of mud in the prototype reactor, a region of dead space occurred in the models for the fluid flow pattern in this reactor. The dimension of the prototype reactor have been chosen according to several recommendations obtained from work with the pilot plant (e.g., scale-up should be done by increasing the cross section of the reactor; one influent point should be applied per 5 m(2) bottom surface). The results presented here clearly show the value of these recommendations.     

Performance of staged and non-staged up-flow anaerobic sludge bed (USSB and UASB) reactors treating low strength complex wastewater

The use of anaerobic processes to treat low-strength wastewater has been increasing in recent years due to their favourable performance-costs balance. For optimal results, it is necessary to identify reactor configurations that are best suited for this kind of application. This paper reports on the comparative study carried out with two high-rate anaerobic reactor systems with the objective of evaluating their performances when used for the treatment of low-strength, complex wastewater. One of the systems is the commonly used up-flow anaerobic sludge blanket (UASB) reactor. The other is the up-flow staged sludge bed (USSB) system in which the reactor was divided longitudinally into 3, 5 and 7 compartments by the use of baffles. The reactors (9 l) were fed with a synthetic, soluble and colloidal waste (chemical oxygen demand (COD) < 1000 mg/l) and operated at 28°C and 24 h hydraulic retention time. Intermediate flow hydraulics, between plug-flow and completely-mixed, in the UASB and 7 stages USSB reactors allowed efficient degradation of substrates with minimum effluent concentrations. Low number of compartments in the USSB reactors increased the levels of short-circuiting thus reducing substrate removal efficiencies. All reactors showed high COD removal efficiencies (93–98%) and thus can be regarded as suitable for the treatment of low strength, complex wastewater. Staged anaerobic reactors can be a good alternative for this kind of application provided they are fitted with a large enough (≥7) number of compartments to fully take advantage of their strengths. Scale factors seem to have influenced importantly on the comparison between one and multi staged sludge-bed reactors and, therefore, observations made here could change at larger reactor volumes.     

Evaluation of a model for the effects of substrate interactions on the kinetics of reductive dehalogenation

The effects of primary electron-donor and electron-acceptor substrates on the kinetics of TCA biodegradation in sulfate-reducing and methanogenic biofilm reactors are presented. Of the common anaerobic electron-donor substrates that were tested, only formate stimulated the TCA biodegradation rate in both reactors. In the sulfate-reducing reactor, glucose also stimulated the reaction rate. The effects of formate and sulfate on TCA biodegradation kinetics were analyzed using a model for primary substrate effects on reductive dehalogenation. Although some differences between the model and the data are evident, the observed responses of the TCA degradation rate to formate and sulfate were consistent with the model. Formate stimulated the TCA degradation rate in both reactors over the entire range of TCA concentrations that were studied (from 50 g TCA/L to 100 mg TCA/L). The largest effects occurred at high TCA concentrations, where the dehalogenation kinetics were zero order. Sulfate inhibited the first-order TCA degradation rate in the sulfate-reducing reactor, but not in the methanogenic reactor. Molybdate, which is a selective inhibitor of sulfate reduction, stimulated the TCA removal rate in the sulfate-reducing reactor, but had no effect in the methanogenic reactor.     

Continuous steroid biotransformations in microchannel reactors

The use of microchannel reactor based technologies within the scope of bioprocesses as process intensification and production platforms is gaining momentum. Such trend can be ascribed a particular set of characteristics of microchannel reactors, namely the enhanced mass and heat transfer, combined with easier handling and smaller volumes required, as compared to traditional reactors. In the present work, a continuous production process of 4-cholesten-3-one by the enzymatic oxidation of cholesterol without the formation of any by-product was assessed. The production was carried out within Y-shaped microchannel reactors in an aqueous-organic two-phase system. Substrate was delivered from the organic phase to aqueous phase containing cholesterol oxidase and the product formed partitions back to the organic phase. The aqueous phase was then forced through a plug-flow reactor, containing immobilized catalase. This step aimed at the reduction of hydrogen peroxide formed as a by-product during cholesterol oxidation, to avoid cholesterol oxidase deactivation due to said by-product. This setup was compared with traditional reactors and modes of operation. The results showed that microchannel reactor geometry outperformed traditional stirred tank and plug-flow reactors reaching similar conversion yields at reduced residence time. Coupling the plug-flow reactor containing catalase enabled aqueous phase reuse with maintenance of 30% catalytic activity of cholesterol oxidase while eliminating hydrogen peroxide. A final production of 36 m of cholestenone was reached after 300 hours of operation.     

A novel split-cylinder airlift reactor for fed-batch cultures

Conventional airlift reactors are not adequate to carry out variable volume processes since it is not possible to achieve a proper liquid circulation in these reactors until the liquid height is higher than that of the downcomer. To carry out processes of variable volume, the use of a split-cylinder airlift reactor is proposed, in the interior of which two multi-perforated vertical baffles are installed in order to provide several points of communication between the reactor riser and downcomer. This improves the liquid circulation and mixing at any liquid volume. In fed-batch cultures, it is important to know how liquid height affects the hydrodynamic characteristics and the volumetric oxygen transfer coefficient since this impacts on the kinetic behavior of any fermentation. Thus, in the present work, the effect of the liquid height on the mixing time, the overall gas hold-up, and the volumetric oxygen transfer coefficient of the proposed airlift reactor were determined. The mixing time was increased and the volumetric oxygen transfer coefficient decreased due to the increase of the liquid height in the reactor in all the superficial gas velocities tested, which corresponded to a pseudohomogeneous flow regime. The experimental values of the mixing time and the mass-transfer coefficient were properly described through correlations in which the U_GR/H_L ratio was used as the independent variable. Thus, this variable might be used to describe the hydrodynamic behavior and the oxygen transfer coefficient of airlift reactors when such reactors are used in processes where the liquid volume changes with time. However, these correlations are useful for the particular device and for the particular operating conditions at which they were obtained. These empirical correlations are useful to understand some factors that influence the mixing time and volumetric oxygen transfer coefficient, but such correlations do not have a sufficient predictive potential for a satisfactory reactor design. The overall gas hold-up values were not significantly affected when the liquid height was lower than the downcomer height. However, the values decreased abruptly when the reactor was operated with liquid heights over the downcomer height, especially at high superficial gas velocities.     

Oxygen transfer in three-phase airlift and stirred tank reactors using silicone oil as transfer vector

The use of organic liquids as vectors to enhance mass transfer has been applied since the 1970s. However, mass transfer in three-phase reactors is only partially understood. This paper aimed to characterize oxygen transfer in three-phase reactors containing air as gas, silicone oil as vector and water as aqueous phase. A mass transfer model that considers separately air/vector, vector/water and air/water oxygen transfers was developed. The model was used to describe oxygen transfer in airlift and stirred tank reactors containing from 0 to 50% of silicone oil. Under the experimental conditions, silicone oil had a positive effect on the overall oxygen transfer. In both reactor designs, the maximum overall oxygen transfer was observed with 10% silicone oil which was increased by 65 and 84% in the airlift and stirred reactor, respectively, compared to reactors operated without silicone oil. The overall transfer increase was mainly due to an enhanced air/water transfer. With 10% silicone oil, the air/water contribution to the overall oxygen transfer was 94.7 and 93.0% for the airlift and stirred reactor, respectively.     

Anaerobic fluidised bed for the purification of effluents from chemical and mechanical pulping

Anaerobic treatment has seldom been used for wastewaters from the pulp and paper industry and other branches of the chemical industry. Escape of volatile pollutants to the atmosphere, which always occurs during aerobic treatment, is avoided, and much less sludge is being produced than in an aerobic process. The greatest obstacle for using anaerobic treatment in the pulp and paper industry is the large wastewater volume, which necessitates short hydraulic detention times, because the treatment is to occur in an enclosed space. We used solid carrier particles to prevent wash-out of biomass from the reactor at high hydraulic loading, and an up-flow system in order to be able to use very small carrier particles, maximizing the surface area for biomass attachment. In this paper we describe and discuss the results obtained with this type of anaerobic reactor (fluidised bed) at bench and semitechnical scale for wastewaters from pressurized ground wood pulping and paper manufacture, sulphite pulp evaporator condensate and bleach waste. Earlier work with Kraft pulp bleaching effluent and thermomechanical pulping wastewater and evaporator condensates using anaerobic reactors is also discussed. The results obtained thus far show that there are several wastewater streams from the pulping industry, where 60 to 90% of the dissolved organic pollutants (measured as COD_Cr or TOC) was biodegraded within 4 to 24 h. The high strength waste streams (COD_Cr 2000 mg O₂ 1⁻¹) allowed organic space load of 4 to 10 kg COD_Cr m⁻³ reactor volume d⁻¹. With low strength wastes the hydraulic loading was the limiting factor.     

Role of the reactor configuration in the biological detoxification of a dump site-polychlorobiphenyl-contaminated soil in lab-scale slurry phase conditions

 The biotreatability of a xenobiotic contaminated soil is frequently determined through a bioslurry treatment usually performed in lab-scale shaken baffled flasks. In this study, a 3-l unconventional stirred tank reactor was developed and tested in the slurry-phase treatment of a soil heavily contaminated by polychlorobiphenyls (PCBs) derived from an Italian dump site, in the absence and in the presence of biphenyl and of the exogenous PCB aerobically dechlorinating co-culture ECO3. The data obtained were compared with those obtained on the same soil in experiments performed in parallel in 3-l baffled shaken flask reactors. Considerably higher PCB removal and soil detoxification yields (determined through the Lepidium sativum germination test and the Collembola mortality test) were attained in the stirred tank reactors, which generally displayed a higher slurry-phase homogeneity and a higher availability of biphenyl- and chlorobenzoic acid-degrading bacteria compared to the corresponding shaken flask reactors. Moreover, enhanced soil PCB biodegradation and detoxification yields were observed when the developed reactor was supplemented with biphenyl and the exogenous ECO3 bacteria. In conclusion, the results of the soil biotreatability experiments commonly performed in bioslurry lab-scale reactors are significantly infuenced by the reactor configuration; the use of the unconventional stirred tank reactor system developed in this work is recommended. Received: 21 June 1999 / Received revision: 9 September 1999 / Accepted: 10 September 1999     

Attainable region analysis for continuous production of second generation bioethanol

Background

Despite its semi-commercial status, ethanol production from lignocellulosics presents many complexities not yet fully solved. Since the pretreatment stage has been recognized as a complex and yield-determining step, it has been extensively studied. However, economic success of the production process also requires optimization of the biochemical conversion stage. This work addresses the search of bioreactor configurations with improved residence times for continuous enzymatic saccharification and fermentation operations. Instead of analyzing each possible configuration through simulation, we apply graphical methods to optimize the residence time of reactor networks composed of steady-state reactors. Although this can be easily made for processes described by a single kinetic expression, reactions under analysis do not exhibit this feature. Hence, the attainable region method, able to handle multiple species and its reactions, was applied for continuous reactors. Additionally, the effects of the sugars contained in the pretreatment liquor over the enzymatic hydrolysis and simultaneous saccharification and fermentation (SSF) were assessed.

Results

We obtained candidate attainable regions for separate enzymatic hydrolysis and fermentation (SHF) and SSF operations, both fed with pretreated corn stover. Results show that, despite the complexity of the reaction networks and underlying kinetics, the reactor networks that minimize the residence time can be constructed by using plug flow reactors and continuous stirred tank reactors. Regarding the effect of soluble solids in the feed stream to the reactor network, for SHF higher glucose concentration and yield are achieved for enzymatic hydrolysis with washed solids. Similarly, for SSF, higher yields and bioethanol titers are obtained using this substrate.

Conclusions

In this work, we demonstrated the capabilities of the attainable region analysis as a tool to assess the optimal reactor network with minimum residence time applied to the SHF and SSF operations for lignocellulosic ethanol production. The methodology can be readily modified to evaluate other kinetic models of different substrates, enzymes and microorganisms when available. From the obtained results, the most suitable reactor configuration considering residence time and rheological aspects is a continuous stirred tank reactor followed by a plug flow reactor (both in SSF mode) using washed solids as substrate.

     

An innovative online VFA monitoring system for the anerobic process, based on headspace gas chromatography

A new method for online measurement of volatile fatty acids (VFA) in anerobic digesters has been developed based on headspace gas chromatography (HSGC). The method applies ex situ VFA stripping with variable headspace volume and gas analysis by gas chromatography-flame ionization detection (GC-FID). In each extraction, digester sample was acidified with H(3)PO(4) and NaHSO(4), then heated to strip the VFA into the gas phase. The gas was sampled in a low friction glass syringe before injected into the GC for measurement. The system has been tested for online monitoring of a lab-scale CSTR reactor treating manure for more than 6 months and has shown good agreement with off-line analysis. The system is capable of measuring individual VFA components. This is of advantage since specific VFA components such as propionic and butyric acid can give extra information about the process status. Another important advantage of this sensor is that there is no filtration, which makes possible application in high solids environments. The system can thus be easily applied in a full-scale biogas reactor by connecting the system to the liquid circulation loop to obtain fresh sample from the reactor. Local calibration is needed but automatic calibration is also possible using standard addition method. Sampling duration is 25-40 min, depending on the washing duration, and sensor response is 10 min. This is appropriate for full-scale reactors, since dynamics within most biogas reactors are of the order of several hours.     

Optimization of Operating Temperature for Continuous Immobilized Glucose Isomerase Reactor with Pseudo Linear Kinetics

In this work, the optimal operating temperature for the enzymatic isomerization of glucose to fructose using a continuous immobilized glucose isomerase packed bed reactor is studied. This optimization problem describing the performance of such reactor is based on reversible pseudo linear kinetics and is expressed in terms of a recycle ratio. The thermal deactivation of the enzyme as well as the substrate protection during the reactor operation is considered. The formulation of the problem is expressed in terms of maximization of the productivity of fructose. This constrained nonlinear optimization problem is solved using the disjoint policy of the calculus of variations. Accordingly, this method of solution transforms the nonlinear optimization problem into a system of two coupled nonlinear ordinary differential equations (ODEs) of the initial value type, one equation for the operating temperature profile and the other one for the enzyme activity. The ODE for the operating temperature profile is dependent on the recycle ratio, operating time period, and the reactor residence time as well as the kinetics of the reaction and enzyme deactivation. The optimal initial operating temperature is selected by solving the ODEs system by maximizing the fructose productivity. This results into an unconstrained one‐dimensional optimization problem with simple bounds on the operating temperature. Depending on the limits of the recycle ratio, which represents either a plug flow or a mixed flow reactor, it is found that the optimal temperature of operation is characterized by an increasing temperature profile. For higher residence time and low operating periods the residual enzyme activity in the mixed flow reactor is higher than that for the plug flow reactor, which in turn allows the mixed flow reactor to operate at lower temperature than that of the plug flow reactor. At long operating times and short residence time, the operating temperature profiles are almost the same for both reactors. This could be attributed to the effect of substrate protection on the enzyme stability, which is almost the same for both reactors. Improvement in the fructose productivity for both types of reactors is achieved when compared to the constant optimum temperature of operation. The improvement in the fructose productivity for the plug flow reactor is significant in comparison with the mixed flow reactor.     

The phases of small networks of chemical reactors and neurons

We present an experimental study of the phase relationships observed in small reactor networks consisting of two and three continuous flow stirred tank reactors. In the three-reactor network one chemical oscillator is coupled to two other reactors in parallel in analogy to a small neural net. Each reactor contains an identical reaction mixture of the excitable Belousov-Zhabotinsky reaction which is characterized by its bifurcation diagram, where the electrical current is the bifurcation parameter. Coupling between the reactors is electrical via Pt-working electrodes and it can be either repulsive (inhibitory) or attractive (excitatory). An external electrical stimulus is applied to all three reactors in the form of an asymmetric electrical current pulse which sweeps across the bifurcation diagram. As a consequence, all three reactors oscillate with characteristic oscillation patterns or remain silent in analogy to the firing of neurons. The observed phase behavior depends on the type of coupling in a complex way. This situation is analogous to the in vivo measurements on single neurons (local neurons and projection neurons) performed by G. Laurent and co-workers on the olfactory system of the locust. We propose a simple neural network similar to the reactor network using the Hodgkin-Huxley model to simulate the action potentials of the coupled single neurons. Analogies between the reactor network and the neural network are discussed.     

生物膜法和SBR法相结合处理难降解制药废水的研究

采用生物膜法和SBR法相结合的废水处理工艺处理含抗生素类等难降解的制药废水 ,对生物膜的耐冲击负荷能力、生物膜对进水可生化性的影响、生物膜对好氧SBR活性污泥性能的影响、pH对系统去除效果的影响等工艺条件进行研究 ,并通过与传统SBR处理工艺的对比试验 ,进一步揭示了生物膜法和SBR法相结合的处理工艺强的耐冲击负荷能力。     

Monitoring of alpha-ketoglutarate in a fermentation process using expanded bed enzyme reactors.

A bienzyme flow injection system is presented for the monitoring of alpha-ketoglutarate produced in a fermentation process, using glutamate dehydrogenase (GDH) and glutamate oxidase (GlOx) immobilised in two serially connected expanded bed reactors. The use of expanded bed resulted in unhindered passage of the bacterial cells through the columns, and thereby the need of a separate filtering step (e.g. microdialysis) was avoided. In the first reactor, alpha-ketoglutarate was converted to L-glutamate by GDH in the presence of ammonia and NADH. In the following reactor, L-glutamate was converted by GlOx to alpha-ketoglutarate, ammonia and hydrogen peroxide, which was detected in an electrochemical flow-through cell at +650 mV vs. Pt/(0.1 M KCl). The detection limit of alpha-ketoglutarate in the coupled packed bed reactors was 1 microM (defined as 3 S/N), the linear range 0-100 microM, and the sensitivity 0.80 nA/microM (R(2) 0.99). In the coupled expanded bed reactors, the detection limit of alpha-ketoglutarate was 7 microM (defined as 3 S/N), the linear range and the sensitivity being 0-500 microM and 0.11 nA/microM (R(2) 1.00), respectively. The response time (defined as the time between peak rise and return to baseline) was 5 min for coupled packed beds (injection of supernatant), and 12 min for coupled expanded beds (injection of sample containing cellular and particulate matter). Several other parameters, such as reactor stability, flow rate dependency, bed expansion, glutamate interference, etc. were investigated and characterised. When analysing real samples from a fermentation broth, the same results were obtained independent of the nature of the reactor system (packed or expanded bed). The hereby described system can easily be automatised and controlled from a personal computer.