Venko Beschkov

Biotransformation of glucose to free gluconic acid by Gluconobacter oxydans: substrate and product inhibition situations

Abstract Microbial oxidation of glucose to free gluconic acid by Gluconobacter oxydans batch cultures was studied with a special reference to substrate and product inhibition. Process kinetics were evaluated by comparing different inhibition models. Substrate inhibition can be best described by the equation of Tseng and Wayman predicting linear inhibition of growth above a characteristic threshold glucose concentration. Non-linear inhibition kinetics was observed, however, for product inhibition and the model of Levenspiel was found to fit the experimental data. The biokinetic model parameters, productivities and conversion degrees on media with different glucose and gluconic acid concentrations were calculated and compared regarding the principal limits of process efficiency. Probable mechanisms of inhibition and possible techniques for their minimization are also discussed.

Bacterial denitrification of waste water stimulated by constant electric field

The present paper considers the effect of constant electric field on the denitrification performance of the bacterium Pseudomonas denitrificans. It is shown that when constant electric field is applied on a denitrifying culture, the reduction of nitrate to elemental nitrogen runs faster than at a traditional microbial process due to the lack of a long lag-phase as it is in the case of traditional microbial dentrification. The process rate and the microbial specific activity are sensitive to the constant cathode potential if the latter is close to the one of an electro-chemical reduction process. However, evidence from the current yield in the experiments described here and from a comparison with data for electrochemical reduction without bacteria shows that a simple electrochemical process cannot explain the observed effect

Biological treatment of mining wastewaters by fixed-bed bioreactors at high organic loading

Acid wastewaters contaminated with Fe - 1000 mg L(-1) and Cu - 100 mg L(-1) were remediated by microbial sulfate-reduction at high organic loading (theoretical TOC/SO4(2-) ratio 1.1) in a laboratory installation. The installation design includes a fixed-bed anaerobic bioreactor for sulfate-reduction, a chemical reactor, a settler and a three-sectional bioreactor for residual organic compounds and hydrogen sulfide removal. Sulfate-reducing bacteria are immobilized on saturated zeolite in the fixed-bed bioreactor. The source of carbon and energy for bacteria was concentrated solution, containing ethanol, glycerol, lactate and citrate. Heavy metals removal was achieved by produced H2S at sulfate loading rate 88 mg L(-1)h(-1). The effluent of the anaerobic bioreactor was characterized with high concentrations of acetate and ethanol. The design of the second bioreactor (presence of two aerobic and an anoxic zones) makes possible the occurrence of nitrification and denitrification as well as the efficiently removal of residual organic compounds and H2S.

Biodegradation of monochloroacetic acid used as a sole carbon and energy source by Xanthobacter autotrophicus GJ10 strain in batch and continuous culture.

AbstractBatch and continuous mode degradation of monochloroacetic acid used as a sole carbon and energy source in the concentration range of 0.9–48.4 mM by pure culture of Xanthobacter autotrophicusGJ10 was investigated. The substrate was completely degraded in each flask in batch system. Partial substrate inhibition occurred at the concentrations exceeding 25.4 mM. Temporary accumulation of glycolic acid in the medium indicated that dehalogenation was undergoing faster than further utilization of glycolate. Three different carbon substrates were used for inoculum preparation – 1,2-dichloroethane, tri-sodium citrate and a nutrient broth. The fastest growth on monochloroacetate occurred for 1,2- dichloroethane-grown inoculum. The assays of haloacid dehalogenase in crude extract indicated that the bacteria grown on 1,2-dichloroethane possessed higher level of the enzyme. The response of the GJ10 culture towards spikes of 20 mM monochloroacetate was tested in 2.5-l continuously stirred tank fermentor. The substrate was readily utilized within 7–8 h. Continuous degradation of monochloroacetate in the fermentor was demonstrated for monochloroacetate concentration of 20 mM and dilution rate 0.016 h−1. Quantitative agreement between the amount of monochloroacetate introduced and chloride released was found. The results demonstrated that the strain X. autotrophicus GJ10 might be suitable for biodegradation of monochloroacetate contaminated media.

A modified method for determination of diffusivities of low molecular substances in non-Newtonian liquids

A modified method for determination of diffusivities of low molecular substances in non-Newtonian liquids described by the power-law model has been proposed. It is based on the dissolution of Geiss body, with a parameter m=1/3 rotating in an infinite fluid. In this case, the solution of the differential equations of motion and mass transfer is available as an analytical formula for calculating the diffusivity coefficient.The method allows the extension of the variety of media and diffusing species. It has been illustrated with dissolving of gypsum in water and five non-Newtonian liquids. The results obtained have been interpreted taking into account the interaction between calcium ions and polymer molecules of the non-Newtonian system, as well as the heterogeneity of the system near to the dissolving surface.

Modelling of Biogas Production from Glycerol by Anaerobic Process in a Baffled Multi-Stage Digestor

ABSTRACT The anaerobic conversion of glycerol by the bacterium Klebsiella sp. is a well-known process. According to its metabolism glycerol is converted into different products, e.g. 2,3-butanediol, 1,3-propanediol, acetic acid, formic acid, lactic acid and biogas. The latter is produced by two competitive ways: decarboxylation of acetic acid or self-reduction of carbon dioxide by hydrogen, acetic acid, carbon dioxide and hydrogen being intermediate products of one of the competitive metabolic routes. Which of these compounds is a desired final product depends on the purpose of the fermentation. In the present work a non-structured mathematical model is developed to describe these competitive processes. The model consists of nine ordinary non-linear differential equations for the kinetics of some selected more important reactions from the metabolic pathway. The model takes into account the microbial growth, the pH drop caused by the organic acid formation and the resulting inhibition of the methanogenic microbial activity. The pH-optima of the enzymes are approximated by Gaussian distribution. The enzyme sensitivity toward pH-variations is presented by the half-width of the Gaussian curve. A large set of experimental data for glycerol anaerobic digestion by Klebsiella sp. in a baffled multistage reactor was used for verification of the model. The comparison of the experimental data with the model results revealed the number of reactor stages, sufficient for complete methanization and to determine the optimum conditions for 2,3-butanediol or methane production.

Stability analysis of a bioreactor model for biodegradation of xenobiotics

We consider an ecological model for biodegradation of toxic substances in aquatic and atmospheric biotic systems. The model, which is described by a nonlinear system of four ordinary differential equations, is known to be experimentally validated. We compute the equilibrium points of the model and study their asymptotic stability. Basic properties of the solutions like uniform boundedness and uniform persistence are established. Global asymptotic results are also developed. Numerical simulation results are presented to demonstrate the theoretical studies.

International Conference on Mathematical Methods and Models in Biosciences (Biomath) 2011

ABSTRACT The 2011 International Conference on Mathematical Methods and Models in Biosciences and School for Young Scientists (BIOMATH 2011) held in Sofia, 15–18 June 2011, was an international meeting that gathered researchers from four different continents and 16 different countries. BIOMATH 2011 continues a tradition of scientific meetings on biomathematics held at the Bulgarian Academy of Sciences (BAS) in Sofia since 1990. This issue of “Biotechnology and Biotechnological Equipment” contains a selection of 14 scientific papers focused on biological, biotechnological and biomedical studies.

A Method for the Simultaneous Cleansing of H2S and SO2

A method for the simultaneous electrochemical purification of hydrogen sulfide and sulfur dioxide from sea water or industrial wastes is proposed. Fundamentally the method is based on the electrochemical affinity of the pair H2S and SO2. The reactions (oxidation of H2S and reduction of SO2) proceed on а proper catalyst in a flow reactor, without an external power by electrochemical means. The partial curves of oxidation of H2S and reduction of SO2 have been studied electrochemically on different catalysts. Following the additive principle the rate of the process has been found by intersection of the curves. The overall process rate has been studied in a flow type reactor. Similar values of the process rate have been found and these prove the electrochemical mechanism of the reactions. As a result the electrochemical method at adequate conditions is developed. The process is able to completely convert the initial reagents (concentrations ), which is difficult given the chemical kinetics.

Numerical Evaluation of Immobilized Cell Contribution in Bioreactors

Immobilized microbial cells can grow and detach from their carrier and grow independently as free ones in the liquid media. The present work proposes a numerical approach for evaluation the contribution for certain microbial conversions of immobilized cells and the free ones separately. For this purpose experimental data are required. This approach consists in mathematical models considering the microbial growth both of the free and immobilized cells, the cell detachment from the carrier and the kinetics of substrate consumption and product formation. The mathematical models are based on ordinary differential equations for cells attached to solid supports and of partial ones for entrapped cells. The cell release into the broth is taken into account introducing a cell detachment rate factor. Four different processes are considered: biodegradation of 1,2-dichloroethane by bacteria ( Klebsiella oxytoca ), fixed on activated carbon; the same substrate by Xanhobacter autotrophicus , entrapped in polyacrylamide gel; the production of cyclodextrin glucanotransferase by free and immobilized cells of Bacillus circulans ; the lactic acid fermentation by cells of Lactobacillus rhamnosus immobilized in polyacrylamide gel. The cell detachment factor for each process could be evaluated from experimental data using the proposed models in an identification procedure. It is shown that in different cases the cell detachment factor could be different and the contribution of the immobilized and the free cells may vary depending on the microbial culture.