Optimization of biological sulfide removal in a CSTR bioreactor

In this study, biological sulfide removal from natural gas in a continuous bioreactor is investigated for estimation of the optimal operational parameters. According to the carried out reactions, sulfide can be converted to elemental sulfur, sulfate, thiosulfate, and polysulfide, of which elemental sulfur is the desired product. A mathematical model is developed and was used for investigation of the effect of various parameters on elemental sulfur selectivity. The results of the simulation show that elemental sulfur selectivity is a function of dissolved oxygen, sulfide load, pH, and concentration of bacteria. Optimal parameter values are calculated for maximum elemental sulfur selectivity by using genetic algorithm as an adaptive heuristic search. In the optimal conditions, 87.76% of sulfide loaded to the bioreactor is converted to elemental sulfur.     

Hydrogen sulfide removal by immobilized autotrophic and heterotrophic bacteria in the bioreactors

Summary Autotrophic Thiobacillus sp. CH11 and heterotrophic Pseudomonas putida CH11 were isolated from piggery waste water. Extensive tests, including removal action, removal efficiency, removal capacity, and adaptability to perturbed conditions were conducted on autotrophic and heterotrophic biofilters. The results clearly demonstrate the application potential of autotrophic and heterotrophic biofilters for the continuous H₂S removal under different perturbed conditions.     

Mathematical modeling of biological sulfide removal in a fed batch bioreactor

In this study, biological sulfide removal is investigated in a fed batch bioreactor. In this process, sulfide is converted into elemental sulfur particles as an intermediate in the oxidation of hydrogen sulfide to sulfate. The main product is sulfur at low dissolved oxygen or at high sulfide concentrations and also more sulfates are produced at high dissolved oxygen. According to the carried out reactions, a mathematical model is developed. The model parameters are estimated and the model is validated by comparing with some experimental data. The results show that, the proposed model is in a good agreement with experimental data. According to the experimental result and mathematical model, sulfate and sulfur selectivity are sensitive to the concentration of dissolved oxygen. For sulfide concentration 0.2 (mM) in the bioreactor and dissolved oxygen of 0.5 ppm, only 10% of sulfide load is converted to sulfate, while it is 60% at the same sulfide concentration and dissolved oxygen of 4.5 ppm. At high sulfide load to the bioreactor, the concentration of uneliminated sulfide increases; it leads to more sulfur particle selectivity and consequently, less sulfate selectivity.     

Continuous antibiotic production by an immobilized cyanobacterium in a seaweed-type bioreactor

The filamentous cyanobacterium,Scytonema sp. TISTR 8208, which produces a cyclic peptide antibiotic, was cultivated for 20 d in a seaweed-type bioreactor containing anchored polyurethan foam strips. Cells immobilized onto the foam strips produced the antibiotic for only several days, and the secreted antibiotic disappeared very rapidly from the medium. Cells accumulated the antibiotic intracellularly in a growth-related manner, and secreted it in the stationary phase. Since the antibiotic has a stable physico-chemical nature, the cells seem to take it up and metabolize it. When continuous cultivation was attempted, stable production of the antibiotic was maintained in the bioreactor for 16 d at a dilution rate of 0.01 h^–1. Three times more antibiotic was produced in the continuous culture than in the batch culture by the 16th day.     

Hydrogen sulfide removal by immobilized Thiobacillus novellas on SiO2 in a fluidized bed reactor

The removal of hydrogen sulfide (H2S) from aqueous media was investigated using Thiobacillus novellas cells immobilized on a SiO2 carrier (biosand). The optimal growth conditions for the bacterial strain were 30 degrees C and initial pH of 7.0. The main product of hydrogen sulfide oxidation by T. novellus was identified as the sulfate ion. A removal efficiency of 98% was maintained in the three-phase fluidized-bed reactor, whereas the efficiency was reduced to 90% for the two-phase fluidized-bed reactor and 68% for the two-phase reactor without cells. The maximum gas removal capacity for the system was 254 g H2S/m3/h when the inlet H2S loading was 300 g/m3/h (1,500 ppm). Stable operation of the immobilized reactor was possible for 20 days with the inlet H2S concentration held to 1,100 ppm. The fluidized bed bioreactor appeared to be an effective means for controlling hydrogen sulfide emissions.     

Toluene removal in an automated cyclical bioreactor

A control scheme was developed for the automation of toluene removal in a cyclical bioreactor. Toluene was added to the self-cycling fermentor by diffusion across a silicone membrane. Transient dissolved oxygen, carbon dioxide evolution, and oxidation-reduction potential (ORP) were screened as potential control variables. Through experimentation, ORP was deemed most effective. Control algorithms based on real-time estimates of the first and second derivatives of the ORP signal were tested. Although both approaches resulted in stable operation of the reactor, average toluene removal efficiencies of 95% were realized when control was based on the second derivative. This was significantly higher than the 77% efficiencies obtained when the control scheme centered on the first derivative of the transient ORP signal. The system developed was self-regulating, ensuring that a high toluene removal rate, on the order of 1.1 g h(-1), was maintained from cycle to cycle.     

Hydrogen sulfide production from elemental sulfur by Desulfovibrio desulfuricans in an anaerobic bioreactor

Feasibility of elemental sulfur reduction by Desulfovibrio desulfuricans in anaerobic conditions in a stirred reactor was studied. Hydrogen was used as energy source, whereas the carbonated species were bicarbonate and yeast extract. Attention was paid to reactor engineering aspects, biofilm formation on the sulfur surface, hydrogen sulfide formation rate and kinetics limitations of the sulfur reduction. D. desulfuricans formed stable biofilms on the sulfur surface. It was found that active sulfur surface availability limits the reaction rate. The reaction rate was first order with respect to sulfur and hydrogen velocity had no effect in the reaction rate for the range 8.2 x 10(-2) to 4.1 x 10(-1) Nm(3) m(-2) min(-1). At a superficial gas velocity (u(G)) = 3.1 x 10(-2) Nm(3) m(-2) min(-1), H(2)S(g) production rate decreased due to a deficient H(2)S stripping. A maximum H(2)S(g) production rate of 2.1 g H(2)S L(-1) d(-1) was achieved during 5 days with an initial sulfur density of 4.7% (w/v).     

Decolorization of Direct Red 28 by mixed bacterial culture in an up-flow immobilized bioreactor

Aerobic mixed bacterial culture comprised of five isolates (Bacillus vallismortis, B. pumilus, B. cereus, B. subtilis and B. megaterium) identified by 16srDNA analysis was developed from wastewater samples from the aeration tank of an effluent treatment plant of a textile and dyeing industry and evaluated for its ability to decolorize azo dye Direct Red 28 in an up-flow immobilized packed bed bioreactor using marble chips as support matrix. The bioreactor was operated under two parameters: an aeration rate of 0.4 and 0.6 mmol/min at a flow rate of 60, 90 and 120 ml/h, respectively. At a constant aeration rate of 0.4 mmol/min and with flow rates of 60, 90 and 120 ml/h, optimum decolorization of 91, 75 and 72% was observed, while at an aeration rate of 0.6 mmol/min and flow rates of 60, 90 and 120 ml/h, optimum decolorization of 93, 78 and 72% was observed over 10 days. The study concluded that across the two aeration rates and the respective flow rates, the higher aeration rate of 0.6 mmol/min along with a flow rate of 60 ml/h was best suited to decolorize Direct Red 28 in the packed bed bioreactor. Spectral changes of the input and output of the bioreactor by UV–visible spectroscopy indicated decolorization of the dye solution by degradation in addition to the visual observation of the biosorption process.     

Trehalose production at high temperature exploiting an immobilized cell bioreactor

The enzymatic production of trehalose from dextrins was studied as a series reaction in a packed bed reactor containing immobilized recombinant Escherichia coli cells, expressing either the Sulfolobus solfataricus (strain MT4) trehalosyl-dextrin forming enzyme (TDFE) or the trehalose-forming enzyme (TFE). The cells, subjected to thermal treatments to increase cell permeability and to inactivate the unwanted host proteins, were entrapped separately or together in a calcium alginate polymeric matrix. The biocatalyst beads were used to pack a tubular glass reactor that was operated in a recycle mode. The performances of a bioreactor containing alternate layers of EcTFE and EcTDFE alginate beads were evaluated and compared with the performance of the co-immobilized biocatalysts. The latter showed a superior throughput, therefore the bioreactor packed with the co-entrapped biocatalysts was tested for the production of trehalose from concentrated dextrin solutions (10%-30% w/v) and a conversion up to 90% was obtained. This conversion corresponded to a production of 127 g trehalose h(-1) kg(-1) of biocatalyst. The results obtained suggest that the bioprocess described may be of interest in the development of a large-scale industrial process for trehalose production at high temperature.     

An immobilized Thiobacillus thioparus biosensing system for monitoring sulfide hydrogen; optimized parameters in a bioreactor

A microbial biosensing system for detection of hydrogen sulfide has been developed by using immobilized Thiobacillus thioparus TK-m in poly vinyl alcohol matrix, together with a dissolved oxygen sensor. Parameters of immobilization (poly vinyl alcohol concentration and amount of wet cell) were optimized by using statistical software. The obtained values for concentration of poly vinyl alcohol and wet cell weight were 11.3% (w/v) and 45 mg, respectively, where the response time of biosensor was 80 s. Calibration of oxygen concentration based on hydrogen sulfide concentration was investigated between 1 mg/L and 20 mg/L. The effect of pH and temperature were investigated in specific range of experimental conditions as well. Some parameters including operational stability and detection limit were studied in detail for characterization of biosensing system. In order to determine the operational stability, bio-sensing system at optimized working conditions was used to distinguish viability of microorganisms in polymer beads in period of time.     

A horizontal bioreactor for ethanol production by immobilized cells

A mathematical model which describes ethanol formation in a horizontal tank reactor containing Saccharomyces cerevisiae immobilized in small beads of calcium alginate has been developed. The design equations combine flow dynamics of the reactor as well as product formation kinetics. The model was verified for 11 continuous experiments, where dilution rate, feed glucose concentration and bead volume fraction were varied. The model predicts effluent ethanol concentration and CO₂ production rate within the experimental error. A simplification of the model is possible, when the feed glucose concentration does not exceed 150 kg/m³. The simplification results in an analytical solution of the design equation and hence can easily be applied for design purposes as well as for optimization studies.     

A horizontal bioreactor for ethanol production by immobilized cells

Continuous ethanol production in a three stage horizontal tank bioreactor (HTR) by yeast cells entrapped in Ca-alginate was about 30% higher than in a vertical type of bioreactor and reached 31 kg/(m³ · h) at 95% glucose utilization. Maximum ethanol productivity obtained was 41.2 kg/(m³ · h), however, with 38% of the glucose fed to the HTR being wasted. The higher performance of the HTR had been mainly accounted for the reduction of the adverse CO₂ gas phase effect and the more pronounced plug-flow character. Glucose and ethanol profiles along the HTR revealed that 50–80% of the overall fermentation activity was present in the first stage. Within a test period of 23 d the HTR showed an excellent operational stability.Compared to other continuous ethanol production processes using entrapped yeast cells the HTR presented here belongs to the top ones.     

Continuous neomycin production by immobilized cells of Streptomyces marinensis NUV-5 in an airlift bioreactor

Neomycin production by free and calcium alginate immobilized cells was investigated in an airlift reactor. The average volumetric productivity with continuous fermentation (72.97 mg/l/h) was greater than with free cells (45.05 mg/l/h). The total neomycin produced with continuous fermentation was 62% greater than with that of free cells. Immobilized Streptomyces particles showed a half-life of 42 days during continuous fermentation under airlift conditions.     

A horizontal bioreactor for ethanol production by immobilized cells

The one-parameter-tanks-in-series model was found to be an adequate model for the characterization of flow dynamics in a horizontal immobilized cell reactor, when blue dextran was used as tracer. Isobutanol proved to be inadequate, because it diffused inside the beads and thus caused tailing in RTD. The CO₂ evolution rate displayed the most pronounced effect on axial liquid dispersion. At high CO₂ production rates and low dilution rates each stage of the reactor behaved like a well-mixed reactor. At lower CO₂ evolution rates the number of tanks (N) related to the reactor increased up to 10. The medium flow rate affects axial dispersion to a minor degree. An increase of the dilution rate from 0.328 to 1.34 h^?1 resulted in a slight rise of N from 3.5 to 5 at high CO₂ production and from 4 to 7 at medium CO₂ production rates. Variation in the bead hold up showed the same characteristic axial mixing behavior as reflected by changing the medium flow rate. The quantitative correlation between axial mixing and the most significant fermentation parameters (dilution rate, CO₂ evolution rate and bead hold up) allow to develop an overall model, which besides kinetic expressions also contains terms related to the flow dynamics of the reactor. In the third part of this communication such a model will be presented and compared with actual fermentation data.     

Berberine production by batch and semi-continuous cultures of immobilized Thalictrum cells in an improved bioreactor

A new bioreactor (liquid-gas two-phase system) was devised for berberine-secreting Thalictrum minus cells immobilized in calcium alginate beads, which were alternately soaked in medium, and exposed to air. The highest yield of berberine (875 mg/l) was obtained by setting the cycle of medium supply and air supply for 30 seconds and 2 minutes, respectively, during a culture period of 30 days. Under such conditions of batch culture, the berberine productivity of immobilized cells was as high as that of freely suspended cells. Furthermore, the rate of berberine production by immobilized cells remained constant at a high value (50 mg/l/day) for a period of 60 days of semi-continuous culture achieved by renewal of medium at intervals of 10 days.     

Acetic acid production from lactose by an anaerobic thermophilic coculture immobilized in a fibrous-bed bioreactor

An anaerobic thermophilic coculture consisting of a heterofermentative bacterium (Clostridium thermolacticum) and a homoacetogen (Moorella thermoautotrophica) was developed for acetic acid production from lactose and milk permeate. The fermentation kinetics with free cells in conventional fermentors and immobilized cells in a recycle batch fibrous-bed bioreactor were studied. The optimal conditions for the cocultured fermentation were found to be 58 degrees C and pH 6.4. In the free-cell fermentation, C. thermolacticum converted lactose to acetate, ethanol, lactate, H(2) and CO(2), and the homoacetogen then converted lactate, H(2), and CO(2) to acetate. The overall acetate yield from lactose ranged from 0.46 to 0.65 g/g lactose fermented, depending on the fermentation conditions. In contrast, no ethanol was produced in the immobilized-cell fermentation, and the overall acetate yield from lactose increased to 0.8-0.96 g/g lactose fermented. The fibrous-bed bioreactor also gave a higher final acetate concentration (up to 25. 5 g/L) and reactor productivity (0.18-0.54 g/L/h) as compared to those from the free-cell fermentation (final acetate concentration, 15 g/L; productivity, 0.06-0.08 g/L/h). The superior performance of the fibrous-bed bioreactor was attributed to the high cell density (20 g/L) immobilized in the fibrous-bed and adaptation of C. thermolacticum cells to tolerate a higher acetate concentration. The effects of yeast extract and trypticase as nutrient supplements on the fermentation were also studied. For the free-cell fermentation, nutrient supplementation was necessary for the bacteria to grow in milk permeate. For the immobilized-cell fermentation, plain milk permeate gave a high acetate yield (0.96 g/g), although the reactor productivity was lower than those with nutrient supplementation. Balanced growth and fermentation activities between the two bacteria in the coculture are important to the quantitative conversion of lactose to acetic acid. Lactate and hydrogen produced by C. thermolacticum must be timely converted to acetic acid by the homoacetogen to avoid inhibition by these metabolites.     

Simultaneous removal of carbon and nitrate in an airlift bioreactor

This paper presents the integrated removal of carbon (measured as chemical oxygen demand i.e. COD) and NO(x)-N by sequentially adapted sludge, studied in an airlift reactor (ALR). Simultaneous removal of COD and nitrate occurs by denitrification (anoxic) and oxidation (aerobic). Aerobic (riser) and anoxic (remaining part) conditions prevail in different parts of the reactor. Studies were carried out in a 42 L ALR operated at low aeration rate to maintain anoxic and aerobic conditions as required for denitrification and COD removal, respectively. The sludge was adapted sequentially to increasing levels of NO(x)-N and COD over a period of 45 days. Nitrate removal efficiency of the sludge increased due to adaptation and degraded 900 ppm NO(3)-N completely in 2h (initially the sludge could not degrade 100 ppm NO(3)-N). The performance of the adapted sludge was tested for the degradation of synthetic waste with COD/N loadings in the range of 4-10. The reduction of COD was significantly faster in the presence of NO(x)-N and was attributed to the availability of oxygen from NO(x)-N and distinct conditions in the reactor. This hypothesis was justified by the material balance of COD.     

A novel bioreactor for immobilized phototrophs

A novel configuration of photobioreactor is described in which filaments of alginate containing immobilized cells of a leaky mutant of Dunaliella parva are wound round a central light well which is located within a glass outer chamber so that a liquid medium is caused to flow in the annular space between the outside chamber and the alginate filaments. Glycerol production by D. parva was maintained for 700 h and the highest concentration of glycerol attained was approx. 12 mg l⁻¹.