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Alternative to the use of fossil fuels are biofuels (e.g., bioethanol, biodiesel and biogas), which are more environmentally friendly and which can be produced from different renewable resources. In this investigation, bioethanol production from raw sugar beet cossettes (semi-solid substrate) by yeast Saccharomyces cerevisiae in a horizontal rotating tubular bioreactor (HRTB) was studied. Obtained results show that HRTB rotation mode (constant or interval) and rotation speed have considerable impact on the efficiency of bioethanol production in the HRTB. The main goal of this research was to develop a non-structural mathematical model of bioethanol production from raw sugar beet cossettes in the HRTB. The established mathematical model of bioethanol production in the HRTB describes substrate utilization and product formation (glycerol, ethanol and acetate) and presumes negative impact of high substrate concentration on the working microorganism (substrate inhibition) by using Andrews inhibition kinetics. All simulations of bioethanol production in the HRTB were performed by using Berkeley Madonna software, version 8.3.14 (Berkeley Madonna, Berkeley, CA, USA). The established non-structural bioprocess model describes relatively well the bioethanol production from raw sugar beet cossettes in the HRTB.

BACKGROUND: Sugar beet pulp (SBP) is a promising feedstock for the production of 2nd generation biofuels, but efficientenzymatic hydrolysis remains a key challenge ; therefore, new process designs and/or bioreactor designs are crucial to overcomethis hurdle. In this regard, horizontal rotating tubular bioreactors (HRTB) offer the advantage of high substrate loadings whileminimizing the space and energy demand compared with conventional stirred tank reactors. Here, a statistical approach is usedto optimize the hydrolysis of sugar beet pulp in laboratory experiments, and it is shown that such a process can be implementedin a HRTB.RESULTS:Usingthedesignofexperiments(DOE)method, thereactionconditionsoffourcommercialenzymemixtures(UltrazymAFP-L, Viscozyme L, Pectinase and Cellulase) was optimized for the degradation of SBP in small-scale experiments. UsingUltrazymAFP-Lasthemostefficientmixture, a10 LscaleconversionwasperformedinaHRTB.Atasubstrateloadingof135 g L−1and optimized conversion parameters (enzyme load, pH and rotating speed of the reactor), 0.525 W dm−3were needed toachieve solubilisation of 30% of the total mass of initial SBP after 24 h.CONCLUSION: DOE was found to be an easy-to-apply method that allowed optimizing the conditions for enzymatic hydrolysisof SBP, resulting in a higher sugar yield. The results could be transferred to an HRTB, which is a suitable system for enzymaticconversion and efficient saccharification of semi-solid or solid substrates with relatively low energy consumption.

In this work, the heterotrophic cultivation of bacterium Paracoccus denitrificans has been studied in a horizontal rotating tubular bioreactor (HRTB).After development of a microbial biofilm on the inner surface of the HRTB, conditions for one-step removal of acetate and ammonium ion were created. The effect of bioreactor process parameters [medium inflow rate (F) and bioreactor rotation speed (n)] on the bioprocess dynamics in the HRTB was studied. Nitrite and nitrogen oxides (NO and N_2O) were detected as intermediates of ammonium ion degradation. The biofilm thickness and the nitrite concentration were gradually reduced with increase of bioreactor rotation speed when the medium inflow rate was in the range of 0.5– 1.5 l h^– 1. Further increase of inflow rate (2.0– 2.5 l h^– 1) did not have a significant effect on the biofilm thickness and nitrite concentration along the HRTB. Complete acetate consumption was observed when the inflow rate was in the range of 0.5– 1.5 l h^– 1 at all bioreactor rotation speeds. Significant pH gradient (cca 1 pH unit) along the HRTB was only observed at the highest inflow rate (2.5 l h^– 1). The results have clearly shown that acetate and ammonium ion removal by P. denitificans can be successfully conducted in a HRTB as a one-step process.

BacteriumParacoccus denitrificansis capable to obtain metabolic energy by oxidation of ammonia (chemolitotrophic metabolism) and organic compounds (chemoorganotrophic or heterotrophic metabolism). It can also perform nitrate and nitrite respiration. When cultivated in certain conditions P. denitrificans can conduct the complete transformation of ammonia into gaseous nitrogen and consequently the whole nitrogen removal process can be carried out in one step. These characteristics make it suitable for successful treatment of wastewater with high nitrogen content. In this work the heterotrophic cultivation of P. denitrificans was studied in continuous stirred tank reactor (CSTR) and in horizontal rotating tubular bioreactor (HRTB). Bioprocess dynamics, efficiency and stability in both bioreactors were examined at different dilution rates (D = 0.0333‐0.1667 h � 1 ). In both cases, bioprocess dynamics was monitored by the analysis of biomass, substrate and metabolites concentrations in the liqiud outflow. The bioprocess efficiencyand stability was much higher in HRTB than in CSTR. Advantages of HRTB over CSTR were more obvious at higher dilution rates as a consequence of bacterial biofilm formation on the inner surface of HRTB that enabled better biomass retention. # 2006 Elsevier Ltd. All rights reserved.

Industrial wastewater contaminated with toxic heavy metals is big ecological and environmental problem. The use of biological materials for effective removal and recovery of heavy metals from contaminated wastewaters has emerged as a potential alternative method to conventional treatment techniques. The aim of this paper was the study of biosorption of toxic metals from aqueous solution by the application of microorganisms, isolated from polluted (metal-laden) soil. Bioadsorption process for heavy metals removal was conducted in a new pilot scale horizontal rotating tubular bioreactor (HRTB). This bioreactor provides conditions for microorganisms’ growth in a form of suspended cells and biofilm. Due to the biofilm properties, it can protect microorganisms from interaction with toxic metals in the surrounding environment. Three metals were selected as model examples: cations of manganes and cobalt, and hexavalent chromium (an oxyanion). Optimized bioreactor condition [medium inflow rate (F) and bioreactor rotation speed (n)] for metal removal was monitored, and, under optimized bioreactor conditions, promising results were obtained.

Pollution by heavy metals is a major environmental problem because they are not subject to natural degradation and therefore it comes to their bioaccumulation in tissues, what can cause serious health problems. Generally used bioreactors for metal removal from waste water include rotating biodisc reactors, trickle filters, fixed bed and fluidized beds, air- lift reactors and other biofilm reactors [1]. In this research, removal of metal (Co, Cr and Mn) by biosorption process in horizontal rotating tubular bioreactor (HRTB), was monitored. HRTB is designed as a combination of thin-layer and biodisc reactor and therefore has characteristics of both reactors. Discs are used as carriers for microbial biomass, which removes metals present in water. Biosorption can be defined as the removal of metal or metalloid species, compounds and particles from solution by biological material [2] and it was monitored by thin-layer chromatography. Before analysis of waste water samples, it was necessary to determine the optimal chromatographic system for separating metal ions. In optimization procedure, binary, ternary and quaternary solvent systems with different volume shares were used. Total mixture volume was 10 mL. Therefore, standard solutions of metal salts in distilled water were prepared in range of 0.1-1.0 g/L. These solutions were used for optimization and metal ion identification during real-sample analysis. Based on the obtained chromatograms, calculated retention factors (RF values) and plate development time, the adequate chromatographic system for separation and identification of analysed metal ions was chosen.

In this work, the bioadsorption process was studied in a pilot scale horizontal rotating tubular bioreactor (HRTB). On the basis of the results obtained via inductively coupled plasma sector field mass spectrometry an integrated mathematical model of heavy metal removal (Co2+, Cr6+ and Mn2+) in HRTB was established. It combines hydrodynamics (mixing) and kinetics in order to define bioprocess conduction in the bioreactor. Mixing in HRTB was described by a structured cascade model and metal ion removal by a combined diffusion-bioadsorption model, respectively. Differences between experimental and simulated metal ion concentrations were in the range of the experimental uncertainty, demonstrating that the established bioprocess model successfully predicts metal ion concentrations in HRTB over a wide range of process parameters [rotation speed (n = 5 - 30 min-1) and inflow rate (F = 0.5 - 2.0 L h-1)]. Furthermore, the obtained results also pointed out that the established model is accurate and robust and has therefore a high potential for use in a scale-up procedure.

Mixed microbial culture was isolated from heavy metal-contaminated ground soils located inside iron, vinyl and cement factory area. Isolated mixed microbial culture was used for the heavy metal ions (Fe2+, Cu2+, Ni2+ and Zn2+) removal process in horizontal rotating tubular bioreactor (HRTB). In this research, the effect of bioreactor process parameters on the bioprocess dynamics in the HRTB was studied. Results of this research have shown that profiles of heavy metals concentration were gradually reduced along HRTB at all combinations of bioreactor process parameters [inflow rates (0.5-2.0 L h^-1) and rotation speed (5-30 min^-1)]. Hydrodynamic conditions and biomass sorption capacity have main impact on the metal ions removal efficiency that was varied in the range of 38.1% to 95.5%. Notable pH gradient (cca 0.7 pH unit) along the HRTB was only observed at the inflow rate of 2.0 L h^-1. On the basis of obtained results, it is clear that medium inflow rate (F) has higher impact on the heavy metal removal process than bioreactor rotation speed (n) due to the fact that increase of inflow rate was related to the reduction of equilibrium time for all examined metal ions. Furthermore, equilibrium times for all metal ions are significantly shorter than medium residence times at all examined combinations of bioreactor process parameters. The main impact on the biofilm sorption capacity has covalent index of metal ions and biofilm volumetric density. The sorption capacity of suspended microbial biomass is closely related to its concentration. Results of this research have also shown that the removal of heavy metals ion can be successfully conducted in an HRTB as a one-step process.

Several regulations were adopted for industrial wastewater discharges. Thus wastewater treatment prior to discharge to a water body (stream, river, bay, etc.) is necessary to meet the conditions given in the respective regulations. Different bioreactor types with microbial biofilm are the most promising systems for the treatment of metal contaminated waters. A horizontal rotating tubular bioreactor (HRTB) is designed as a combination of a thin layer and a biodisc reactor and its use for heavy metals removal from wastewater was investigated. Heavy metals removal process in HRTB was studied in continuos mode and wastewater samples were drawn along the bioreactor length. In order to make decisions for the further optimization steps a fast and simple as well as on-site applicable method for the determination of the given metals was needed. The UV-VIS spectrophotometric determination of Cu, Fe, Ni and Zn using sodium diethyldithiocarbamate, 1, 10-phenathroline, dimethylglyoxime, and 2-{;[alpha-(2-Hydroxy-5-sulfophenylazo)-benzylidene]-hydrazino};-benzoic acid monosodium salt (=zincon monosodium salt) as reagents respectively, was optimized and validated. To avoid inferences by the yellowish hue of the samples there were diluted 1:20 with distilled water prior to complexation. The limits of detection are 0.045, 0.04, 0.04, and 0.01 mg/L for Cu, Fe, Ni and Zn, respectively. The recovery for all elements is in range from 98 % up to 104 %. The uncertainty of measurement for all metals under study was approx. 5 %. The same samples were then analyzed by inductively coupled plasma optical emission spectrometry. The results obtained by both methods were compared using a paired t-test. No statistically significant differences were found.

In this investigation simultaneous nitrification and denitrification with bacterium Paracoccus denitrificans has been studied in horizontal rotating tubular bioreactor (HRTB). The interior of HRTB is characterized by O - ring shaped partition walls what together with bioreactor wall performed an area suitable for microbial biofilm development. After microbial biofilm development on the inner surfaces of HRTB conditions for simultaneous nitrification and denitrification as one step process were created. The main goal of this investigation was to study the effect of bioreactor process parameters [medium inflow rate (F = 0, 5 - 2, 5 L h^-1) and HRTB rotation speed (n = 5 - 30 min^-1)] on the dynamics of bioprocess in HRTB. In this investigation acetate consumption was in the range of 50, 0 - 96, 6 % by all combination of bioreactor process parameters (n, F). Nitrite, nitrogen (II) oxide and nitrogen (I) oxide were detected as intermediers of ammonium degradation during simultaneous nitrification and denitrification. pH gradient (cca 1 pH unit) along HRTB were observed only for medium inflow rate of 2, 5 L h^-1. Biofilm thickness and nitrit concentration were gradually reduced with increase of bioreactor rotation speed when medium inflow rate was in the range of 0, 5 - 1, 5 L h^-1. Further increase of medium inflow rate (2, 0 - 2, 5 L h^-1) did not have significant effect on the biofilm thickness and nitrit concentration along HRTB. On the bases of obtained results it can be concluded that simultaneous nitrification and denitrification can be successfully done in HRTB as one step process.

Simultaneous nitrification and denitrification is essential bioprocess for successful treatment of wastewater with high nitrogen content. Bacterium Paracoccus denitrificans is capable of the complete transformation of ammonia to gaseous nitrogen and therefore it is suitable for conduction of simultaneous nitrification and denitrification in one step. During this investigation simultaneous nitrification and denitrification with bacterium Paracoccus denitrificans has been studied in horizontal rotating tubular bioreactor (HRTB) and in continuous stirred tank reactor (CSTR). In this research the effect of dilution rate (D = 0.0333 - 0.1667 h^-1) on the bioprocess dynamics was examined. Bioprocess dynamics was monitored by the analysis of acetate, ammonium ion, nitrate, nitrite and biomass in suspension and nitrogen (I) oxide and nitrogen (II) oxide in gas outflow, respectively. Obtained results show that higher bioprocess efficiency was observed in HRTB that in CSTR. Advantages of HRTB over CSTR were more obvious at higher dilution rates most probably due to developed bacterial biofilm on inner surfaces of HRTB.

Horizontal rotating tubular bioreactor (HRTB) is designed as a combination of the "biodisc" reactor and the "thin-layer" bioreactor. The interior of the HRTB is divided by O-ring shaped partition walls, which together with tube wall are supposed to serve as carriers for active layer of biomass. HRTB is horizontally placed on the bearings that enable rotation of the whole bioreactor. The performance of HRTB was investigated with a real biological system in non-sterile conditions. For this purpose a mixed culture was cultivated in simple glucose/yeast extract medium with C:N:P ratio 100:10:4. During the experiment, microbial biofilm was formed on the bioreactor wall and partition walls as well. The dynamics of biological system in HRTB was monitored by withdrawing the samples from five positions along the bioreactor and samples were analysed for glucose and products of metabolism (ethanol and lactate). The bioconversion was investigated in a wide range of process parameters (medium in fow rate 1-10 L h^ -1 and HRTB rotation speed 5-30 min^ -1). Investigation in HRTB showed rapid and efficient glucose conversion into products of metabolism, ethanol and lactate. The rate of glucose consumption along the HRTB depended on medium inflow rate, while rotation speed of the bioreactor did not have significant influence. Complete glucose conversion in HRTB was achieved at inflow rate of up to 6.5 L h^ -1, with its maximum value of 26 g glucose per hour and litre of bioreactor working volume. In this case, ethanol and lactate made 40 - 60 %, of total organic carbon in outflow of HRTB. The results showed that concept of HRTB can he successfully applied in anaerobic and microaerophilic microbial bioprocesses.

Recently, the structured "spiral flow" model was developed to describe the mixing performance of the horizontal rotating tubular bioreactor (HRTB). The adjustable parameters of the model were optimised by Monte-Carlo method. In order to apply the mixing model for a semifundamental scale-up procedure it was necessary to make a relation between the adjustable model parameters and bioreactor process parameters usually expressed as dimensionless numbers. The sets of mathematical equations which relate adjustable model parameters with dimensionless numbers were developed by using non-linear and surface regression methods. These sets of equations were applied for developing the prediction systems of adjustable model parameters. In total, nine systems of equations for the prediction of the adjustable model parameters were established and examined by simulation. Three of them were selected as adequate to describe the mixing performance of HRTB in a wide range of process conditions.

Investigations in the horizontal rotating tubular bioreactor (HRTB) were done to explain the influences of the liquid levels (H_M = 0.05 - 0.08 m) and the distances between the partition walls (D_S = 0.02 - 0.07 m) on the mixing performance of bioreactor. The mixing in HRTB was described by a "spiral flow" model. The adjustable model parameters were optimised by the Monte-Carlo method. The three sets of mathematical equations were selected which related the adjustable model parameters with dimensionless numbers. These prediction systems were modified by incorporating the influence of the liquid level in the bioreactor (H_M) and the distance between partition walls (D_S) as additional parameters. Such modified prediction systems (SC-2A, SC-6A and SC-9A) were used for mixing simulation. Prediction system SC-9A was distinguished as the most suitable to describe the mixing performance of HRTB. This prediction system was incorporated as a part of the scale-up procedure of HRTB, which was based on physiological, hydrodynamic and geometric similarity.

Investigations of mixing properties in the horizontal rotating tubular bioreactor (HRTB) by using the temperature step method resulted in an evaluation of the structured "spiral flow" model (1,2). The adjustable parameters of the model were optimised by "Monte-Carlo" method. For a semifundamental scale-up procedure it is necessary to make a relation between the adjustable model parameters and the process parameters of the bioreactor (usually expressed as a dimensionless numbers). The dimensional analysis was done by Gauss-Jordan reduction method. Mathematical equations which relate adjustable model parameters with dimensionless numbers were developed by a surface regression based on Maclaurin power series. The equations with the highest correlation coefficients were applied further for developing the scale-up procedure of HRTB.

A horizontal rotating tubular bioreactor (HRTB) was designed as a combination of a "thin-layer bioreactor" and a "biodisc" reactor. Its interior was divided by O-ring shaped partition walls which could be used as a carriers for the film of microbial biomass (1,2). Mixing properties of this new type of bioreactor were investigated by using a temperature step method and different mixing models. A dispersion model for mixing which use Bodenstein number as a parameter was compared with a structured "spiral flow" model. The obtained results show that structured "spiral flow" model (containing four adjustable parameters) is more capable to describe mixing properties in HRTB than the one parameter dispersion model.

A horizontal rotating tubular bioreactor (HRTB) was designed as a combination of a "thin-layer bioreactor" and a "biodisc" reactor whose interior was divided by O-ring shaped partition walls (1). Mixing properties of this new type of bioreactor were investigated by using a temperature step method and different mixing models. A dispersion model for mixing with Bodenstein number as a parameter was compared with a structured "spiral flow" model (2) by using following experimental and mathematical procedures : -experimental results were obtained by measuring the response to temperature step of six Pt-100 sensors (t_90 response time 0.08 s and resolution 0.002 K) for wide range of process conditions (eight dilution rates combined with seven rotation speeds). - the two sets of differential equations for heat balances were established based on two mathematical models, the axial dispersion model and structured model with "spiral flow", respectively. - the above systems were simulated by using Runge-Kutta-Fehlberg integration and optimised by the "Monte-Carlo" method. - the comparison of mixing models was based on mathematical criteria (the sum of temperature differences between measured and simulated results for all six sensors and whole experimental time range). The results obtained with above procedure show that structured "spiral flow" model (containing four adjustable parameters) is more capable to describe mixing properties in HRTB than the one parameter axial dispersion model. 1. B. Šantek et al.; Bioprocess Eng.; 14; 195-203; 1996 2. B. Šantek et al.; Bioprocess Eng.; 14; 223-229; 1996

In previous work (1,2) we studied the mixing properties in the horizontal rotating tubular bioreactor (HRTB) by using the temperature step method which resulted in evaluation of the structured "spiral flow" model. The adjustable parameters of the model were optimised by "Monte-Carlo" method. For an application of a mixing model and dimensional analysis in a semifundamental scale-up procedure it is necessary to make a relation between the adjustable model parameters and the process parameters (usually expressed as dimensionless numbers) of the bioreactor. The dimensional analysis was done by Gauss-Jordan reduction method. Because of the special interior structure of HRTB the modified dimensionless numbers were used: Reynolds rotation number (ReN), Reynolds axial flow number (ReD), Froude rotation number (FrN) and Froude axial flow number (FrD), respectively. Mathematical equations which relate adjustable model parameters with dimensionless numbers were developed by a surface regression based on Maclaurin power series. The equations with the highest correlation coefficient were selected and applied further for developing the scale-up procedures of HRTB. 1. B. Šantek et al.; Bioprocess Eng.; 14; 195-203; 1996 2. B. Šantek et al.; Bioprocess Eng.; 14; 223-229; 1996