Basil C. Baltzis

Biofiltration of toluene vapor under steady-state and transient conditions: Theory and experimental results

Abstract Removal of toluene vapor from airstreams was studied in a vapor phase biological reactor known as a biofilter. The reactor was packed with a mixture of peal and perlite particles on which a mixed microbial population (consortium) was immobilized and formed a biolayer. The reactor was operated over a period of 11 months under various inlet-airstream toluene concentrations and flow rates of the contaminated airstream. Except at start-up, no supplemental nutrients were provided to the column, which remained active and never exhibited any significant pressure drop build-up. The process was modeled with general mass balance equations which take into account reaction, mass transfer, and adsorption of the pollutant onto the packing material. The model equations were solved numerically and the predicted concentration profiles agreed very well with the experimental data, for both steady-state and transient operation. Predicted concentration profiles for the biofilm indicate that toluene gets depleted before oxygen in a thin layer of the order of 35 μm. This finding is opposite to what has been reported for hydrophilic solvents where oxygen is depleted before the contaminant in the biolayer. The model equations have been used in parameter sensitivity studies that have revealed the parameters which need to be accurately known for predicting the performance of a biofilter.

Nanofiltration-based diafiltration process for solvent exchange in pharmaceutical manufacturing

Athermal solvent exchange from one organic synthesis step to the next step is highly desirable in bulk pharmaceutical manufacturing due to the thermally labile nature of the active intermediates. Diafiltration (DF) was employed using methanol as the solvent needed in the next synthesis step to drastically reduce the concentration of ethyl acetate used as the solvent in the previous synthesis step. Ethyl acetate was reduced to the level of a low concentration impurity in methanol by both batch and continuous DF using solvent resistant nanofiltration membranes MPF-50 and MPF-60; the latter has a high rejection of around 96% for the solute, erythromycin, representing an active intermediate. Nanofiltration-based diafiltration for exchanging the solvent methanol for ethyl acetate was demonstrated. The membrane rejection of the solute needs to be higher. Membrane compaction has to be considered in the design of the process for both membranes.

An experimental and modeling study on the removal of mono-chlorobenzene vapor in biotrickling filters

Removal of mono-chlorobenzene (m-CB) vapor from airstreams was studied in a biotrickling filter (BTF) operating under counter-current flow of the air and liquid streams. Experiments were performed under various values of inlet m-CB concentration, air and/or liquid volumetric flow rates, and pH of the recirculating liquid. Conversion of m-CB was never below 70% and at low concentrations exceeded 90%. A maximum removal rate of about 60 gm-3-reactor h-1 was observed. Conversion of m-CB was found to increase as the values of liquid and air flow rate increase and decrease, respectively. The effects of pH and frequency of medium replenishment on BTF performance were also investigated. The process was successfully described with a detailed mathematical model, which accounts for mass transfer and kinetic effects based on m-CB and oxygen availability. Solution of the model equations yielded m-CB and oxygen concentration profiles in all three phases (airstream, liquid, biofilm). It is predicted that oxygen has a controling effect on the process at high inlet m-CB concentrations. From independent, suspended culture, experiments it was found that m-CB biodegradation follows Andrews inhibitory kinetics. The kinetic constants were found to remain practically unchanged after the culture was used in BTF experiments for 8 months. Copyright 1998 John Wiley & Sons, Inc.

Kinetics of phenol biodegradation in the presence of glucose.

The kinetics of utilization of glucose, phenol, and their mixtures by Pseudomonas putida (ATCC 17514) were studied with a continuously aerated, jacketed batch reactor operating at 28°C and pH 7.2. It was found that when glucose is the sole carbon and energy source, the culture utilizes it following Monod kinetics. When phenol is the sole carbon and energy source, the culture biodegrades it following Andrews (inhibitory) kinetics. When both glucose and phenol are present in the medium, the culture uses them simultaneously but with lower specific rates. Reduction of the specific substrate utilization rates indicates that the two substances are involved in a cross‐inhibitory pattern which can be classified as uncompetitive. The values of the kinetic interaction constants suggest that glucose inhibits the specific rate of phenol removal much more than phenol inhibits the specific rate of glucose utilization. The results suggest that substitutable substrates which are dissimilar in origin and molecular structure may be involved in an uncompetitive cross‐inhibitory interaction when they are simultaneously removed. It is also concluded that the use of easily degradable substrates may not enhance the per‐unit amount of biomass removal of compounds which are classified as toxic. A general classification of kinetic interactions between substitutable resources is proposed.

Biofiltration of volatile organic compound (VOC) mixtures under transient conditions

Abstract In this study, a transient biofilter model for binary volatile organic compound (VOC) mixture is developed. In the model, general mixing, oxygen limitation aspects, multi-component adsorption phenomena, Monod- and Andrews-type kinetics with interference between the components are considered. Multicomponent adsorption isotherms are developed and used in the solution of the model. The model has been solved by two approaches, (approximate and general) and validated with biofiltration of benzene-toluene mixture data of previous work. The approximate model is based on the quasi-steady-state approximation in the biofilm, but the general model is solved as it is without any simplifications. The results show that the predictions by the general model are closer to the actual experimental results. It was also found that inlet oxygen composition plays an important role in the biofiltration of mixed VOC removal, specially at high inlet concentrations.

The growth of pure and simple microbial competitors in a moving distributed medium

The dynamics of pure and simple competition between two microbial species are examined for the case of interaction arising in a distributed and nonstagnant environment. The environment is modeled as a tubular reactor. It is shown that for relatively small values of the dispersion coefficient (i.e., for small, but nonzero, backmixing of the medium), the two competing populations can coexist in a stable steady state. It has been assumed that the species grow uninhibited and that if there are maintenance requirements they are satisfied from endogenous sources. From numerical studies it has been found that a necessary condition for coexistence is that the net specific growth rate curves of the two competitors cross each other at a positive value of the concentration of the rate-limiting substrate. The model equations have been numerically solved by using the methods of orthogonal and spline collocation.

Biodegradation of wastes in a cyclically operated reactor: Theory, experimental verification and optimization studies

Abstract A cyclically operated biological reactor employed for treatment of liquid wastestreams containing a single pollutant was mathematically modeled and analyzed, using the principles of bifurcation theory for forced systems. The analysis showed that the system has two qualitatively different solutions (washout and culture survival), and that there are regions in the operating parameter space where multistability occurs. The theoretical predictions were experimentally verified with a 5-liter continuously operated reactor. Phenol was used as the model pollutant, and the biomass consisted of a pure culture of Pseudomonas putida (ATCC 31800). In all cases, an excellent agreement was found between experimental data and model predictions, both at a qualitative and a quantitative level. The model was subsequently used in numerical studies, the objective of which was to find optimal operating parameter values for maximizing the productivity (volumetric efficiency) of the reactor. The results indicate that in all cases, optimal parameter value sets exist. The system considered here is a variation of the sequencing batch reactors (SBRs) used in wastewater treatment applications. The results of this study suggest a methodology for optimal bioreactor design for environmental applications.

Utilization of substitutable substrates in a bioreactor under cycling: A study on process dynamics and optimization

The process of simultaneous utilization of two substitutable substrates for growth purposes in a continuously operated cyclic bioreactor was described with a mathematical model. The model assumes that all required nutrients except for the two substitutable resources are present in the reactor in excess at all times. Hence, the kinetics of the process depend only on the availability of the two substitutable substrates. Cyclic operation is imposed by the periodic harvesting of a fraction of the microbial suspension and replenishment of the harvested volume with an equal volume of fresh medium. The proposed model has been experimentally validated for its basic predictions by using a relatively simple system involving a pure culture of Pseudomonas putida (ATCC 17514) and media containing phenol and glucose as carbon and energy sources. The mathematical model was subjected to a detailed analysis of its dynamics through the use of computer algorithms based on the bifurcation theory for forced systems. It was found that self-substrate and cross-substrate inhibition terms which introduce nonlinearities in the denominator of the specific growth-rate expressions lead to regions of multistability in the operating parameter space. The effects of all five operating parameters, namely, concentration of the two substrates in the medium, dilution rate, fraction of reactor contents harvested, and fraction of cycle time devoted to replenishing the harvested suspension with fresh medium, on the dynamics of the system were investigated, and the results are presented in the form of two-dimensional projections of the operating diagram. Optimization studies were also performed and their results show that, in most cases, operating parameters can be selected in ways which maximize the working capacity of the reactor while achieving a desired conversion of at least one of the substrates.

Evaluation of batch and semi-batch reactor operation for enzymatic reactions with inhibitory kinetics

Two enzymatic reaction sequences in which one of the intermediate products is the desired one were examined under conditions of batch and semi-batch reactor operation. Semi-batch operation involves a non-instantaneous filling phase during which reaction also occurs. The kinetics were assumed to involve substrate inhibition. The effect of various operating parameters -feeding rate, reactant concentration in the feed-stream, and minimum to maximum volume of reactor contents- on the performance of semi-batch operation was examined. It was found that operating parameters can be optimally selected in order to maximize the yield of the desired product. Semi-batch operation under optimal conditions was found to be superior to batch operation on the basis of desired product yield. Implications of the results for waste minimization in pharmaceutical and specialty chemicals production are discussed.

Biodegradation of Mixed Wastes, in Continuously Operated Cyclic Reactors

The problem of simultaneous biodegradation of two dissimilar substrates in a continuously operated cyclic reactor was studied both at the theoretical and experimental levels using a simple model system. The system involved media containing mixtures of glucose and phenol as carbon sources. A pure culture ofPseudomonas putida (ATCC 17514) was employed. Independent kinetic experiments have revealed that glucose and phenol are involved in a crossinhibitory uncompetitive kinetic interaction. The dynamics of a cyclically operated reactor were analyzed using the principles of bifurcation theory for forced systems. Experimental results have confirmed the theoretical predictions. Implications of the results for the design of waste-treating facilities are discussed.