C.A. Kent

Biodegradation of potato slops from a rural distillery by thermophilic aerobic bacteria

A study has been made of thermophilic aerobic biodegradation of the liquid fraction of potato slops (distillation residue) from a rural distillery. The COD of this fraction ranged from 49 to 104 g O2/l, the main contributions to the COD coming from organic acids, reducing substances, and glycerol. It was found that biodegradation could be divided into the following stages: organic acids were removed first, followed by reducing substances and glycerol. The extent of removal varied according to the process temperature. At 50 degrees C, acetic and malic acids were removed completely, but the amount of isobutyric acid increased. At 60 degrees C, organic acid removal ranged from 51.2% (isobutyric acid) to 99.6% (lactic acid). Removals of glycerol and reducing substances were 86.2% and 87.4%, respectively. COD reduction was also temperature dependent, the highest removal efficiency (76.7%) being achieved at 60 degrees C. Dissolved oxygen may have limited the biodegradation process, as indicated by the DOT-versus-time profile.

Biomass production for the removal of heavy metals from aqueous solutions at low pH using growth-decoupled cells of a Citrobacter sp.

Abstract A Citrobacter sp. accumulates heavy metals via the activity of an acid-type phosphatase that produces inorganic phosphate, HPO42−. This ligand precipitates with heavy metals (M) as MHPO4, which is retained at the cell surface. Continuous metal deposition has been coupled to the removal of heavy metals from metal-laden solution. The pH optimum of the mediating phosphatase is 5·0–8·0, with 55% and 70–80% retention of activity at pH 4·0 and 4·5, respectively. Metal accumulation was reduced at pH 5·0, attributable to increased metal phosphate solubility and reduced metal phosphate precipitation, but this was overcome using cells of higher phosphatase activity. A 3·25-fold overproduction of enzyme compensated for a 100-fold increase in the concentration of H+. Preliminary tests enabled prediction of the increased phosphatase activity required to treat a target waste stream containing uranyl ion at pH 4·5. Enzyme over-production was achieved by growth of a phosphatase constitutive variant in a lactose-based medium, but enzyme activity was reduced at the high carbon concentration required for a high biomass yield. The latter requirement was fulfilled with enhanced enzyme production by the use of fed-batch culture, with substrate addition regulated via feedback analysis of the off-gases. The biomass removed uranyl ion efficiently from a challenge solution at pH 4·5 in a batch contactor. Lactose-grown immobilized cells also removed uranyl ion from an acidis simulated industrial wastewater.

A genetic algorithm based approach to intelligent modelling and control of pH in reactors

Abstract The present work reports a novel genetic algorithm (GA) based strategy for designing efficient ‘global’ pH controllers in highly nonlinear neutralisation reactors, wherein linear internal model control (IMC) methodology and generic nonlinear compensators defined from acid–base principles are applied. In the study, the GA was used to optimise the IMC and nonlinear compensator parameters by evaluating a first-principles model developed to simulate a pH reactor. The proposed methodology was tested for assumed first- and second-order IMC transfer function models, initially for generality on a highly buffered pH reactor. A variant was developed for the problem of severe reactor nonlinearity, by including one of four defined nonlinear compensators in the optimisation process. Simulation results showed that the general use of the genetic algorithm based internal model control (GAIMC) strategy automated controller tuning that improved control performance, while its use with nonlinear compensators for the severely nonlinear reactor achieved tighter pH control for multiple operating regimes.

A structured model for penicillin production on mixed substrates

Abstract A structured kinetic model previously developed to describe the growth, differentiation, and penicillin production of Penicillium chrysogenum has been enhanced and extended in order to apply it to a mixed carbon source fermentation. The filamentous hyphae are divided into four distinct regions on the basis of their activities and the physiological structure (i.e., vacuolation) of the hyphal compartments: viz., actively growing (mainly apical) regions, non-growing or penicillin producing regions, vacuoles, and degenerated or metabolically inactive regions. A simple approach is taken to give quantitative descriptions of hyphal extension, branch formation, vacuolation and differentiation. The fermentation medium contained glucose and lactose monohydrate as the main carbon sources. The source of the lactose was whey powder used in excess in the inoculum medium, whilst glucose was fed continuously throughout the fermentation. Lactose, a disaccharide, is hydrolysed to two monosaccharides, glucose and galactose, when the residual glucose concentration in the medium drops to a very low level. The utilisation of glucose and that of galactose following the hydrolysis of lactose were observed to occur simultaneously. This allowed the assumption of simple lactose utilisation kinetics in which lactose hydrolysis could be considered as producing an equivalent amount of glucose. The model has been used for successful predictions of fed-batch penicillin fermentations using an industrial P. chrysogenum strain under different glucose feed rates. Quantitative information on proportions of the hyphal regions was obtained from image analysis measurements and the parameters of the model were identified. When the glucose feed rate to the production culture was switched between a high and a low value, the model successfully predicted the dynamic changes of differentiation and the resulting penicillin production caused by the variations in the nutrient conditions. The use of image analysis to characterise differentiation as a basis for structured modelling of the penicillin fermentation appears to be very powerful, and such models have great potential for use in process simulation and control of antibiotic fermentations.

Effect of temperature on the efficiency of the thermo- and mesophilic aerobic batch biodegradation of high-strength distillery wastewater (potato stillage)

The objective of the study was to assess the effect of temperature on the extent of aerobic batch biodegradation of potato stillage with a mixed culture of bacteria of the genus Bacillus. The experiments were performed in a 5-l stirred-tank reactor at 20, 30, 35, 40, 45, 50, 55, 60, 63 and 65 degrees C with the pH of 7. Only at 65 degrees C, no reduction in chemical oxygen demand (COD) was found to occur. Over the temperature range of 20-63 degrees C, the removal efficiency was very high (with an extent of COD reduction following solids separation that varied between 77.57% and 89.14% after 125 h). The process ran at the fastest rate when the temperature ranged from 30 to 45 degrees C; after 43 h at the latest, COD removal amounted to 90% of the final removal efficiency value obtained for the process. At 20, 55, 60 and 63 degrees C, a 90% removal was attained after 80 h. Two criteria were proposed for the identification of the point in time when the process is to terminate. One of these consists in maximising the product of the extent of COD reduction and the extent of N-NH4 content reduction. The other criterion is a simplified one and involves the search for the minimal value of N-NH4 concentration.

Thermophilic bioremediation strategies for a dairy waste

Two double-staged strategies for cheese whey bioremediation are described and compared, involving the use of indigenous thermotolerant microorganisms found in whey in the first stage, followed by a second stage employing an added thermophilic mixed population of Bacillus sp., isolated from a fruit-and-vegetable waste. In strategy 1, both stages were operated mesophilically, whereas in strategy 2, the second stage involved thermophilic conditions. Strategy 2 produced greater reductions in COD, lactose, and protein than the solely mesophilic strategy, whilst operating at twice the rate. Reaction schemes for both the stages are proposed.

Thermophilic bioremediation of whey: effect of physico-chemical parameters on the efficiency of the process

A two-stage process for the bioremediation of blue Stilton whey has been developed. It employs both naturally occurring thermotolerant organisms found in whey (lactic acid bacteria and yeast) and a thermophilic isolate (Bacillus sp.). Thermophilic digestion occurred only at neutral pH. Multiple substrates were consumed simultaneously under mesophilic but not thermophilic conditions.

Evaluation of cell numbers and viability of Saccharomyces cerevisiae by different counting methods

Four counting methods (two flow cytometry, one Coulter principle, one microscopic) are compared for measuring cell density and viability of batch-grown yeast. All gave adequate precision in measuring total cell density with no systematic difference between methods. However, the promise of flow cytometry as a rapid means of determining both quantity and quality of a cell population is shown.

An Approach to Robust and Flexible Modelling and Control of pH in Reactors

Preliminary investigations into the potential application of static feed forward neural networks in the dynamic modelling of pH in complex, time-varying systems have been carried out. To assist in network training and testing, a simplified, ‘global first principles’ (FP) model of the pH of such systems was developed, and used successfully to simulate input-output data. Neural networks with input information vectors enhanced by the introduction of auxiliary variables derived from acid-base principles were trained and tested on this data, using both Levenberg-Marquardt (L-M) and heuristic training algorithms. Both algorithms produced good predictions, but the heuristic algorithm required data pre-treatment to minimize its error. However, it trained much faster than the standard, L-M algorithm.

Towards the design of an optimal strategy for the production of ergosterol from Saccharomyces cerevisiae yeasts

The total yield of ergosterol produced by the fermentation of the yeast Saccharomyces cerevisiae depends on the final amount of yeast biomass and the ergosterol content in the cells. At the same time ergosterol purity—defined as percentage of ergosterol in the total sterols in the yeast—is equally important for efficient downstream processing. This study investigated the development of both the ergosterol content and ergosterol purity in different physiological (metabolic) states of the microorganism S. cerevisiae with the aim of reaching maximal ergosterol productivity. To expose the yeast culture to different physiological states during fermentation an on‐line inference of the current physiological state of the culture was used. The results achieved made it possible to design a new production strategy, which consists of two preferable metabolic states, oxidative‐fermentative growth on glucose followed by oxidative growth on glucose and ethanol simultaneously. Experimental application of this strategy achieved a value of the total efficiency of ergosterol production (defined as product of ergosterol yield coefficient and volumetric productivity), 103.84 × 10−6 g L−1h−1, more than three times higher than with standard bakers yeast fed‐batch cultivations, which attained in average 32.14 × 10−6 g L−1h−1. At the same time the final content of ergosterol in dry biomass was 2.43%, with a purity 86%. These results make the product obtained by the proposed control strategy suitable for effective down‐stream processing.