M. Del Borghi

Effects of temperature, inoculum size and starch hydrolyzate concentration on butanediol production by Bacillus licheniformis

An optimization study has been performed on 2,3-butanediol production by Bacillus licheniformis NCIMB 8059 from different carbon sources (glucose, sucrose and cornstarch hydrolyzate), alternately varying temperature (34<T<40 degrees C), inoculum size (0.5<X(0)<10 gl(-1)), and starting substrate concentration (20<S(0)<70 gl(-1)). The results of average volumetric productivity obtained from tests at variable temperature have been worked out according to Arrhenius to estimate the thermodynamic parameters of both 2,3-butanediol formation (Deltah(*)=69.5 kJ mol(-1); Deltas(*)=-0.12 kJ mol(-1)K(-1)) and thermal inactivation equilibrium (Deltah( composite function )(D)=179 kJ mol(-1); Deltas( composite function )(D)=0.73 kJ mol(-1)K(-1)). The highest butanediol yield on starting glucose (Y=0.87 mol x mol(-1)) and average diol plus acetoin productivity (nu(av)=0.58 gl(-1)h(-1)) were obtained on cornstarch hydrolyzate at T=37 degrees C, pH 6.0, X(0)=10 gl(-1); S(0)=30 gl(-1) which suggests some stimulation in this raw material of the fermentative metabolism of B. licheniformis. Therefore, cornstarch hydrolyzate can be proposed as an alternative carbon source for industrial production of 2,3-butanediol with no need for growth factor addition.

Cadmium, Zinc, Copper, Silver and Chromium(III) removal from wastewaters by Sphaerotilus natans

Abstract Living cells of Sphaerotilus natans are used for heavy metals (Cd, Zn, Cu, Ag, and Cr) removal from aqueous solutions simulating the polluting power of acid industrial wastewaters. At low metal concentrations (<25 mg/l) this microorganism is able to remove within 8–15 days Cd, Zn, Cu, and Ag with excellent yields (from 81 to 99%) often increasing with starting metal concentration. The yield observed for Cr(III) removal, never exceeding 60%, is not appreciably influenced by the starting biomass level; in addition, the time necessary to reach the equilibrium concentration is always remarkably longer (>30 days) than for the other metals. At much higher concentrations, the removal of all the metals is strongly affected in terms of both yield reduction and increase in the time necessary to reach the equilibrium concentrations. Under the hypothesis of mass transfer limitation, the kinetic study of batch runs suggests that metal diffusion from the bulk to the surface of S. natans clumps could be responsible not only for the simple biosorption of the tested metallic micronutrients or abiotic metals, but even for the cell penetration by ions of biological significance, like Mg2+ and Fe3+.

A technique for solution of the equations for fluid—solid reactions with diffusion

Abstract The kinetic equations for several types of models for fluid—solid heterogeneous non-catalytic reactions with diffusional limitations have a similar mathematical structure. These coupled nonlinear partial differential equations possess very few analytical solutions, and are even difficult to solve numerically because of the “wave-like” nature of the concentration profiles. A transformation has been formulated that reduces these problems to a single, nonlinear diffusion-reaction equation in a new variable. Then, the multitude of results available for this type of problem can be used. With pseudo-steady-state conditions, the conversion of solid becomes analogous to an “effectiveness factor” in the transformed variable. For slab geometry, especially simple results are obtained, leading to analytical or semi-analytical results.

Xylitol recovery by crystallization from synthetic solutions and fermented hemicellulose hydrolyzates

Preliminary xylitol separation tests were carried out using solutions with relativity high concentrations of xylose and xylitol to simulate the actual composition of hemicellulose hydrolyzates. Xylitol was recovered by a crystallization methodology consisting of dilute solution evaporation up to supersaturation, supersaturated solutions cooling, separation of crystals by centrifugation, and final filtration. Two sets of tests were performed on xylitol–xylose synthetic solutions and an additional one on fermented hardwood hemicellulose hydrolyzate. The best results in terms either of crystallization yield (0.56) or purity degree (1.00) were obtained with quite concentrated solutions (730 g/l) at relatively high temperature (−5 ◦ C). Besides, xylitol solubility limits in the solution, which are very important for future scale-up of the process, were estimated at different crystallization temperatures. Product yields and crystal purity were calculated and crystallization kinetics were investigated.

Increase in phosphate removal from wastewater by alternating aerobic and anaerobic conditions

Abstract Increase in the efficiency of phosphorus removal from wastewater is becoming of outmost importance in order to comply with the quality standards foreseen by the international codes. In fact, efficiencies not exceeding 30% are usually ensured by traditional wastewater treatment plants through settling and removal of excess sludge. The Phostrip, Bardenpho and A O processes, recently developed and implemented in industrial plants, have allowed for substantial improvements in the biological removal of phosphate. Aim of the present paper is to describe how the efficiency of the original A O process has been optimized in a laboratory-scaled single basin, through an alternate sequence of aerobic and anaerobic phases. Using ratio of aerobic/anaerobic phases duration of 0.6, phosphorus removal yields higher than 90% have been ensured feeding wastewater containing phosphorus levels up to 100 mg/l.

The fluidized bed reactor in the anaerobic treatment of wine wastewater

The aim of the present work is the performance evaluation of a fluidized bed reactor in the anaerobic treatment of a wastewater deriving from the washing operations of the wine industry. The results are in agreement with the ones obtained using a mixture of municipal and food processing wastewaters containing high organic contents. A comparison with other liquid wastes shows that no subtrate inhibition phenomenon occurs with the above substrates. A saturation kinetic model is also presented for describing the dependence of the COD removal rate on the organic loading rate.

Evaluation of glucose diffusion coefficient through cell layers for the kinetic study of an immobilized cell bioreactor

Abstract The aim of this study is the evaluation of the diffusion coefficient of glucose through Saccharomyces cerevisiae biofilms, with thickness ranging from 0.02 to 1.60 mm, in order to carry out the macrokinetic study of alcoholic fermentations in immobilized cell bioreactors provided with matrices with varying porosity. Effective average diffusivities of 1.39 × 10 −6 and 1.44 × 10 −6 cm 2 s −1 have been calculated at 18 and 30°C, respectively, with no appreciable dependence on biofilm thickness. These values are about 20% of those calculated in water at the same temperatures. The glucose diffusion coefficient through the biofilm at 30°C has been used to carry out a comparison between diffusion, convection and bioreaction mass velocities along a fixed-bed column fed with starch hydrolysate solutions. Although diffusion through the biofilm is the limiting step, biomass grows so abundantly within the support pores at high residence time that the most superficial active layers of biofilm are enough to transform nearly completely the substrate fed. At low residence time the system is not able to stand an evident situation of substrate overloading.

Influence of organic loading rate on the anaerobic treatment of high strength semisynthetic waste waters in a biological fluidized bed

Abstract The results of anaerobic digestion in a fluidized bed reactor fed with municipal waste waters enriched with glucose are presented. Several process conditions have been tested by varying the influent chemical oxygen demand (COD) from 4 to 13 kg m −3 and the residence time from 5.0 to 68.2 h. The degradation efficiency and methane production rate are substantially affected by an increase in organic loading rate from 4 to 24 kgCOD i m −3 day −1 , suggesting a maximum operational value for this parameter. Using a Monod-type kinetic model, a maximum theoretical specific degradation rate of 1.76 kgCOD r kgVSS −1 day −1 has been calculated, which is very similar to values calculated for other effluents from food industry activities.

Acid hemicellulose hydrolysates: Physical treatments and continuous immobilized-cell fermentations

Hardwood hemicellulose acid hydrolysates, enriched in pentoses through countercurrent operations, have been fermented by Pachysolen tannophilus NRRL Y2460 in batch reactors. In order to evaluate the effects of pentose as well as inhibitor concentrations on the overall ethanol yield, several materials have been used for the physical treatment of this hydrolysate: Fly ash, a mixture of ionexchange resins, a porous material constituted by SiO2 and Al2O3 and a mixture of organic solvents. The best combination, consisting of an overliming pretreatment followed by a secondary treatment in a column filled with the Si-Al material, has assured an increase in ethanol concentration of 15%. Finally, the data of continuous fermentations of these hydrolysates in an immobilized-cell reactor are presented and discussed.

In-situ ethanol recovery and substrate recycling during continuous alcohol fermentation

In order to reduce the inhibiting effect of product on ethanol fermentation and to exploit at best the sugar substrate, a system continuously recycling the unfermented sugars and recovering produced ethanol is proposed in this paper. Unacceptable increases of unfermentable polysaccharides and ions in the broth up to inhibiting levels have been evidenced after about 40 d of continuous recycling. The accumulation of these substances has been overcome by installing in the production cycle two subsequent separated stages for polysaccharide enzymatic hydrolysis and ion bioaccumulation, respectively.