Stavros Pavlou

Hydrogenotrophic denitrification of potable water: A review

Several approaches of hydrogenotrophic denitrification of potable water as well as technical data and mathematical models that were developed for the process are reviewed. Most of the applications that were tested for hydrogenotrophic process achieved great efficiency, high denitrification rates, and operational simplicity. Moreover, this paper reviews the variety of reactor configurations that have been used for hydrogen gas generation and efficient hydrogen delivery. Microbial communities and species that participate in the denitrification process are also reported. The variation of nitrate concentration, pH, temperature, alkalinity, carbon and microbial acclimation was found to affect the denitrification rates. The main results regarding research progress on hydrogenotrophic denitrification are evaluated. Finally, the commonly used models and simulation approaches are discussed.

Single cell oil production from rice hulls hydrolysate.

Rice hull hydrolysate was used as feedstock for microbial lipids production using the oleaginous fungus Mortierella isabellina. Kinetic experiments were conducted in C/N ratios 35, 44 and 57 and the oil accumulation into fungal biomass was 36%, 51.2% and 64.3%, respectively. A detailed mathematical model was used in order to describe the lipid accumulation process. This model was able to predict reducing sugar and nitrogen consumption, fat-free biomass synthesis and lipid accumulation. Neutral lipids constitute the predominant lipid fraction, while the major fatty acids were oleic, palmitic and linoleic acid. Fatty acids of long aliphatic chain were not detected, thus the microbial oil produced is a promising feedstock for biodiesel production.

Semi-solid state fermentation of sweet sorghum for the biotechnological production of single cell oil.

A semi-solid fermentation process for the production of biodiesel from sweet sorghum is introduced. The microorganism used is the oleaginous fungus Mortierella isabellina, which is able to transform efficiently sugar to storage lipid. Kinetic experiments were performed at various water content percentages. The fungus consumed simultaneously sugars and nitrogen contained in sorghum and after nitrogen depletion the biomass growth was completed and oil accumulation began. Water content of 92% presented the highest oil efficiency of 11 g/100 g dry weight of substrate. The semi-solid process is shown to have certain advantages compared to liquid cultures or solid-state fermentation and gives oil of high quality.

Modeling of single‐cell oil production under nitrogen‐limited and substrate inhibition conditions

Sweet sorghum extract was used as substrate for lipid accumulation by the oleaginous fungus Mortierella isabellina in batch cultures. Various initial sugar (13–91 g/L) and nitrogen (100–785 mg/L) concentrations resulting in various C/N (43–53) ratios were tested. Oil accumulation ranged between 43% and 51% corresponding to oil production from 2.2 to 9.3 g/L. A detailed mathematical model was developed. This model is able to adequately predict biomass growth, lipid accumulation, and sugar and nitrogen consumption. The model assumes that fungus growth is inhibited at high sugar concentrations. A set of kinetic experiments was used for model kinetic parameters estimation, while another set of experiments was used for model validation. The developed model could be generalized for similar systems of lipid accumulation and become a useful tool for reactor design for biofuel production. Bioeng. 2011; 108:1049–1055.

Coexistence of three competing microbial populations in a chemostat with periodically varying dilution rate

Coexistence of three microbial populations engaged in pure and simple competition is not possible in a chemostat with time-invariant operating conditions under any circumstances. It is shown that by periodic variation of the chemostat dilution rate it is possible to obtain a stable coexistence state of all three populations in the chemostat. This is accomplished by performing a numerical bifurcation analysis of a mathematical model of the system and by determining its dynamic behavior with respect to its operating parameters. The coexistence state obtained in the periodically operated chemostat is usually periodic, but cases of quasi-periodic and chaotic behavior are also observed.

Development of a dynamic model describing nitritification and nitratification in trickling filters

A dynamic model which describes nitritification and nitratification in trickling filters has been developed. The model predicts the concentration profiles of ammonia, nitrite and nitrate along the filter depth and along the biofilm depth, as a function of the operating parameters, in batch as well as in continuous operation. The model can also predict the biofilm thickness as a function of filter depth and time. The batch version of the model predicts that the best moment to start the continuous operation is the moment when Nitrobacter concentration is maximum and nitrite concentration is minimum, while the continuous version of the model predicts that nitrite accumulation would be observed only when a significant ammonia concentration is observed at the filter outlet. A pilot-scale trickling filter was used in order to verify the predicted results. In all cases there was very good agreement between predicted and experimental results.

Biodegradation of p-aminoazobenzene by Bacillus subtilis under aerobic conditions

Biological oxidation of organic dyes is important for textile industry wastewater treatment. The aim of this work was to assess the biodegradation kinetics of a specific azo-dye, p-aminoazobenzene. The degradation of p-aminoazobenzene by Bacillus subtilis was examined through batch experiments in order to investigate the effect of p-aminoazobenzene on the bacterial growth rate and elucidate the mechanism of dye degradation. The results proved that B. subtilis cometabolizes p-aminoazobenzene in the presence of glucose as carbon source, producing aniline and p-phenylenediamine as the nitrogen–nitrogen double bond is broken. The azo-dye was found to act as an inhibitor to microbial growth. A mathematical model was developed that describes cellular growth, glucose utilization, p-aminoazobenzene degradation and product formation.

Chaotic dynamics of a food web in a chemostat

We analyze a mathematical model of a simple food web consisting of one predator and two prey populations in a chemostat. Monods model is employed for the dependence of the specific growth rates of the two prey populations on the concentration of the rate-limiting substrate and a generalization of Monods model for the dependence of the specific growth rate of the predator on the concentrations of the prey populations. We use numerical bifurcation techniques to determine the effect of the operating conditions of the chemostat on the dynamics of the system and construct its operating diagram. Chaotic behavior resulting from successive period doublings is observed. Multistability phenomena of coexistence of steady and periodic states at the same operating conditions are also found.

Visualization experiments of biodegradation in porous media and calculation of the biodegradation rate

Biodegradation in porous media is studied with carefully controlled and well-characterized experiments in model porous media constructed of etched glass. Porous media of this type allow visual observation of the phenomena that take place at pore scale. An aqueous solution of five organic pollutants (toluene, phenol, o-cresol, naphthalene and 1,2,3-trimethylbenzene) was used as a model NAPL (representing creosote). The bacteria used were Pseudomonas fluorescens, which are indigenous (even predominant) in many contaminated soils. The maximum aqueous concentrations of the specific organic substances, below which biodegradation becomes possible, were determined as a function of temperature from toxicity experiments. Visualization experiments were made under various flow velocities and organic loadings to study the morphology and thickness of the biofilm as a function of the pore size and the distance from the entrance, and the efficiency of biodegradation. The efficiency of biodegradation decreased as the aqueous concentration of NAPL at the inlet increased and/or as the flow velocity increased. The thickness of biofilm decreased as the distance from the inlet increased and/or the pore diameter decreased. A quasi-steady-state theoretical model of biodegradation was used to calculate the values of the mesoscopic biochemical rates and to predict the profile of NAPL concentration in the porous medium and the thickness of biofilm in pores. The agreement between experimental data and model predictions is quite satisfactory.

Chaotic dynamics of a microbial system of coupled food chains

We analyze a mathematical model of a simple microbial system consisting of two microbial populations competing for a single nutrient and two predator populations, each one feeding upon one competitor, in a chemostat. Monods model is employed for the specific growth rates of all the microbial populations. We use numerical bifurcation techniques to determine the effect of the operating conditions of the chemostat on the dynamics of the system and construct its operating diagram. We demonstrate that the system exhibits chaotic behavior and multistability. Two different routes to chaos are observed. Chaotic behavior is reached either through a sequence of period doublings or through birth and breaking of quasi-periodic states, as the operating conditions are varied. In some cases, transition from periodic to chaotic behavior is accompanied at certain parameter values by limit-point bifurcations of periodic states, the effect being multistablity, i.e. coexistence of stable periodic states with other stable periodic, quasi-periodic or chaotic states. The results demonstrate the importance of the interaction of food chains with regard to the dynamics exhibited by systems of microbial species inhabiting a common environment.