Kaushik Nath

Improvement of fermentative hydrogen production: various approaches

Fermentation of biomass or carbohydrate-based substrates presents a promising route of biological hydrogen production compared with photosynthetic or chemical routes. Pure substrates, including glucose, starch and cellulose, as well as different organic waste materials can be used for hydrogen fermentation. Among a large number of microbial species, strict anaerobes and facultative anaerobic chemoheterotrophs, such as clostridia and enteric bacteria, are efficient producers of hydrogen. Despite having a higher evolution rate of hydrogen, the yield of hydrogen [mol H2 (mol substrate−1)] from fermentative processes is lower than that achieved using other methods; thus, the process is not economically viable in its present form. The pathways and experimental evidence cited in the literature reveal that a maximum of four mol of hydrogen can be obtained from substrates such as glucose. Modifications of the fermentation process, by redirection of metabolic pathways, gas sparging and maintaining a low partial pressure of hydrogen to make the reaction thermodynamically favorable, efficient product removal, optimum bioreactor design and integrating fermentative process with that of photosynthesis, are some of the ways that have been attempted to improve hydrogen productivity. This review briefly describes recent advances in these approaches towards improvement of hydrogen yield by fermentation.

Modeling and optimization of fermentative hydrogen production.

Biohydrogen is a sustainable energy resource due to its potentially higher efficiency of conversion to usable power, non-polluting nature and high energy density. The purpose of modeling and optimization is to improve, analyze and predict biohydrogen production. Biohydrogen production depends on a number of variables, including pH, temperature, substrate concentration and nutrient availability, among others. Mathematical modeling of several distinct processes such as kinetics of microbial growth and products formation, steady state behavior of organic substrate along with its utilization and inhibition have been presented. Present paper summarizes the experimental design methods used to investigate effects of various factors on fermentative hydrogen production, including one-factor-at-a-time design, full factorial and fractional factorial designs. Each design method is briefly outlined, followed by the introduction of its analysis. In addition, the applications of artificial neural network, genetic algorithm, principal component analysis and optimization process using desirability function have also been highlighted.

Microbial regeneration of spent activated carbon dispersed with organic contaminants: mechanism, efficiency, and kinetic models

Background and purposeRegeneration of spent activated carbon assumes paramount importance in view of its economic reuse during adsorptive removal of organic contaminants. Classical thermal, chemical, or electrochemical regeneration methods are constrained with several limitations. Microbial regeneration of spent activated carbon provides a synergic combination of adsorption and biodegradation.MethodsMicroorganisms regenerate the surface of activated carbon using sorbed organic substrate as a source of food and energy. Aromatic hydrocarbons, particularly phenols, including their chlorinated derivatives and industrial waste water containing synthetic organic compounds and explosives-contaminated ground water are the major removal targets in adsorption–bioregeneration process. Popular mechanisms of bioregeneration include exoenzymatic hypothesis and biodegradation following desorption. Efficiency of bioregeneration can be quantified using direct determination of the substrate content on the adsorbent, the indirect measurement of substrate consumption by measuring the carbon dioxide production and the measurement of oxygen uptake. Modeling of bioregeneration involves the kinetics of adsorption/desorption and microbial growth followed by solute degradation. Some modeling aspects based on various simplifying assumptions for mass transport resistance, microbial kinetics and biofilm thickness, are briefly exposed.ResultsKinetic parameters from various representative bioregeneration models and their solution procedure are briefly summarized. The models would be useful in predicting the mass transfer driving forces, microbial growth, substrate degradation as well as the extent of bioregeneration.ConclusionsIntraparticle mass transfer resistance, incomplete regeneration, and microbial fouling are some of the problems needed to be addressed adequately. A detailed techno-economic evaluation is also required to assess the commercial aspects of bioregeneration.

Super phosphoric acid catalyzed esterification of Palm Fatty Acid Distillate for biodiesel production: physicochemical parameters and kinetics

Abstract In the present study the esterification of palm fatty acid distillate (PFAD), a by-product from palm oil industry, in the presence of super phosphoric acid (SPA) catalyst was studied. The effects of various physico-chemical parameters such as temperature, PFAD to methanol molar ratio and amount of catalyst on the conversion of biodiesel were investigated. The percent conversion of FFA and properties of the biodiesel were determined following standard methodologies. Percent conversion of biodiesel was found to increase with the increase in PFAD to methanol molar ratio and at 1:12 molar ratio and 70°C temperature 95% conversion was achieved. Thermodynamic parameters were also evaluated in terms of Gibbs free energy, enthalpy and entropy at different molar ratio and temperatures. Both pseudo first and second order irreversible kinetics were applied to a wide range of experimental data. However, according to regression coefficient (R2) the second order described better experimental behavior of kinetic data.

Modeling of permeate flux and mass transfer resistances in the reclamation of molasses wastewater by a novel gas-sparged nanofiltration

A semi-empirical model has been applied to predict the permeate flux and mass transfer resistances during the cross flow nanofiltration of molasses wastewater in flat-sheet module. The model includes laminar flow regime as well as flow in presence of gas sparging at two different gas velocities. Membrane hydraulic resistance (Rm), osmotic pressure resistance (Rosm) and the concentration polarization resistance (Rcp) were considered in series. The concentration polarization resistance was correlated to the operating conditions, namely, the feed concentration, the trans-membrane pressure difference and the cross flow velocity for a selected range of experiments. There was an appreciable reduction of concentration polarization resistance Rcpspar in presence of gas sparging. Both the concentration polarization resistance Rcplam and osmotic pressure resistance Rosm decreased with cross-flow velocity, but increased with feed concentration and the operating pressure. Experimental and theoretical permeate flux values as a function of cross flow velocity for both the cases, in the presence and absence of gas sparging, were also compared.

Mitigation of Flux Decline in the Cross-Flow Nanofiltration of Molasses Wastewater under the Effect of Gas Sparging

Gas sparging has been shown to significantly affect the performance of nanofiltration of molasses wastewater using a hydrophilized polyamide membrane (molecular weight cut-off 250) in a flat sheet module. Sparging of nitrogen at two different gas velocities was capable of appreciable alleviation of permeate flux decline over a period of time under the present experimental conditions when compared with the conventional unsparged nanofiltration. Critical flux (Jcrit), limiting flux (Jlim), and shear stress number (Ns) were determined at different cross-flow velocities for both the cases in the presence and absence of gas sparging. Increase in Ns with increasing cross-flow velocity was more pronounced in presence of gas sparging. Almost 1.5-2 fold increase in Ns was observed using cross-flow velocity of 0.8 ms−1 with gas sparging, compared to the case of “no sparging.” However, there was no significant improvement of rejection behavior of the solutes in presence of gas sparging. The value of mass transfer coefficient was considerably higher with increasing cross-flow velocity, while gas sparging was on, as against the case of “no sparging.” In terms of percentage increase in permeate flux with increase in trans-membrane pressure (TMP), gas sparging could achieve higher enhancement where concentration polarization is expected to be severe, that is, at high TMP and high feed concentrations. Some practical issues and technical limitations of gas sparging are also discussed.

Performance of polyamide and polyethersulfone membranes in the nanofiltration of reactive dye-salt mixtures on pilot scale

AbstractThin film composite polyamide (PA-NF) and polyethersulfone (PES-NF) membranes were used to study their performance towards reclamation of an aqueous solution of reactive red 198 and NaCl in a nanofiltration pilot plant. It was found that there were significant differences between the performance of the two membranes, and that the membrane properties played an important role in the dye removal rate, stable permeate flux, and salt rejection of the membrane. The experimental results showed that both the permeate flux and observed rejection decreased with an increase in dye as well as salt concentration in the feed. The maximum flux obtained with PA-NF membrane at 12.5 mg/l feed concentration was 11.4 × 10−6 m3/m2 s, whereas with PES-NF the flux was 5.5 × 10−6 m3/m2 s. The flux decline as a function of time for PES-NF membrane was much less compared to PA-NF. The contact angles of PA-NF and PES-NF membranes were found out to be 52o and 73o, respectively. Atomic force microscopy images revealed that th...

Analysis of molar flux and current density in the electrodialytic separation of sulfuric acid from spent liquor using an anion exchange membrane

Separation of sulfuric acid from a dilute solution involved a plate and frame type electrodialysis unit using a commercial anion exchange membrane. Experiments were conducted in batch with catholyte concentrations ranging from 1 to 5 wt%. Effect of applied current density, initial catholyte concentration and initial concentration difference of catholyte and anolyte on the molar flux was studied extensively. The maximum molar flux was estimated to be 10.52×10-8 mol cm-2s-1 at 4.45 wt% catholyte concentration and applied current density of 30 mA cm-2. Current efficiencies were observed to be 75 to 85% at lower current density, which rose to more than 100% at 20 and 30mA cm-2, at equal initial concentration of catholyte and anolyte. Diffusive flux and flux due to membrane potential contributed very less compared to total flux in presence of applied electric current. An equation was developed to predict the practical molar fluxes, which fitted satisfactorily with minor standard deviation. Pristine and used membrane specimens were characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).