Mauro Rovatti

MERCURY REMOVAL FROM WATER BY ION EXCHANGE RESINS ADSORPTION

In this paper a detailed experimental and theoretical analysis of the adsorption process of mercury by ion exchange resins is presented. Experiments have been performed to study adsorption efficiency, the effect of pH on the uptake of mercury and the adsorption kinetics. The experimental apparatus was a batch mechanically stirred reactor (volume 1.5 dm3), under almost isothermal conditions (±0.1°C). The resin used in this study is Duolite GT-73, a chelating resin, macroreticular with thiol (S-H) functional groups. The studied resin has a very high adsorption efficiency, reaching 30–40% in weight and the efficiency decreases, decreasing pH, due to competition between ions H+ and Hg2+. Such a high efficiency confirms previous results and justifies the great interest for the application of ion exchange resins in water treatment plants. As in previous works, measurements of process kinetics show that the adsorption rate decreases as the initial mercury concentration is increased. This fact suggests that intraparticle diffusion rate can be the controlling step for the adsorption process. To verify this, a simplified mathematical model has been identified, accounting for a diffusional resistance inside solid particles and where the equilibrium relationship between Hg concentration in the liquid and in the solid is described by the Freundlich isotherm, neglecting H+ competition: this model is very effective in the prediction of the change in the adsorption kinetics with the initial Hg concentration. Hence this preliminary approach can be held as the reference starting point for the adsorption model: further developments will concern the equilibrium thermodynamics (H+ competition).

Biological removal of phosphorus from wastewaters by alternating aerobic and anaerobic conditions

Abstract A new process alternating aerobic and anaerobic conditions has been tested for continuous removal of phosphorus from wastewaters. Preliminary batch runs have been carried out at different temperatures (5 ÷ 35°C) to study the effect of stress related to changes of temperature, inoculating a synthetic medium with the biomass from the same plant operating in continuous mode. The continuous system, which appeared capable of ensuring phosphorus removals higher than 90% at relatively high phosphate levels, is now studied mainly during the transitions between aerobic and anaerobic stages. The present results show, for the first time, a direct influence of dissolved oxygen level on pH and allow reasonable hypotheses on the mechanisms for both carbon and phosphorus routes.

Kinetic and process considerations on biological reduction of soluble and scarcely soluble sulfates

Abstract Discharging of sulfur compounds, which may occur in both liquid and solid phases, can cause severe environmental pollution problems. Biological sulfate removal processes are demonstrated to be suitable even for industrial effluents or industrial derived sulfate-rich solutions. In this work a culture from anaerobic treatment plant has been employed to reduce soluble sulfate (Na2SO4) and scarcely soluble sulfate (CaSO4) using propionate as carbon source. The Pr−/SO42− inlet ratio influence on the process was studied and when a complete selection of sulfate reducing bacteria (SRB) reached a ratio value equal to the stoichiometrical one, it was registered (about 1). A removal yield higher than 98% with a hydraulic retention time of 2 days was observed. The increased selection in SRB biomass allowed a sulfate reduction rate of 0.7 kgSO4/m3 per day which corresponds to a vmax of 3.2 gSO4/gvss per day and to a Ks of 24.5 mgSO4/l.

Terminal settling velocity and bed-expansion characteristics of biofilm-coated particles.

Fluid dynamic behavior of biofilm-coated particles in ambient water has been investigated. New experimental results are presented and compared with published data. From experimental measurements of the single particle terminal settling velocity the corresponding drag coefficient was found to be larger (by a factor of 1.6) than that for a smooth, rigid sphere at the same Reynolds number. A new simple correlation describing this finding is suggested. For multiparticle systems the Richardson-Zaki equation, derived empirically for rigid particles, provided a satisfactory description of biological beds. Of the two numerical parameters characterizing the expansion law, i. e. the slope n and the extrapolation to voidage equal one ui, the first was found to be similar to that suggested by Richardson and Zaki (1954), whereas ui gave results smaller than the single-particle terminal settling velocity, in contrast with the mentioned work but in agreement with more recently published behavior.

Mathematical modeling of monolith reactors for photocatalytic oxidation of air contaminants

A distributed parameter model for photocatalytic oxidation of air contaminants in monolith reactors is presented. Heat and mass balance equations for monolith structure are combined with a model of irradiation from a light source and a kinetic model for photon adsorption and chemical reaction to describe the processes of heat, mass and photon transfer within the system and the heterogeneous chemistry at the catalyst surface. The model accounts for interaction between light and matter at the catalyst surface, convective and interphase gas-solid heat and mass transfer, reaction at the catalyst surface and heat conduction within the solid structure. Together with detailed axial profile of temperature and conversion in the gas phase and at the catalyst surface under different operating conditions (inlet gas temperature, composition and flow rate, light source power, monolith geometry), the model provides the distribution of photon flux along the channels and allows the discrimination between thermal and pure photonic effect on the overall rate of conversion.

Mechanisms of biofilm detachment in fluidized bed reactors

Biofilm detachment in liquid fluidized bed biological reactors was investigated to point out how different mechanisms influence the process. Erosion due to liquid shear and abrasion due to collisions of particles were considered as possible mechanisms of biomass detachment in liquid fluidized beds. A dimensional analysis technique allowed the identification of the significant parameters affecting the process. The influence of these parameters was established on a lab-scale reactor. An empirical model was proposed to correlate the experimental data and to analyze the effect of some characteristic quantities, such as particle Reynolds number, biomass fraction, liquid shear stress and solid concentration, on the detachment rate. Detachment rate strongly increased with fluid velocity while, owing to modifications in biofilm structure and morphology during the biological growth, it slightly decreased with liquid shear stress.

An experimental model of biofilm detachment in liquid fluidized bed biological reactors.

Dimensional analysis was applied for the description of biofilm detachment in liquid fluidized bed biological reactors. This technique allowed the identification of the significant parameters influencing detachment mechanisms and suggested suitable experiments for the characterization of involved phenomena. The influence of the significant variables was established on a lab-scale reactor and an empirical model was proposed to correlate experimental results. The detachment rate was strongly dependent on liquid velocity, while the influence of other parameters, such as solid hold-up and liquid shear stress, was found to be less important.

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.

Biomass concentration in fluidized bed biological reactors

Synthetic wastewater treatment was carried out in a fluidized bed biological reactor with sand as support media. Biomass concentration in the bed was measured as total attached solid and recorded together with the bed expansion characteristics throughout the period of operations. A new method to estimate the reactor biomass concentration was developed and tested on the collected data. The method combines the Richardson and Zaki (1954) law for fluidized bed expansion, the definition of overall bed voidage and available literature data on biofilm dry density and gives an estimate of the biomass concentration by means of a non-linear minimization routine using experimental data of bed height versus fluid superficial velocity. The comparison between the values derived from the experimental measures and those calculated by means of the method has proved reasonably successful.

Influence of the ratio of the initial substrate concentration to biomass concentration on the performance of a sequencing batch reactor

Biomass behaviour and COD removal in a benchscale activated sludge reactor have been studied alternating anaerobic and aerobic conditions. Particular attention has been paid to the influence of the ratio of the initial substrate concentration (S0) to the initial biomass concentration (X0) on the reactor performance. Tests at very low ratios (S0/X0<2) demonstrate the existence of a threshold below which the reactor performance is seriously affected (S0/X0=0.5). Under conditions of total suppression of cell duplication, substrate maintenance requirements have also been calculated for the microbial consortium present in the activated sludges. The results obtained show that stressed biomass can survive conditions of substrate lack better than unstressed biomass.