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Electrocoagulation (EC) process was used as an alternative pretreatment in order to assess its applicability to replace the conventional pretreatments used to mitigate membrane fouling prior to seawater desalination by reverse osmosis process, such as chemical coagulation, chlorination and scale inhibitors. Electrocoagulation was conducted in a batch cell using aluminum electrodes driven in the galvanostatic mode. Absorbance (UV 254nm ) and DOC were measured to follow the effectiveness of EC process for removing organic matter from seawater. Likewise, cultivable heterotrophic bacteria were enumerated to assess the disinfection ability of the process and total hardness was monitored. The influence of various operating parameters, namely mixing speed, current density, initial pH and inter-electrode distance, was investigated. The removal of organic matter from seawater by electrocoagulation was improved with higher current density and lower pH. Electrocoagulation removed 70.8% DOC, while absorbance abatement was 89.7% with a complete removal of microorganisms at high current density. Conversely, the abatement of total hardness was weak, about 10%, so that electrocoagulation could not be used as a softening process. Finally, experimental results showed the high potential of electrocoagulation as a pretreatment method to mitigate potential organic fouling and biofouling of reverse osmosis membrane owing to its ability to remove effectively dissolved organic matter and microorganisms from seawater. [ABSTRACT FROM AUTHOR]

Abstract: The decolorization of a synthetic textile wastewater was investigated using electrocoagulation (EC) in a batch electrochemical cell. Orange II, a typical mono-azo acid dye, was used. Aluminum and iron electrodes were compared. The respective effects of operational parameters (initial dye concentration, current density, electrolysis time) and wastewater properties (initial pH and conductivity) were analyzed by comparing the performance of color removal, energy input, electrode mass consumption and the amount of sludge produced. Operating costs were deduced. A comparison with chemical coagulation (CC) using the same amount of metal cations as in EC was also carried out. Experimental results showed that EC maximized decolorization (up to 98%) in comparison to CC (limited to 53%). For EC, iron electrodes exhibited the highest decolorization yield and minimized simultaneously energy requirements, the amount of floc and operation costs in comparison to aluminium. As voltage vs. current curves did not differ significantly between Al and Fe, higher current was required with Al electrodes to achieve a similar decolorization yield. This was partly explained by differences in the decolorization mechanism, as Al electrodes seemed to promote the electroreduction of the azo bond of Orange II, contrary to Fe electrodes. [Copyright &y& Elsevier]

Abstract: A detailed study of the residence time distribution (RTD) analysis of liquid phase has been performed in an external-loop airlift reactor of 20L nominal volume, regarded as a global unit and discriminating its different sections (riser, gas–liquid separator and downcomer) using the tracer response technique. The reactor was used as an electrochemical reactor in order to carry out the electrocoagulation/electroflotation (EC/EF). The gas phase created in the riser is the hydrogen produced by water electrolysis. In order to use this reactor for a continuous EC/EF, hydrodynamic studies were carried out to control the operating conditions and to help modelling the electrocoagulation. Current density, position of the electrodes in the riser and the volumetric liquid flow (inlet flow) are the key parameters for the hydrodynamics. The experimental results revealed that both in the downcomer and the riser–separator zones, the flow model is axial dispersion. Interesting results were obtained: [–] The superficial liquid velocity (U Ld ) at the downcomer, decreased when the volume inlet flow increased (0

Abstract: The effectiveness of chemical coagulation (CC) was compared to electrocoagulation (EC) with aluminium electrodes for decolourization purpose of a synthetic textile wastewater containing a disperse red dye. For CC, ferric chloride FeCl3 and aluminium sulphate Al2(SO4)3 as the coagulant were compared: the respective effects of initial pH, coagulant dosage, initial dye concentration, ionic strength and mixing conditions were investigated in order to maximize decolourization yield. The comparison between CC and EC is based on recently published data on EC by the same authors. Experimental results showed first that Al2(SO4)3 was far more effective than FeCl3 for colour removal using CC, regardless of operating conditions. A removal yield higher than 90% could be achieved with a 40mg/L dose of Al2(SO4)3 18H2O in a large range of pH from 4 to 8 and for a dye concentration up to 235mg/L. The removal yield could however be enhanced up to 95% using EC for pH values between 6 and 9 at the expense of higher operating costs. Nevertheless, EC presented the additional advantages to be more robust against pH change and to reduce simultaneously equipment costs in comparison to CC. [Copyright &y& Elsevier]

Abstract: A kinetic study of defluoridation of drinking water was carried out using the electrocoagulation/electroflotation process in two batch reactors of identical volume (20L): a stirred tank reactor (STR) and an external-loop airlift reactor (ELALR). When the evolution of fluoride content was independent of stirring speed, experimental results showed that the kinetics of fluoride removal could be modelled using a variable-order-kinetic (VOK) approach coupled with a Langmuir–Freundlich adsorption model in the STR. Conversely, when mixing was less efficient, which is the case in the ELALR, experimental data could be fitted adequately only using a pseudo-first-order model. This constitutes however only an empirical approach based on a lumped parameter that accounts simultaneously for mass transfer, adsorption and electrochemical steps. In this case, only regression analysis could be used to establish a quantitative relationship between the kinetic constant and the operating conditions, such as current density and initial fluoride concentration. [ABSTRACT FROM AUTHOR]

Abstract: Defluoridation using batch electrocoagulation/electroflotation (EC/EF) was carried out in two reactors for comparison purpose: a stirred tank reactor (STR) close to a conventional EC cell and an external-loop airlift reactor (ELAR) that was recently described as an innovative reactor for EC. The respective influences of current density, initial concentration and initial pH on the efficiency of defluoridation were investigated. The same trends were observed in both reactors, but the efficiency was higher in the STR at the beginning of the electrolysis, whereas similar values were usually achieved after 15min operation. The influence of the initial pH was explained using the analyses of sludge composition and residual soluble aluminum species in the effluents, and it was related to the prevailing mechanisms of defluoridation. Fluoride removal and sludge reduction were both favored by an initial pH around 4, but this value required an additional pre-treatment for pH adjustment. Finally, electric energy consumption was similar in both reactors when current density was lower than 12mA/cm2, but mixing and complete flotation of the pollutants were achieved without additional mechanical power in the ELAR, using only the overall liquid recirculation induced by H2 microbubbles generated by water electrolysis, which makes subsequent treatments easier to carry out. [Copyright &y& Elsevier]

Abstract: The performance of a continuous electrocoagulation (EC) process was investigated for decolorization and chemical oxygen demand (COD) abatement of a synthetic textile wastewater using aluminum electrodes. The effects of the relevant wastewater characteristics such as conductivity, influent pH and inlet dye concentration, but also of the key operating conditions such as current density and residence time were studied in order to optimize process performance. The results showed that color induced by a red dye was effectively removed (with a removal yield higher than 85%) for wastewater with a COD of 2500mg/L and a dye concentration lower than 200mg/L when pH ranged from 6 to 9, residence time was 14min, current density was 31.25mA/cm2 and water conductivity was 2.4mS/cm for an inter-electrode distance of 1cm. Under these conditions, the COD abatement was also higher than 80%. [Copyright &y& Elsevier]

The electrocoagulation (EC) process was developed to overcome the drawbacks of conventional wastewater treatment technologies. This process is very effective in removing organic pollutants including dyestuff wastewater and allows for the reduction of sludge generation. The purposes of this study were to investigate the effects of the operating parameters, such as pH, initial concentration (C 0), duration of treatment (t), current density (j), interelectrode distance (d) and conductivity (κ) on a synthetic wastewater in the batch electrocoagulation–electroflotation (EF) process. The optimal operating conditions were determined and applied to a textile wastewater and separation of some heavy metals. Initially a batch-type EC–EF reactor was operated at various current densities (11.55, 18.6, 35.94, 56.64, 74.07 and 91.5mA/cm2) and various interelectrode distance (1, 2 and 3cm). For solutions with 300mg/L of silica gel, high turbidity removal (89.54%) was obtained without any coagulants when the current density was 11.55mA/cm2, initial pH was 7.6, conductivity was 2.1mS/cm, duration of treatment was 10min and interelectrode distance was 1cm. The application of the optimal operating parameters on a textile wastewater showed a high removal efficiency for various items: suspended solid (SS) 86.5%, turbidity 81.56%, biological oxygen demand (BOD5) 83%, chemical oxygen demand (COD) 68%, and color over 92.5%. During the EC process under these conditions, we have studied the separation of some heavy metal ions such as iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), lead (Pb) and cadmium (Cd) with different initial concentrations in the range of 50–600mg/L and initial pH between 7.5 and 7.8. This allowed us to show that the kinetics of electrocoagulation–electroflotation is very quick (<15min), and the removal rate reaches 95%. [Copyright &y& Elsevier]

Abstract: An innovative application of external-loop airlift reactors as electrocoagulation/electroflotation cells with Al electrodes for defluoridation of drinking water was developed. Liquid overall recirculation and mixing were induced only by hydrogen microbubbles electrochemically generated from the cathode. This application was carried out in a 20L external-loop airlift reactor both under semi-batch and continuous flow conditions. Results showed that liquid recirculation could be correlated to current density and gas–liquid dispersion height in the separator. Experimental data obtained at optimum conditions that favored simultaneously mixing and flotation confirmed that concentrations lower than 1.5mg/L could be achieved when initial concentrations were between 10 and 20mg/L. The effects of conductivity and pH agreed with the literature. Conversely, the low electrode surface vs. reactor volume ratio merged the formation of fluoroaluminum microflocs near the electrodes to fluoride adsorption on these particles in the riser and the separator sections, which differed from conventional EC cells. Consequently, defluoridation could be achieved at lower energy and electrode consumptions than in the literature. An optimum current density was defined at j =6mA/cm2 for pH 5, accounting simultaneously for mixing, reaction time, yield and operating costs. A promising attempt of transposition from batch to continuous process was also reported in this work, as flotation avoids the need for a downstream settling unit. [Copyright &y& Elsevier]

Abstract: Although ferrous iron removal from drinking water by aeration has been studied for a while, there is still an uncertainty on the performance and the physicochemical mechanisms of iron(II) oxidation [S.K. Sharma, B. Petrusevski, J.C. Schippers, J. Water. Supply Res. Technol. Aqua, 54 (2005) 239–247]. A possible reason is the autocatalytic effect of ferric hydroxide particles, but this assumption is never validated quantitatively in practice because this catalytic effect has been investigated only under batch laboratory-controlled conditions. In this work, iron(II) oxidation has been studied on synthetic waters in a 63L split-rectangular airlift reactor, the hydrodynamics and the mass transfer properties of which were described previously [N. El Azher, B. Gourich, C. Vial, M. Belhaj Soulami, A. Bouzidi, M. Ziyad, Biochem. Eng. J., 23 (2005) 161–167]. Experiments were carried out both under semi-batch and continuous flow conditions. The kinetic parameters derived from the experiments were consistent with the literature both under unsteady and steady-state conditions. Experimental results showed that the airlift reactor allowed simultaneously good mixing, mass transfer and pH control despite the strong sensitivity of the oxidation kinetics to pH. Data confirmed that recycling about 50mg/L of ferric hydroxide particles in a slurry phase could decrease drastically the time necessary to reach the minimum admissible concentration of iron(II). For example, residence time could be reduced by a factor six at pH 7.0 under steady-state conditions, which may avoid the need for a further pH increase. [Copyright &y& Elsevier]

Abstract: Hydrodynamic parameters (gas-induced flow rate and gas hold-up) and mass transfer characteristics (k L a, k L and a) have been investigated in a gas–liquid reactor denoted “Emulsair” in which the distributor is an emulsion-venturi and the gas phase is self-aspired by action of the kinetic energy of the liquid phase at the venturi throat. Two configurations, respectively cocurrent downflow and cocurrent upflow were compared. A chemical method involving the dispersion of a CO2–air mixture in a monoethanolamine (MEA) aqueous solution was used to measure mass transfer parameters. Experimental results showed that only the homogeneous bubbling regime prevailed in the upward configuration, while an annular regime could also be observed for cocurrent downflow at low liquid flow rate. Gas-induced flow rate and gas hold-up were usually smaller for cocurrent upflow, both at constant liquid flow rate and specific power input. The same stood for mass transfer properties. Conversely, specific power requirements were lower at constant liquid flow rate and mass transfer characteristics were enhanced at constant gas-induced flow rate for cocurrent upflow. A comparison with other gas–liquid contacting devices showed that the Emulsair reactor is a versatile tool avoiding the presence of mechanically moving parts when high and quickly adaptable dissolved gas supply is required. The cocurrent upflow configuration can be preferred when high gas flow rates are desired because the evolutions of gas-induced flow rate and mass transfer characteristics exhibit a stronger dependence on specific power input in the homogeneous bubbling regime for this configuration. [Copyright &y& Elsevier]

Abstract: The usual gassing-in and gassing-out method was applied in a high aspect ratio bubble column using both tap water and coalescence-inhibiting liquid mixtures that represent the coalescence behaviour of biological media. The oxygen concentration curves were analyzed on the basis of different hydrodynamic models available from the literature, taking into account several parameters such as sensor dynamics, mixing in both phases, oxygen depletion in the gas phase or the axial evolution of pressure in the column. The sensitivity of K L a L values to the assumptions of each model was examined first in relation to physical considerations, but also as a function of the sensitivity of the optimization procedure. The results showed that the conventional method consisting in cutting the beginning of the oxygen response curves along with an assumption of perfect mixed reactor had to be avoided both in tap water and coalescence-inhibiting mixtures, except at low superficial gas velocity. They showed also that this simple model could not be improved by taking sensor dynamics into account. Conversely, several characteristic times and a dimensionless pressure ratio were established in order to define general quantitative criteria able to determine the best model for reliable K L a L estimation from dynamic oxygen response curves with the aim to contribute to design and scale-up of bubble column bioreactors. [Copyright &y& Elsevier]

Abstract: The aim of this paper is first to investigate the influence of operating conditions and design parameters on the hydrodynamics and the mass transfer properties of an emulsion loop–venturi reactor and then to compare its performance to that of the conventional gas–liquid contactors, such as airlift and bubble column reactors or aerated stirred tanks. The results have shown that emulsion venturi reactors should be preferentially operated as gas-inducing devices. In this case, hydrodynamics and mass transfer depend strongly on the clear liquid height. This parameter exhibits an optimum value, as its increase improves mass transfer, but reduces the maximum gas-induced flow rate. Conversely, the respective influences of the tank diameter at constant clear liquid height when D >0.3m and of the surface area of the orifices at the venturi throat were not significant in this work. Finally, emulsion loop–venturi contactors appear to be versatile tools able to carry out both fast and slow chemical reactions, which is not the case for the three other kinds of reactors. They are therefore useful when high and quickly adaptable oxygen supply is required, as they avoid also the presence of mechanically moving parts and present lower shear levels than mechanically stirred tanks. [Copyright &y& Elsevier]

Abstract: An original procedure has been established for estimating the overall volumetric mass transfer coefficient using the oxygen concentration curves resulting from the usual gassing-in and gassing-out method. This procedure was applied to experimental data obtained in a small scale bubble column using both tap water and a coalescence-inhibiting liquid mixture that represents the coalescence behavior of biological media. It is based on the analysis of the characteristics times of the system, including those of the hydrodynamics of the two phases, the sensor dynamics and the system inertia when the gas composition is modified. A numerical procedure was developed to estimate the characteristic time of the system inertia , using the assumption that this inertia is nearly independent of superficial gas velocity . The calculations confirmed that the optimized value was nearly independent of and of the coalescence behavior of the liquid phase. Additionally, the resulting values for tap water were closer to the correlation of Shah et al. [1982. Design parameters estimations for bubble column reactors. A.I.Ch.E. Journal 28, 353–379] than those of other conventional models. Finally, the original procedure was also reported to reduce significantly the square sum deviation between the predicted and the measured oxygen response curves. [Copyright &y& Elsevier]

Abstract: Hydrodynamics and gas–liquid mass transfer have been investigated in an “Emulsair” reactor with cocurrent downflow of gas and liquid. This consists of a cylindrical tank with conical bottom topped by an emulsion-venturi as the gas–liquid distributor in which the gas is self-aspired by action of the kinetic energy of the liquid recirculation. An original tracer dynamic technique using the CO2–N2/water system that enables the simultaneous measurement of overall gas hold-up and overall volumetric mass transfer coefficient has been developed and validated using conventional techniques, such as volume expansion for gas hold-up and dynamic oxygenation for mass transfer. It has been shown that gas hold-up and can be deduced from the moments of experimental response curves using a CO2 pulse in the gas feed. Experimental results have proved that hydrodynamics and mass transfer in the Emulsair reactor are strongly influenced by the flow regime in the divergent. Two different regimes have been observed depending on the liquid recirculation flow rate: annular and homogeneous bubbling flows. In both regimes, self-aspired gas flow rate, gas hold-up and have been reported to increase with the liquid flow rate. The operation effectiveness, estimated from the gas-to-liquid flow rate ratio, has been shown to pass through a maximum around 0.59 as a function of liquid recirculation. A comparison with the literature proved that this maximum is higher than those observed for other kinds of gas–liquid reactors equipped with a venturi. Correlations for mass transfer estimation have been derived and are in agreement with the literature. [Copyright &y& Elsevier]

Abstract: Hydrodynamics and gas–liquid mass transfer in a rectangular airlift loop reactor with an internal baffle has been studied in this work. Measurements were carried out for two- and three-phase systems using air–water and air–water–polystyrene particles. The effects of the superficial gas velocity and solid loading up to 15% (v/v) were examined and correlations describing the hydrodynamics and gas–liquid mass transfer behaviours were proposed. The experimental results of mass transfer were compared with those of the literature. It was found in all cases that the gas hold-up in the riser and downcomer and volumetric mass transfer coefficient increased with increasing the superficial gas velocity, while the liquid velocity in the downcomer increased with U gr until 0.033ms−1 and then decreased to reach a plateau value for U gr >0.04ms−1. It was also found that the liquid velocity and the volumetric mass transfer coefficient decreased only with the initial addition of solid for U gr higher than 0.02ms−1 and that the gas hold-up in the downcomer was significantly influenced by the presence of solid for a concentration above 10% (v/v), while the gas hold-up was almost not affected in the riser. The aeration efficiency was also calculated and was found to be in the range 2.54–4.41kg O2/kWh. [Copyright &y& Elsevier]

Abstract: In this work, the effect of alcohol addition on gas hold-up, liquid circulation velocity and gas–liquid volumetric mass transfer coefficient was studied in a split-rectangular airlift bioreactor using air-water as a system to which propanol was added in concentrations ranging from 0.01 to 0.1% (v/v). In order to compare the effect of the propanol with other alcohols, methanol and butanol were also used. The experimental results showed that, at high superficial gas velocities, the addition of small amounts of alcohols decreases the difference between the gas hold-up in the riser and the downcomer, which is due to smaller bubble diameter than with the tap water system. This behaviour is enhanced by alcohols with long carbon chain lengths, which has been explained on the basis of surface tension effects. As a result, the addition of alcohol solutions results in a significant decrease in the liquid circulation velocity, which is mainly due to the decrease in the circulation driving force, but also in a strong decrease in the volumetric gas–liquid mass transfer coefficient for superficial gas velocities higher than 0.033m/s. This surprising result has been explained by a strong decrease in KL values due to the presence of alcohol and by oxygen depletion due to the increasing amount of small recycled bubbles at high superficial gas velocity. [Copyright &y& Elsevier]

Abstract: This paper studied the efficiency of electrocoagulation/electroflotation in removing colour from synthetic and real textile wastewater by using aluminium and iron electrodes in an external-loop airlift reactor of 20L. The disperse dye is a mixture of Yellow terasil 4G, Red terasil 343 150% and Blue terasil 3R02, the reactive dye is a mixture of Red S3B 195, Yellow SPD, Blue BRFS. For disperse dye, the removal efficiency was better using aluminium electrodes, whereas, the iron electrodes showed more efficiency for removing colour for reactive dye and mixed synthetic dye. Both for disperse, reactive and mixed dye, 40mAcm−2 and 20min were respectively the optimal current density and electrolysis time. 7.5 was an optimal initial pH for both reactive and mixed synthetic dye and 6.2 was an optimal initial pH for disperse dye. The colour efficiency reached in general 90%. The results showed also that Red and Blue disappeared quickly comparatively to the Yellow component both for reactive and disperse dyes. The real textile wastewater was then used. Three effluents were also used: disperse, reactive and the mixture. The colour efficiency is between 70 and 90% and COD efficiency reached 78%. The specific electrical energy consumption per kg dye removed (E dye) in optimal conditions for real effluent was calculated. 170kWh/kgdye was required for a reactive dye, 120kWh/kgdye for disperse and 50kWh/kgdye for the mixture. [ABSTRACT FROM AUTHOR]

Abstract: A 20L external-loop airlift reactor was used as an electrochemical cell in order to carry out water depollution using batch electrocoagulation (EC) without mechanical agitation, pumping requirements or air injection. Mixing and complete flotation of the pollutants were achieved using only the overall liquid recirculation induced by H2 microbubbles generated by water electrolysis. A red dye from the Moroccan textile industry was used in a case study to validate this innovative application of airlift reactors. Experimental results showed that the axial position of the Al electrodes and the residence time in the separator section were the key parameters to achieve good mixing conditions, to avoid bubbles/particles recirculation in the downcomer and to prevent floc break-up/erosion by hydrodynamic shear forces. Such optimum conditions corresponded to an optimum liquid overall recirculation velocity that was correlated to current, electrode position and dispersion height. Operation time required to achieve 80% COD and 80% color removal efficiencies was modeled as a function of current density. Similarly, specific energy and electrode consumptions were correlated to current, electrode gap and conductivity, which provided the necessary tools for scale-up and process optimization. Operation time and removal efficiencies were similar to those reported in conventional EC cells, but specific energy and electrode consumptions were even smaller without the need for mechanical agitation, pumping requirements and air injection, which could not be achieved in other kinds of conventional gas–liquid contacting devices. [Copyright &y& Elsevier]

Brackish water desalination by reverse osmosis membranes is energy-driven process. With the purpose of improving the energy performance of such unit a full-scale BWRO desalination plant located in Canary Island (Spain) has been working under intermittent operating conditions for 14 years was assessed with conventional and enhanced (exogenous and endogenous/avoidable and unavoidable exergy destructions) exergy analysis. The exergies across the major components of the plant are calculated. The conventional exergy analysis reveals that, largest irreversibilities were identified in the RO system (membrane modules), the high pressure pump and the feed pump which amounts to about 64.28%, 40.84% and 38.48% respectively. Results of advanced exergy analyses shows that 70.61%, 92.94% and 7.83% of the total exergy destruction in the high pressure pump, feed pump and the RO system respectively are avoidable. Moreover, the highest endogenous avoidable exergy destruction rate was determined owing to the feed and high pressure pumps. Using these findings, instructions were proposed to ensure optimal effectiveness of the RO system and to improve the exergy destruction in the pumps currently in use. • Performing an enhanced exergy analysis for a full-scale BWRO desalination plant • Analyzing the performance of main plant components exergetically • Dividing irreversibilities into exogenous and endogenous/avoidable and unavoidable • Determining inefficiencies within the desalination plant for planning future improvements [ABSTRACT FROM AUTHOR]

Electrocoagulation (EC) has been studied to assess its applicability as a denitrification process for drinking water. The objective was to investigate the mechanisms of nitrate removal. Electrolysis has been driven in the discontinuous mode with aluminum electrodes, using a synthetic water representative of drinking water. The respective effects of mixing, initial nitrate concentration, current and initial pH have also been analyzed. Experimental results have shown that EC removes effectively the nitrate anions, following first-order kinetics. The rate of denitrification is proportional to current. The removal of nitrate anions results primarily from their electroreduction into ammonium, but total nitrogen decreases simultaneously in water and follows zero-order kinetics. Nitrogen mass balance has shown that the formation of N2 gas is negligible and that the secondary depollution mechanism is adsorption onto the flocs. Adsorption experiments on preformed flocs highlight a preferential adsorption of ammonium. A numerical model able to simulate nitrate removal has been established. The analysis of operating costs has shown, however, that EC is an expensive method, except for waters exhibiting very high nitrate contents. Consequently, EC should be preferentially used as a pretreatment step for biological denitrification when implemented to eliminate simultaneously other types of pollution. [ABSTRACT FROM AUTHOR]