Leen Braeken

Ozone oxidation for the alleviation of membrane fouling by natural organic matter: A review

Membrane fouling by natural organic matter is one of the main problems that slow down the application of membrane technology in water treatment. O(3) is able to efficiently change the physico-chemical characteristics of natural organic matter in order to reduce membrane fouling. This paper presents the state-of-the-art knowledge of the reaction mechanisms between natural organic matter and molecular O(3) or *OH radicals, together with an in-depth discussion of the interactions between natural organic matter and membranes that govern membrane fouling, inclusive the effect of O(3) oxidation on it.

Flux decline in nanofiltration due to adsorption of organic compounds

Abstract The water flux for two nanofiltration membranes (UTC-20 and NF70) was measured using aqueous solutions of 11 organic compounds in different concentrations. The flux declined, compared to the pure water flux, by more than 50% for solutions containing less than 1 g/l of some organic compounds. Flux decline as a function of the concentration of the organic compound showed the typical shape of a Freundlich isotherm. Comparison to adsorption experiments indicated that flux decline, for the solutions used, was related to adsorption on the membrane material. The dipole moment, the octanol–water partition coefficient and the water solubility were examined as parameters to explain adsorption on nanofiltration membranes. A clear correlation was found between the octanol–water partition coefficient and adsorption; adsorption also appeared to be related to the dipole moment and the water solubility. This shows that both the surface charge and hydrophobicity of the membrane can play a role in the adsorption. When flux decline is related to octanol–water partition coefficient, the molecular size (molecular weight) should be taken into account as well. A subset of molecules with a molecular weight in a narrow range, somewhat below the cut off value of the membranes, was selected. Within this subset, a clear correlation between the octanol–water partition coefficient and flux decline was observed.

Evaluation of parameters describing flux decline in nanofiltration of aqueous solutions containing organic compounds

Abstract One of the major drawbacks for the introduction of membrane technology is the possible occurrence of fouling. Especially in newer processes such as nanofiltration, understanding of the mechanisms of flux decline and fouling is limited. Water fluxes obtained with nanofiltration of pure water are usually different from those obtained with real feed solutions. Depending on the feed composition, a flux decline ranging from a few percent to a complete loss of water flux can be found. For aqueous solutions containing organic components, in the absence of e.g. suspended solids or high concentrations of ions that may cause scaling, adsorption of organic material on the membrane surface is the major fouling mechanism. Identification of the parameters that play a role in the process of adsorption on the membrane surface should lead to a better understanding of the mechanism that results in a hindrance of the water flux, and eventually to pore blocking. In this study, the following parameters were selected for a detailed investigation of the adsorption process: the dipole moment, the polarisability, the dielectric constant, the solubility in water, the octanol—water partition coefficient, the contact angle membrane/water, the Small number, the modified Small number, molecular size, pKa, and the Taft parameter. All of these parameters were evaluated in the framework of adsorption on nanofiltration membranes from an aqueous solution. The use of each parameter for describing adsorption will be discussed. The pKa was rejected on theoretical grounds; other parameters such as the solubility in water proved to be impractical. Eventually, the dipole moment, the octanol—water partition coefficient, and the molecular size were selected. Correlations between adsorption on nanofiltration membranes and these interaction parameters show that there is a clear influence on adsorption.

How a microfiltration pretreatment affects the performance in nanofiltration

Abstract The use of a well‐chosen pretreatment system is a key element to avoid fouling in nanofiltration (NF). Among the different possibilities for pretreatment systems, microfiltration (MF) emerges as the most compatible with NF. This article explores the influence of a MF pretreatment by comparing the performance of three NF membranes (UTC‐20, Desal 51 HL, and NF‐PES‐10) with and without MF pretreatment for the purification of two types of wastewaters from the brewery of Westmalle, Belgium. The wastewaters studied were bottle‐rinsing water and rinsing water from the fermentation tanks, respectively. The results indicate that MF pretreatment had no influence for UTC‐20, a considerable influence for Desal 51 HL, and a dramatic influence for NF‐PES‐10. No correlation with the membrane roughness, determined by AFM, was found, but the results support the assumption that particle fouling is mainly determined by the hydrophobicity of the membrane. The effect of pretreatment was largest for the hydrophobic NF‐PES‐10 membrane, smallest for the hydrophilic UTC‐20 membrane, and intermediate for Desal 51 HL. The effect of the composition of the feed solution was considerably smaller than the effect of the membrane used, although, small differences where found depending on the size and concentration of the particles.

The effects of ultrasound on micromixing.

The Villermaux-Dushman reaction is a widely used technique to study micromixing efficiencies with and without sonication. This paper shows that ultrasound can interfere with this reaction by sonolysis of potassium iodide, which is excessively available in the Villermaux-Dushman solution, into triiodide ions. Some corrective actions, to minimize this interference, are proposed. Furthermore, the effect of ultrasonic frequency, power dissipation, probe tip surface area and stirring speed on micromixing were investigated. The power and frequency seem to have a significant impact on micromixing in contrast to the stirring speed and probe tip surface area. Best micromixing was observed with a 24kHz probe and high power intensities. Experiments with different frequencies but a constant power intensity, emitter surface, stirring speed, cavitation bubble type and reactor design showed best micromixing for the highest frequency of 1135kHz. Finally, these results were used to test the power law model of Rahimi et al. This model was not able to predict micromixing accurately and the addition of the frequency, as an additional parameter, was needed to improve the simulations.

Ultrasound precipitation of manganese carbonate: The effect of power and frequency on particle properties

The influence of ultrasonic frequency and intensity on particle shape, tap density and particle size distribution was investigated during the precipitation of manganese carbonate. For the first time, a broad frequency range of 94 till 1135 kHz was studied in one single reactor setup. Smaller and more spherical particles were observed during sonication compared to silent conditions. Lower frequencies and increased intensities result in smaller and more spherical particles. The most spherical particles with superior tap densities are obtained at the lowest frequency and most elevated intensity. Moreover, the results indicate that a particle size threshold exists, below which the particle size cannot be reduced by a further increase of the ultrasonic intensity or reduction of the frequency. Sonication of already formed spherical powders resulted in particles with smaller sizes but unaffected shapes. Finally, one test with pulsed ultrasonic irradiation resulted in equally sized particles with similar sphericity as the ones produced under continuous sonication.

Influence of dissolved gases on sonochemistry and sonoluminescence in a flow reactor

In the present work, the influence of gas addition is investigated on both sonoluminescence (SL) and radical formation at 47 and 248 kHz. The frequencies chosen in this study generate two distinct bubble types, allowing to generalize the conclusions for other ultrasonic reactors. In this case, 47 kHz provides transient bubbles, while stable ones dominate at 248 kHz. For both bubble types, the hydroxyl radical and SL yield under gas addition followed the sequence: Ar>Air>N2>>CO2. A comprehensive interpretation is given for these results, based on a combination of thermal gas properties, chemical reactions occurring within the cavitation bubble, and the amount of bubbles. Furthermore, in the cases where argon, air and nitrogen were bubbled, a reasonable correlation existed between the OH-radical yield and the SL signal, being most pronounced under stable cavitation at 248 kHz. Presuming that SL and OH originate from different bubble populations, the results indicate that both populations respond similarly to a change in acoustic power and dissolved gas. Consequently, in the presence of non-volatile pollutants that do not quench SL, sonoluminescence can be used as an online tool to qualitatively monitor radical formation.

Characterization of stable and transient cavitation bubbles in a milliflow reactor using a multibubble sonoluminescence quenching technique

The bubble type, generated by an ultrasonic field, was studied in a batch and flow reactor using a multibubble sonoluminescence (MBSL) quenching technique with propanol and acetone. The influence of frequency and transducer configuration was evaluated using the same piezoelectric element in both setups. Results show that the bubble type not only depends on the frequency, but also on the input power or transducer configuration. Additionally, the effect of flow on sonoluminescence yield and bubble type was studied in the continuous setup at various frequencies. As the flow becomes turbulent, the sonoluminescence signal reaches a plateau for three out of four frequencies, and a transition from transient to stable cavitation occurs for frequencies below 200 kHz.

AOX removal from industrial wastewaters using advanced oxidation processes: assessment of a combined chemical–biological oxidation

In this paper, the abatement of adsorbable halogenated organic compounds (AOX) from an industrial wastewater containing relatively high chloride concentrations by a combined chemical and biological oxidation is assessed. For chemical oxidation, the O(3)/UV, H(2)O(2)/UV and photo-Fenton processes are evaluated on pilot scale. Biological oxidation is simulated in a 4 h respirometry experiment with periodic aeration. The results show that a selective degradation of AOX with respect to the matrix compounds (expressed as chemical oxygen demand) could be achieved. For O(3)/UV, lowering the ratio of O(3) dosage to UV intensity leads to a better selectivity for AOX. During O(3)-based experiments, the AOX removal is generally less than during the H(2)O(2)-based experiments. However, after biological oxidation, the AOX levels are comparable. For H(2)O(2)/UV, optimal operating parameters for UV and H(2)O(2) dosage are next determined in a second run with another wastewater sample.

Effect of fluid properties on ultrasound assisted liquid-liquid extraction in a microchannel

When immiscible liquids are subjected to an ultrasonic field, they form emulsions. This principle has been used to improve the mass transfer characteristics of a liquid-liquid extraction process in microreactor systems. The formation of emulsion and its characteristics are prominently dependent on the properties of the liquids used and this also holds true for emulsion brought about by ultrasound. This paper focuses on the properties of fluids that are reported to have an influence on the cavitation behaviour, namely viscosity, interfacial tension and vapour pressure. These properties were examined by changing the solvent of the organic phase in the hydrolysis of p-nitrophenyl acetate. The study is performed by comparing pairs of solvents that are different in one property but similar in the other two. The pairs selected are toluene - chlorobenzene for viscosity, toluene - methyl Isobutyl ketone for interfacial tension and methyl isobutyl ketone - 2-Methyl tetrahydrofuran for vapour pressure effects. A qualitative study was performed with a high-speed camera in flow to understand the emulsification initiation mechanisms and behaviours. These findings were further explored by performing the sonicated emulsion in a batch-sonicated reactor. The quantitative analysis of the fluid properties was evaluated and compared based on the relative percentage increase in yield upon sonication with respect to their individual silent conditions. The quantitative results were further supported by the quantification of the emulsion performed with an FBRM probe. The results indicate a two times improvement in yield with solvent of lower viscosity as 2 times more droplets were formed in the emulsion. Both the solvent systems with higher interfacial tension and vapour pressure had an improved yield of 1.4 times owing to larger number of droplets formed.