Corinne Cabassud

Vacuum membrane distillation of seawater reverse osmosis brines.

Seawater desalination by Reverse Osmosis (RO) is an interesting solution for drinking water production. However, because of limitation by the osmotic pressure, a high recovery factor is not attainable. Consequently, large volumes of brines are discharged into the sea and the flow rate produced (permeate) is limited. In this paper, Vacuum Membrane Distillation (VMD) is considered as a complementary process to RO to further concentrate RO brines and increase the global recovery of the process. VMD is an evaporative technology that uses a membrane to support the liquid-vapour interface and enhance the contact area between liquid and vapour in comparison with conventional distillation. This study focuses on VMD for the treatment of RO brines. Simulations were performed to optimise the operating conditions and were completed by bench-scale experiments using actual RO brines and synthetic solutions up to a salt concentration of 300 g L(-1). Operating conditions such as a highly permeable membrane, high feed temperature, low permeate pressure and a turbulent fluid regime allowed high permeate fluxes to be obtained even for a very high salt concentration (300 g L(-1)). For the membrane studied, temperature and concentration polarisation were shown to have little effect on permeate flux. After 6 to 8 h, no organic fouling or biofouling was observed for RO brines. At high salt concentrations, scaling occurred (mainly due to calcium precipitation) but had only a limited impact on the permeate flux (24% decrease for a permeate specific volume of 43L m(-2) for the highest concentration of salt). Calcium carbonate and calcium sulphate precipitated first due to their low solubility and formed mixed crystal deposits on the membrane surface. These phenomena only occurred on the membrane surface and did not totally cover the pores. The crystals were easily removed simply by washing the membrane with water. A global recovery factor of 89% can be obtained by coupling RO and VMD.

Water desalination using membrane distillation: comparison between inside/out and outside/in permeation☆

This work focuses on vacuum membrane distillation (VMD) for seawater desalination. The aim of the work was to compare two hollow fibre module configurations (inside/out and outside/in). Experiments were carried out with pure water and with 15 g/L up to 300 g/L NaCl solutions and for two different material and structure of fibres. Pure water permeability and global heat transfer coefficient were compared for the two configurations. The influence of hydrodynamics on global heat and mass transfer coefficients is discussed.

Interests and limitations of nanofiltration for the removal of volatile organic compounds in drinking water production

Most of volatile organohalide compounds (HOVs) present in surface waters and ground waters are carcinogenic. In drinking water, they represent a danger for human health and are present in water resources. The objective of this study was to characterize the performances of nanofiltration as a refining step for removing these compounds at trace level in water. Three HOV compounds which are toxic and highly present in natural waters were selected: trichloroethylene, tetrachloroethylene and chloroform. Performances of different nanofiltration organic membranes to remove these compounds are presented. High rejection ratios were obtained. Influence of pressure on selectivity is low. Rejection mechanisms are discussed. Separation seems partially governed by steric effects, but another phenomenon was noticed and characterized: HOVs absorb, diffuse and then desorb from organic membranes. This phenomenon was especially observed for chloroform. As a consequence of desorption in the filtrate, the regulation limits for drinking water were not always reached. Moreover, some modifications of membrane properties were observed by chemical ageing tests.

Adsorption combined with ultrafiltration to remove organic matter from seawater.

Organic fouling and biofouling are the major severe types of fouling of reverse osmosis (RO) membranes in seawater (SW) desalination. Low pressure membrane filtration such as ultrafiltration (UF) has been developed as a pre-treatment before reverse osmosis. However, UF alone may not be an effective enough pre-treatment because of the existence of low-molecular weight dissolved organic matter in seawater. Therefore, the objective of the present work is to study a hybrid process, powdered activated carbon (PAC) adsorption/UF, with real seawater and to evaluate its performance in terms of organic matter removal and membrane fouling. The effect of different PAC types and concentrations is evaluated. Stream-activated wood-based PAC addition increased marine organic matter removal by up to 70% in some conditions. Moreover, coupling PAC adsorption with UF decreased UF membrane fouling and the fouling occurring during short-term UF was totally reversible. It can be concluded that the hybrid PAC adsorption/UF process performed in crossflow filtration mode is a relevant pre-treatment process before RO desalination, allowing organic matter removal of 75% and showing no flux decline for short-term experiments.

Side-stream membrane bioreactors: influence of stress generated by hydrodynamics on floc structure, supernatant quality and fouling propensity.

This work aims to characterise the impact of hydrodynamics on sludge properties and consequently on fouling mechanisms in side-stream membrane bioreactors (MBRs). Two side-stream processes which generate very different shear stresses are compared, without filtration. This operating mode permits specific quantification of the impact of the external loop (and of induced shear stress) on floc structure/morphology, supernatant quality and fouling propensity. The study shows that low constraints (7 x 10(-3) Pa) generated on submerged side-stream hollow fibre modules have no significant impact on sludge properties. In contrast, high shear stresses (72 Pa) associated with a crossflow configuration induce very significant modifications of the mixed liquor which increases its fouling propensity (measured in a standard filtration cell). A theoretical explanation of the role of turbulence on the floc size distribution is given. Based on a Kolmogorov microscale calculation, it seems possible to predict the mean floc size reached in both filtration systems, for a given shear stress intensity. Disaggregation is characterised by a two-step kinetic: first a short-term breakage attributed to fragmentation and loss of weak strength bonds, and secondly a longer-term breakage probably due to erosion phenomena and removal of high strength bonds. Only the second step induces a significant release and an accumulation of soluble protein-like substances. Soluble organic matter strongly enhances the fouling propensity of the mixed liquor. This seems to be amplified by the concentration of protein-like substances. The importance of considering the protein and carbohydrate content as well as floc size is also pointed out in the paper.

Determination of struvite crystallization mechanisms in urine using turbidity measurement

Sanitation improvement in developing countries could be achieved through wastewater treatment processes. Nowadays alternative concepts such as urine separate collection are being developed. These processes would be an efficient way to reduce pollution of wastewater while recovering nutrients, especially phosphorus, which are lost in current wastewater treatment methods. The precipitation of struvite (MgNH(4)PO(4)∙6H(2)O) from urine is an efficient process yielding more than 98% phosphorus recovery with very high reaction rates. The work presented here aims to determine the kinetics and mechanisms of struvite precipitation in order to supply data for the design of efficient urine treatment processes. A methodology coupling the resolution of the population balance equation to turbidity measurement was developed, and batch experiments with synthetic and real urine were performed. The main mechanisms of struvite crystallization were identified as crystal growth and nucleation. A satisfactory approximation of the volumetric crystal size distribution was obtained. The study has shown the low influence on the crystallization process of natural organic matter contained in real urine. It has also highlighted the impact of operational parameters. Mixing conditions can create segregation and attrition which influence the nucleation rate, resulting in a change in crystals number, size, and thus final crystal size distribution (CSD). Moreover urine storage conditions can impact urea hydrolysis and lead to spontaneous struvite precipitation in the stock solution also influencing the final CSD. A few limits of the applied methodology and of the proposed modelling, due to these phenomena and to the turbidity measurement, are also discussed.

A new method for permeability measurement of hydrophobic membranes in Vacuum Membrane Distillation process

In this paper, a new method for permeability measurement of hydrophobic membranes used in Vacuum Membrane Distillation, instead of common measurement methods, was proposed. As VMD is a pressure and temperature driven process, the idea of this work is to propose a new water vapour permeability measurement method based on variation of feed temperature at a fixed vacuum pressure. This new method showed a greater stability and simplicity than the existing pressure variation method by not only allowing a wide range of feed temperature (25 °C ÷ 60 °C) to be scanned continuously, but also avoiding fluctuations of the system as observed in the pressure variation test. Permeabilities of two different kinds of hydrophobic membranes were measured by this new method and also by the existing pressure variation test. A comparison between these two methods was also presented to assess the feasibility and applicability of this new method.

Performance of NF/RO process for indirect potable reuse: interactions between micropollutants, micro-organisms and real MBR permeate

Micropollutants and micro-organisms are major concerns for indirect potable reuse of municipal wastewater. In this study, the retention of pesticides, polycyclic aromatic hydrocarbons (PAHs), metals and micro-organisms in real membrane bioreactor (MBR) permeate matrix and in ultrapure water matrix by nanofiltration (NF) or reverse osmosis (RO) process, and the impact of their retention on NF/RO membrane fouling, were investigated. NF 90 and ESPA2 membranes were chosen for this work. The RO membrane showed high retention for most of the molecules tested, whereas the NF membrane exhibited some variation in the retention of pesticides. The retention of pesticides and metals was enhanced by their interactions with the MBR permeate, while an almost complete retention of PAHs and micro-organisms was observed in both matrices. The presence of PAHs in the MBR permeate increased the permeability of the RO membrane, whereas no such effect was observed for pesticides, metals and micro-organisms. Also, phage leakage was detected when the NF/RO membrane was contaminated by bacteria, which may have caused a slight membrane damage. Pesticide rejection was found to be related to LogKow and molecular weight in ultrapure water at low concentrations (mu g/L level).

Fouling and its reversibility in relation to flow properties and module design in aerated hollow fibre modules for membrane bioreactors

Nowadays, most membrane bioreactors are using membranes submerged in the biomass and aeration in the concentrate compartment to limit or to control fouling. An important issue for the design of modules or membrane bundles in MBRs is to understand how the air/liquid flow is behaving and influencing fouling and its reversibility in relationship to the module properties. This paper focuses on an innovative and very specific process, in which HF membranes are put in a cartridge outside the activated sludge tank and a recycling loop is associated to the cartridge in order to decrease concentration of foulant species at the membrane surface and mass transfer resistance. Recycling operates with a very low liquid velocity in the module (a few cm.s(-1)) which constitutes a specificity of this process in terms of filtration operation. The aim of this study is to characterise two-phase flow and its effects on fouling and fouling reversibility at the scale of a semi-industrial bundle of outside/in hollow fibres, and as a function of bundle properties (packing density, fibre diameter), using specific methods to characterise the flow and fouling effects. Two modules were used showing a different packing density. Filtration was operated at constant permeate flux with clay suspension at 0.65 g.l(-1) in the same hydrodynamic conditions. Fouling kinetics and irreversibility were characterised by an adapted step method, and gas and liquid flows were characterised at global scale by residence time distribution analyses and gas hold-up. Fouling velocities are clearly influenced by gas velocity. The proportion of dead to total volume in the module is mainly affected by the liquid flow velocity and module design. The module with the higher fibre diameter and the lower packing density showed better performances in terms of fouling which was correlated with better flow properties.

Activated carbon selection for the adsorption of marine DOC and analysis of DOC fractionation

AbstractAdsorption on granular activated carbon (GAC) and fractionation of marine dissolved organic carbon (DOC) was investigated with the objective of mitigating RO membrane biofouling in the context of a reverse osmosis process for seawater pretreated by ultrafiltration. The behaviours of six commercial GAC compounds were compared. Batch kinetics experiments showed that 80% of DOC in seawater was adsorbed within four hours. Adsorption kinetics fitted the Ho and McKay model well, and isotherms followed the Freundlich model. GAC A and GAC F had the best adsorption capacities. For GAC A, the adsorption mechanism was mainly related to high occupation of the porous volume, whereas for GAC F, the adsorption was mainly related to electrostatic interactions. GAC A was selected for a fixed bed column experiment continuously fed with seawater as GAC F released acidic groups into the feed water. This filtration set-up enabled more than 70% of DOC to be removed from seawater. Moreover, LC-OCD analysis showed that G...