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The thermo-fluid dynamic behavior of spacer-filled channels for membrane distillation was investigated experimentally. Several different geometry were investigated thanks to customized reference spacers manufactured using a 3D printer. In particular, two sets of experiments were conducted: in the first set, cylindrical filaments were orthogonally arranged and the flow attack angle was made to vary from 0o to 90o; in the second set, the flow attack angle was kept symmetrical and the filament angle was made to vary from 30° to 150°. Each spacer was tested for Reynolds numbers between 200 and 900 in the hot channel, while maintaining a constant temperature difference of 13 °C between the hot and the cold channels. Thermochromic Liquid Crystal (TLC) sheets were used for surface temperature measurements. Results showed that, for all spacers, the heat transfer coefficient increases with hot feed flow rate and that the combination of attack angle 45° and filament angle 90° gave the highest heat transfer. Pressure drop measurements showed that spacers could be categorized into two groups relevant to the values of the Darcy friction coefficients (f). One group, associated with spacer designs with filament angles greater than 90°, led to high f values, while the other, associated with filament angles less than 90°, led to moderate f values.

Wastewater has become one of the most important pollutants for the natural eco-system such as ocean, rivers, lakes and as well as groundwater aquifer. Among the various options to remedy this problem, there is membrane crystallization (MCr) which presents a promising alternative to achieve a high conversion ratio of the feed stream and to produce valuable minerals. The present study focuses on MCr for the production of crystals of sodium sulfate (Na2SO4) and high quality of water from concentrated solution ([Na2SO4] = 2 M). The experiments were conducted at permeate temperature set to 25 °C and variable feed and crystallization temperatures. The permeate and feed flow rates are kept fixed to 2 L/h. The results show that the recovery ratio reaches up to 80%. The salt production was about 100 kg/m3 of Na2SO4 crystals. The flux data shows decreasing - increasing cycles. The cycles were due to the phenomenon, induced in MCr, which boosts the concentration until reaching super-saturation (critical levels) and crystallization phenomenon initialization. The hyperbolic trend is specific to the membrane processes and it is attributed to the scaling phenomena of the membrane surface. The most important magnitude cycle was recorded for Tf = 70 °C. It had, as a ∆ J, equals to 7.81 kg/m2 h for a ∆ C about 1 M. Among the three adopted modes (SH, SC, CHS), the CHS was more efficient and safe. The seeding, with a rate of 1 g/h, enabled the process to control the crystal growth phenomenon. The cross-session SEM observations show that the heating-seeding combination can increase the membrane longevity. The particle size analysis proves that this process produces well-controlled size distribution (CSD) crystals. The X-ray diffraction analysis shows that crystals formed are the anhydrous form with an orthorhombic shape. This form is called thenardite.

In this paper, we focused our work on the direct contact membrane distillation (DCMD) capability to treat hyper-saline solution. The governing operative model for mass transfer was investigated. The measured flux has been well predicted by the Knudsen-molecular mechanism model. The effects on the DCMD flux of polarization phenomena TP and CP were underlined. The optimum operating parameters were defined: the hot and cold stream temperatures were set, respectively at 59 and 20 °C, and the feed and permeate velocities were fixed both to 0.046 m s− 1. With regard to membrane performance to treat the reverse osmosis brine, a long-term experiment was carried out under the optimal experimental conditions. The increase in feed RO brine concentration provoked a noticeably decrease in flux from 8.43 to 4.06 kg m− 2 h− 1. The RO brine experiments proved that the DCMD process was capable to concentrate the solution tell to reach concentration factor (CF) further than four times, which corresponded to the super-saturation of saline solution. Based on the characterization methods, the occurrence of the membrane wetting and scaling was shown and interpreted. These extreme phenomena promote the salt crystallization on the feed side of the membrane. The onset crystallization phenomenon starts when the permeate decreases so fast. Their sudden decline was about 90% for a working period of 20 h.

Theoretical and experimental studies of direct contact membrane distillation (DCMD) using commercially flat sheet hydrophobic polyvinylidene fluoride (PVDF) membrane were investigated to remove boron from simulated and natural seawater. Mass transfer mechanism and theoretical flux were determined using the dusty-gas model (DGM). Mass transfer analysis showed that the combined Knudsen-molecular model is considered to be the dominant mechanism which describes the transport of water vapor through the membrane pores. Then, the effect of various operating parameters such as feed temperature, boron concentration, salt concentration and feed solution pH on DCMD performance was investigated. Experimental runs showed that the permeate flux was enhanced exponentially with feed temperature, and maximum permeate flux of 27.5 kg/m 2 ∙ h was obtained at 74 °C. The permeate flux decreases slightly with feed boron and salt concentration. Feed solution pH had no significant effect on DCMD performance. DCMD process can produce water with high boron rejection of more than 90% even at feed concentration as high as 200 mg/L. Natural seawater, containing 5.37 mg/L of boron, was treated by DCMD process. During 18 h of working period, the permeate flux had showed a reduction of about 15.75%, and permeate boron was kept below 0.47 mg/L. The decreasing of contact angle value can explain the performance decline of DCMD process.

Tunisia is a semi arid country; the shortage of good water quality in particular at the south regions forced the government to desalt brackish water via membrane technology such as RO to supply these regions with potable water. Station of Djerba Isle established in 2000 by SONEDE has a nominal capacity of 15 ⋅ 103m3 per day and recovery rate of 75%; it is fed from an artesian well with the water salinity of 5500 ppm. Three RO units compose station; each one has configured in two stages a 2:1 array with six RO polyamide elements per pressure vessel. Membranes have been cleaned or replaced, during four operating years, as much time. In the aim to determine the reason of membrane fouling, this work occurs to analyse fouling layers. The membrane autopsy has achieved by different analytical methods, such as COT measurement, SEM (scanning electron microscopy), IR analysis, and diffraction by X-ray. Obtained results show that the deposit composed mostly of organic matter, silica, iron and CaCO3. The debris accumulate on upstream of pressure vessel is composed mainly of silica, metallic matter (iron and chromium) and aluminium phosphate. The presence of TOC indicates the presence of microorganism. The metallic products come from stainless steel fittings and internal coating film of cartridges filter that are damaged by the silica particle probably rejected by sand filters. The scale of CaCO3 deposition is owing to the bad anti-scaling organophosphonate-based, which can form aluminium phosphate by reaction with aluminium composing cartridges filter internal coating.

Tunisia, located in a semi-arid zone, has limited water resources. The shortage of good water quality in particular at the south regions required a brackish water desalination to supply these regions with potable water. Zarzis desalination plant, working since 1999, has revealed difficulties related to the loss of membrane performances. In the aim to determine the reason of membrane fouling, foulant layer were analysed. The membrane autopsy has achieved by different methods, such as TOC (total organic carbon) measurement, SEM (scanning electron microscopy), AFM (atomic force microscopy), FTIR (fourier transform infrared spectroscopy) analysis, diffraction by X-ray. Results show that the foulant layer composed mostly of SiO2, clay, organic matter (polysaccharide, protein), CaSiO3, Fe3O4, AlPO4, and CaSO4. The characteristics bonds obtained by IR indicate the presence of polysaccharides and proteins that constitute a source for further microbiological growth; this is the problem of biofouling. The metallic elements results of corroded stainless steel and internal coating of cartridges filter that are damaged by the silica particle rejected by sand filters. The scale of CaSO4 and CaSiO3 deposition is owing to the bad antiscaling efficiency. The results are presented and discussed in the light of new trends in material.

The ion exchange equilibrium between an ion exchange membrane and solutions of different electrolytes was studied. The matrix of the ion exchange membrane used consists of a vinyl polychloride network reinforced by a polyester screen; with quaternary and tertiary ammonium as functional groups. The data, provided by manufacturer, were completed by the experimental determinations of humidity percentage and ion exchange capacity. This paper focuses on the study of exchange reaction between an anion exchange membrane and selected binary and ternary electrolyte solutions. Experimental data are reported for the ion exchange equilibrium of the binary system Cl−/NO3−, Cl−/SO42−, NO3−/SO42− and the ternary system Cl− − NO3− − SO42−. The ion exchange isotherms were established and affinity order of ions was given. The prediction of the ternary system based only on the binary data was consistent with the experimental data obtained for this system. Selectivity coefficients KCl−NO3−, K2Cl−SO42−, K2NO3−SO42− were determined at ionic strength 0.1 M. Ion analyses were performed by ion chromatography, coupled to a conductimetric detector.

In the recent years, membrane processes have drawn more attention because of their strong separation capabilities and exhibiting a great potential for the treatment of waters and wastewaters worldwide. However, these membrane separation processes have some problems due to the formation of polarisation films and by-products which may generate bacteria and fouling. These problems may be overcome by combining two or more processes in the desalination units of the treatment process. The specificity of each process makes their integration more efficient. In this work, we report on the concept and the realisation of an experimental pilot plant which is composed of the following compartments: nanofiltration (NF), reverse osmosis (RO), cartridges filter, pump and measuring instruments. Samples of brackish water were filtered through filtrate cartridges in order to get rid of the suspended matter. A pilot plant equipped with composite RO and NF membranes was operated at 6 bar, ambient temperature and neutral pH with relatively good water recovery. This pilot plant shows the ease with which the combination of both processes is carried out. The main objective is to evaluate the effect of NF and RO membranes as well as their coupling on water quality. The effects of divalent and monovalent cations on operating parameters were studied. The recovery (Y) in NF/RO coupling was improved compared to that obtained in RO alone. This may be explained by a decrease in the input osmotic pressure of the RO membrane. This coupling also improves salt rejection and thus leads to a decrease in the salinity of water product. The results show that the coupling is efficient as it highly reduces water salinity.

In Tunisia, water resources are very limited and have high salinities. Domestic water in rural communities has TDS reaching 2750 ppm. Inhabitants of these areas are scattered and lack proper infrastructure for the implementation of conventional desalination plants. However, these regions have an important potential of renewable energy (RE), especially solar energy where sunshine in southern regions can reach 2500 h per year. RE can be harnessed to power small-scale desalination plants in order to produce the fresh water necessary to cover the basic requirements (drinking and cooking). This paper aims to identify the potential of renewable energies, mainly solar and geothermal. It illustrates the quality of distributed water in rural communities. It presents several studies that were carried out, including solar multiple effects distillation, reverse osmosis driven by photovoltaic panels, and lately membrane distillation using a geothermal resource. The experimental results show technical promise. However, more investigations will need to optimise the operating parameters and improve economic feasibility.

In water treatment, silica is a problem because of its tendency to form deposits. It is often necessary to include reducing silica concentration to permit increased cycles of concentration without scale. The behavior of aqueous silica and the various water treatment processes used for its removal were reviewed. Silica that exists in water in equilibrium with ion silicate can be removed by strong base anion-exchange resins operated in the hydroxide cycle. With the aim of silica removal by ion exchange, some thermodynamic and kinetic aspects were studied at different temperatures. Selectivity coefficients were determined and activity coefficients were calculated from an appropriate model. Silica concentrations were obtained by the measure of the absorbance of yellow or blue-colored silicomolybdic acid. Anion species concentrations were determined by ionic chromatography.

A desalination family prototype has been designed in a laboratory supplying good quality water for drinking and household cooking needs. The available intermittent and fluctuating energy source (solar or wind) has made us explore the coupling of such a source to a prototype that operates itself in an intermittent mode by integrating an energy storage dissipation system in the prototype schema. Such a coupling allows reducing the energy constraint linked to the production, defining an optimal storage capacity and cancelling the resource to complex classic systems of storage that increase the cost of m3 of water produced. This study allows the evaluation of the performances of the prototype and optimises its functioning in terms of energy availability. Thus, we confirmed the contribution of the storage dissipation system when using solar energy for water desalination, and the advantages brought by the use of this system in terms of a continuous operating mode by excluding the effects of source fluctuations.

Tunisia has five major geothermal districts. Recorded hot spring temperatures range from 294 to 340 K with flow rates 0.1–102 L/s. In southern parts available groundwater resources are mostly hard and brackish (3 g/L). Recently, Tunisia has resorted to this resource for agriculture and potable water uses. Cooling towers have been built to lower down the temperature and hardness. Cooled brackish water irrigates greenhouses and feeds desalination plants. The cooling operation of groundwater rejects an important quantity of thermal energy in the atmosphere. However, energy requirements of thermal desalination plants are too excessive to be supplied by a geothermal resource. Membrane distillation (MD) is an emerged desalination technology, which can be driven by a thermal energy at low enthalpy (less than 363 K) as geothermal energy, and a fluidised bed crystalliser can ensure reduction of an important portion of hardness without significant loss of temperature. MD is realised by means of a microporous hydrophobic membrane separating a warm solution from the cooling chamber, which contains either liquid or gas. A fluidised bed crystalliser permits production of from it of a granular crystal calcium carbonate from the consumption rates of calcium ions, using aragonite or sand as seeding materials. The purpose of this paper is to realise experimental assembly constituted of fluidised bed crystalliser (FBC) and a cell of air gap membrane distillation (AGMD). It presents preliminary results related to characterisation of fluidised bed, growth of seeded particles size of CaCO3 and operating parameters of AGMD. In laboratory scale, the technical feasibility has been shown. More investigation is needed to prove the efficiency and availability of coupled AGMD with FBC.

In order to deepen the understanding of the permeate characteristics of membrane distillation considering heat and mass transfers and concentration polarisation, a fully predictive mathematical model was developed. The first part that is devoted to the natural heat and mass transfer in the rectangular cavity, states the problem in Cartesian coordinates system, involves the use of a control-volume method and solves the full vorticity transport equation together with the stream function, mass and energy equations. The temperature fields and streamlines are plotted for various geometrical conditions to show some of the flow field characteristics. The second part was set up to simulate the phenomenon present in the membrane distillation process. The mass transfer was described with combined Knudsen and Maxwell-Stefan equations and coupled to the simultaneous heat transfer. The effect of parameters such as thickness of cavity, temperature and feed flow rate were investigated. The results were compared with the available data and the agreement is satisfactory.

Tunisia has utilized a large portion of its moderate water potential. Needs increase exponentially as a result of population growth and socio-economic development. Since the 80's, Tunisia has resorted to desalination of brackish water and particularly via membrane techniques to supply the traditionally deficient regions with good quality water and come up with a solution for industrial water utilities. The choice and the management of desalination technologies must be studied. With this aim, a national research program has been established to dealing with the optimal management of existing units with a view to propose technological schemes more adapted to the quality of the raw water. In the present work, three plants are studied. The first one is an RO fed by water drawn from a subsoil aquifer. The operating parameters show that partial iron oxidation and chlorine are responsible for the short life of modules. The second deals with the water from a deep geothermal layer. Membrane autopsies show that the fouling is essentially biological and that the shock treatment has led to a significant improvement. The third one is an EDR plant fed by dam water. The main difficulty is the turbidity fluctuation, the instantaneous adjustment of the dose injected into flocculant and coagulant which has led to a substantial improvement of the plant performances. The follow up and diagnosis are presented and discussed in the light of new trends in material.

The membrane distillation (MD) is a process for vapour extraction from aqueous solution at temperature which my be much lower than the boiling point of the solution. This operation is realized by means of a micro porous hydrophobic membrane separating a warm solution from a cooler chamber which contains either liquid or gas. The membrane distillation has been used to concentrate at high level different kinds of solutions, to remove salt from seawater in semi-industrial applications and to separate alcohol-water solutions in biotechnological systems. Recently, this process has been used in water desalination and wastewater treatment. So far, essentially two different configurations of membrane distillation processes have been used, namely direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD). Several authors have performed experimental and theoretical analysis of both processes, and desalination equipment based on DCMD and AGMD is commercially available. The large air gap in AGMD allows for the existence of important temperature and partial pressure gradient. This paper objective is to realize DCMD and AGMD experimental tools and to study the effect of the following operating variables: feed concentration, feed flow, feed temperature and the air gap width. The flux up to 3–5 kg/m2h was achieved and the salt rejection is about 97%. The main advantage of membrane distillation lies in its simplicity and the need for only small temperature differentials. The AGMD configuration is more adapted to desalination of geothermal resources, and the need of energy is lower — only for pumping.