Introduction
1.4 Existing processes for the separation of fluoride, iron and arsenic from drinking water
1.5.5. Floatation
Fe(OH)4-2 between pH 7 and 14. The ferric ion, Fe(III) hydrolyzes much more readily than the ferrous ion, Fe(II). Baes and Mesmer [85] presented diagrams, which show that iron at the range of pH 7–8 is a precipitate. The rates of ferrous ion oxidation by air increase with pH and about 90% conversion may be achieved in a few minutes at a pH of 7 [86]. Precipitation depends on the size and shape of the particle which is formed after coagulation followed by the adsorption on the active surfaces of the coagulants formed during the electrocoagulation process. At the higher pH, removal of iron is achieved mainly by adsorption of iron hydroxide in the form of brown flocks due to the sufficient availability of coagulants in the medium. Therefore the flocks formed were large in size and settled down as a precipitate at the bottom of the container shortly after the completion of the experiment.
The metal cations of As(III) and As(V) react with the OH− ions produced at the cathode during the evolution of hydrogen to yield both soluble and insoluble hydroxides that will react with or adsorb pollutants, respectively, from the solution and also contribute to coagulation by neutralizing the negatively charged colloidal particles that may be present at neutral or alkaline pH. This enables the particles to approach closely and agglomerate under the influence of van der Waals attractive forces.
"electrolytic flotation" and more conventional flotation techniques is the method of bubble production and resultant bubble size. Expertise from other flotation techniques, including electroflotation, dissolved air flotation and air-lift reactors, can be employed to understand the flotation process in electrocoagulation reactors. Electroflotation describes the production of electrolytic gases for the sole purpose of pollutant removal. One of the main advantages of flotation by electrolytic gases is the small size of the bubbles produced. For a given gas volume, a smaller bubble diameter results in both a greater surface area and more bubbles, thereby increasing the probability of collision and the ability to remove fine pollutant particles [87]. Also, as noted, electrolytic bubbles enhance mixing in the bulk solution via their overall upward momentum flux, increasing the likelihood of effective contact between coagulant and pollutant particles.
Bubble movement within a reactor is a function of the bubble density, bubble path and bubble residence time. Current density determines the production rate of electrolytic gas, and thus the bubble density, while reactor geometry (size, height, electrode positioning, effective electrode surface area to volume ratio) determines the bubble path. The average time a bubble spends in the reactor is referred to as its residence time, which is a function of bubble size and path length. It should be noted that shear forces from any mixing source affect the growth of aggregates. Operation at a low current density produces relatively few bubbles, resulting in gentle agitation - conditions that are idea for aggregate growth and flocculation. As the current density increases, however, bubble density and the net upward momentum flux increases. These increase change the reactor’s hydrodynamic behaviour and the degree of mixing. High shear forces induced
by mixing can damage and break flocks apart, reducing the effectiveness of pollutant removal.
Electrochemistry, coagulation and flotation thus form the three foundation stones for electrocoagulation. Each component is a well-studied technology in its own right.
However, it is clear from the published literature that what is lacking is a quantitative appreciation of the way in which these technologies interact to provide an electrocoagulation system.
1.6 Application of membrane technology for the treatment of drinking water Membrane separation process in water treatment has gradually gained popularity because it effectively removes a variety of contaminants from raw waters. While microfiltration (MF) and ultrafiltration (UF) membranes can mainly remove suspended particles, nanofiltration (NF) membranes are an effective technology to remove dissolved organic contaminants with molecular weights (MW) of larger than 200 Da and about 70% of monovalent ions by electrostatic repulsion (charge effect), size exclusion (sieving effect) and a combination of the rejection mechanisms [88]. NF membranes offer an attractive approach to meeting multiple objectives of advanced drinking water treatment, such as the removal of disinfection byproduct precursors, natural organic matter (NOM), endocrine disrupting chemicals and pesticides [89]. However, the decrease of permeate flux (i.e., membrane fouling) is a major obstacle to the application of NF membranes to drinking water treatment. Fouling worsens membrane performance and ultimately shortens membrane life, resulting in the increase of operational cost. Efficient control of membrane fouling is, therefore, required to successful application of NF technology.
Since a broad spectrum of constituents in feed waters can cause membrane fouling, it is important to investigate what types of materials (i.e., foulants) should be removed from feed waters by pretreatments prior to NF process. NF membranes are subject to fouling by dissolved and macromolecular organic substances, sparingly soluble inorganic compounds, colloidal and particulate matter, and microorganisms [90 - 92], which are not primarily removed by the pretreatment process, such as coagulation–flocculation. To date conventional pretreatment (coagulation followed by filtration) and MF are usually used in the drinking water treatment [93-94]. NF process could only be applied directly without pretreatment to the groundwaters containing very low turbidity. Since surface waters contain higher turbidity and the water quality characteristics of surface waters vary significantly, the selection of pretreatment processes is one of the most important factors that determine the success or failure of NF process in the drinking water treatment because membrane fouling is caused by a combined effect of the membrane and feed water properties. Better understanding of the properties of tested waters and the efficiency of different pretreatments to remove foulants from feed waters may help us preventing membrane fouling and designing the best pretreatment process for NF membranes. However, there is little information on the effect of pretreatment processes on the NF membrane fouling in the treatment of actual surface water.