Flocculants process chemicals

The results of this study clearly show the complex dependence of the flocculation process on polymer dosage and charge density. It is seen that the form of dependence varies markedly among the responses monitored. In addition to the factors studied here, it can also be expected to depend upon several other physicochemical conditions of the system, including the type of mixing. The final state of flocculation achieved by a mineral/polymer system will depend upon many interactions in the system as determined by various chemical and hydrodynamic properties of the particles, polymer, dissolved organics and the fluids. 

Fine particles such as clays are normally removed by flocculation using chemicals such as aluminium sulphate. The floes can entrap bacteria and their spores protecting them from chlorination. The vast majority of floe particles are removed during processing but if one or two pass through the system the entrapped bacteria will be unaffected by the final disinfection stage. 

In reviews on the use of in situ sensors" or optical sensor systems" for bioprocesses, UV-vis does not play a major role. An example for the application of at-line UV-vis spectroscopy was presented by Noui et al. The selective flocculation processes of Saccharomyces cerevisiae homogenates were followed with a newly developed direct UV spectrophotometer. The results from a PLS regression model were in good agreement with those from off-line chemical assays. 

Flocculation processes are complicated phenomena because of the varieties of both particle morphology and chemical reactions they encompass.34 A few concepts of a general nature have emerged, however, and they will be the focus of this chapter. From the perspective of kinetics, perhaps the most important of these broad generalizations is the distinction that can be made between transport-controlled and reaction-controlled flocculation, parallel to the classification of adsorption processes described in Section 4.5. Flocculation kinetics are said to exhibit transport control if the rate-limiting step is the movement of two (or more) particles toward one another prior to their close encounter and subsequent combination into a larger particle. Reaction control occurs if it is particle combination instead of particle movement (toward collision) that limits the rate of flocculation. 

The sensitivity of the stability ratio to chemical or particle interaction factors can be illustrated by an examination of the model expression for Wn in Eq. 6.75. For example, if temperature and the particle interaction parameters are fixed, then Wn will vary with the concentration, c (also included in /c), of Z-Z electrolyte. At low values of c, k is also small, and the first equality in Eq. 6.75 indicates that Wu will take on its largest values. (Decreasing c also provokes an increase in dm because of Eq. 6.73, but this effect is dominated by that of k.40) Conversely, as c increases, the value of Wu will drop until it achieves its minimum, Wn = 1.0, when Z dm = 2 (Eq. 6.75). At this concentration, termed the critical coagulation concentration (ccc), or flocculation value, the flocculation process has become transport-controlled and therefore is rapid. Thus in general... 

The electrochemical coagulation is a complex process that can be used to reduce the organic content of many types of industrial wastewaters including those polluted with colloidal particles, macromolecules, or O/W emulsions. This process consists of two sequential processes the dose of the reagent, which is really the more important electrochemical process, and the subsequent coagulation/flocculation processes. In principle, these later processes are not electrochemical but chemical or physical processes. However, the mobility of the pollutants can be greatly improved due to electrophoretic or the electromigration processes, and consequently electrochemistry should also be considered in the description of these processes. 

Dispersion-flocculation processes are generally controlled by double layer swelling, adsorbed hydrolyzed Fe or Al, and chemical bridging (tactoid formation) (Stumm and O Melia, 1968). Once dispersed, clay colloids are kept dispersed by repulsive double layers (Van Olphen, 1971). The force of repulsion is related to the thickness of the double layer (see Chapter 4). This dimension is represented by the ions concentrated near the oppositely charged colloid surface. Any colloid that has a net negative or a net positive charge repulses a like-charged colloid. 

This process is enhanced by the addition of polymers, alum, or other flocculants. This chemical addition provides for coagulation and flocculation of colloidal oils and solids into larger particles that can be easily removed. 

UV spectrum of urban effluent from a chemical treatment plant has thus a featureless shape, like the one of raw wastewater. The absorbance value globally decreases as the wavelength increases. However, the effect of the coagulation-flocculation process can be noted. Indeed, the absorbance value above 250 nm is very low, showing that the concentration of suspended solids and colloids the optical, effect of which being more sensible in this wavelength range (see Chapter 6), is also very low. 

This includes reactions of the polymer groups with metallic sites on the particle surface that may result in the formation of stable or insoluble compounds through covalent, ionic or coordination bonding. Carboxyl flocculants such as polyacrylic acid and carboxyl-methyl cellulose can chemisorb on the surface of calcite and sphalerite which have calcium or zinc sites on them. Certain flocculants, such as cellulose and starch with xanthate and polyacrylamide with dithiocarbamate with high chemically active groups, have been found to exhibit selective reaction with sulfide minerals. Such complexing polymers have been investigated for their use in selective flocculation processes. 

I. Activation of metallic ions in flocculation process Activation of flocculation by metallic ions is similar to that of mineral flotation using collectors discussed earlier. Metallic ions can activate minerals that do not normally have chemical reaction activity with the functional groups of flocculants, which enhance the flocculation effect. For example, for Cu " " -Si02- polymer system, the reaction can be illustrated as below ... 

Both the A1 and Fe trivalent ions form as proton donors, Al(H20)g and Fe(H20)g, which are only stable at low pH [1-3]. At higher pH, they lose protons to form the monomers A1(H20)5(0H) +, A1(H20)4(0H)+, A1(0H)3, and A1(0H)4 progressively as the pH increases. Some of these monomers are unstable and try to form the OH-bridged dimer A1(H20)4(0H)2(H20)4A1 ". These unstable monomers react best at a pH in the range of 5 < pH < 7. This process goes much faster when there are pollutants in the water onto which the dimer can easily form, binding to the pollutant at the same time. The Fe " reactions are similar. These are the standard chemical trivalent ion flocculation processes used in water treated by both chemical and electrocoagulation processes. 

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