Conventional Surface Water Treatment for Drinking Water.
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Conventional Surface Water Treatment for Drinking Water
Coagulation / Flocculation Physical and chemical processes to induce collisions and growth of particles, and to remove some soluble contaminants Coagulation: Formally, means making particles “sticky” (rapid mix step); colloquially, also refers to forming new particles that collect dissolved contaminants Flocculation: Causing collisions among sticky particles (slow mix step), to form fewer, larger ones Suspensions in which most particles remain in the water are “stable,” so coagulation is sometimes referred to as “particle destabilization.”
Particle Removal Regulations EPA’s surface water treatment rules require systems using surface water or groundwater under the direct influence of surface water to (1) disinfect their water, and (2) filter their water or meet criteria for avoiding filtration so that the following contaminants are controlled at the following levels: Cryptosporidium 99 percent removal. Giardia lamblia: 99.9 percent removal/inactivation Viruses: 99.99 percent removal/inactivation Turbidity: Turbidity can never exceed 1 nephelometric turbidity unit (NTU), and must not exceed 0.3 NTU in 95 percent of daily samples in any month
Some Properties of Natural Organic Matter (NOM) One Person’s Idea of an NOM Molecule Some H + ions, especially from COOH groups, “jump” to water molecules, leaving the remaining molecule with a negative charge +H 2 O +H 3 O + O OHO OOCCOO OH OH O COOH COO O OH HOOC O O O HO O O OH - - - -
Some Properties of Natural Organic Matter (NOM) NOM molecules have a strong affinity for surfaces of natural particles and for polyvalent dissolved ions (Fe 3+, Al 3+ ). In most natural waters, the ratio of NOM to particles is large, so particles are substantially coated, and most NOM is dissolved If a little Fe 3+ or Al 3+ is added, it mostly combines with NOM (both dissolved and on solids). O OHO OOCCOO OH OH O COOH COO O OH HOOC O O O HO O O OH - - - - Fe 3+ If a lot of Fe 3+ or Al 3+ is added, it mostly precipitates.
Coagulation Physics: Particle Stability is Caused Primarily by Electrical Charges on Particle Surfaces Evidence: Electrophoretic mobility Origins –Adsorption of dissolved molecules (NOM) –Imperfections/ impurities in solid Sign and Magnitude –Always negative in natural waters, due to adsorption of NOM molecules that have released H + –Surface electrical potential typically a few tens of mV –Electrical force between colloidal particles at separations up to a few m comparable to gravitational force
Particles attract opposite- charged ions, and repel like- charged ions, generating a diffuse layer in which the particle’s charge is neutralized
When the surface potentials are high, the repulsion is strong even at large separations, and particles do not collide When the surface potentials are low, the particles can approach close enough to collide and stick
Like-charged particles experience an electrical repulsion that decreases with distance, and a “Van der Waals” attraction that is even more sensitive to the separation distance. The electrical repulsion can usually be altered; the Van der Waals attraction cannot.
The net effect of the repulsion and attraction is typically a repulsion at large or moderate separation and an attraction at small separation. If approaching particles have enough momentum to bring them to the location of zero net interaction energy, they will spontaneously collide
Reducing the surface potential reduces the magnitude of the energy barrier, and in some cases can eliminate it entirely.
The “zeta potential” is an indicator of the surface electrical potential. When it is zero, the particles have the greatest tendency to collide, and they are therefore easiest to remove.
Coagulation Chemistry Mechanism 1. Alum [Al 2 (SO 4 ) 3 nH 2 O] and ferric chloride (FeCl 3 ) dissociate to release highly charged ions. Al 2 (SO 4 ) 3 nH 2 O 2 Al 3+ + 3 SO 4 2 + nH 2 O FeCl 3 Fe 3+ + 3 Cl These ions tend to bind either directly to particle surfaces (adsorb) or to NOM on those surfaces. Either way, they reduce the negative charge associated with the particles and reduce repulsion. Ideally, the charge is brought to near zero, and repulsion is almost eliminated.
Coagulation Chemistry Under some conditions, adsorption of Al and Fe species can overwhelm the charge contributed by adsorbed NOM and cause “restabilization” of the particles. Mechanism 2. At higher concentrations and pH, solids can precipitate and enmesh the colloids in a “sweep floc”. Al 3+ + 3 OH Al(OH) 3 (s) Fe 3+ + 3 OH Fe(OH) 3 (s)
Mechanism 3. Polymers can induce coagulation by “bridging” between particles Coagulation Chemistry