Theory on Coagulation, Flocculation and Precipitation, Chemistry for Wastewater Treatment

Theory on Coagulation, Flocculation, and Precipitation Suspended Solids removal from wastewater stream Target constituents •

Remove suspended solids at the lower end of size spectrum

Figure 1.

Particle size spectrum - Osmonics

Treatment problem: Suspended Solids •

Particles make up a stable suspension which will not settle. Particles are known as colloids.

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Table 1.

Particle size found in water treatment Material Viruses Bacteria Small colloids Large colloids Soil Sand Floc particle

Particle diameter in micrometer 0.005 – 0.01 0.3 – 3.0 0.001 – 0.1 0.1 – 1 1 – 100 500 100 – 2000

Characteristics of Colloids The unique behavior of colloidal particles is a result of surface phenomena. The principal phenomena controlling the behavior of colloids are electrostatic forces, van der Waals forces, and Brownian motion.

Electrostatic forces: Electrostatic force is the principal force contributing to the stability of the colloidal suspensions. Most colloids are electrically charged; negatively charged colloids predominate in natural waters. A model of a colloidal particle and its double layer is illustrated in Figure 2.

Figure 2.

Model of a colloidal particle and double layer. (a) A negative colloidal particle with its electrostatic field. (b) Distribution of electrostatic potential, shear plane, and zeta potential.

Van der Waals Forces: A force of attraction between any two masses. Van der Waals forces are the antithesis of electrostatic forces. As indicated in Figure 3, the force of repulsion due to the electrical charge will normally repel the colloids before they can move close enough for van der Waals forces to become significant.

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Goal:  magnitude of electrostatic forces must be reduced!

Brownian Motion: Brownian motion can cause two colloids to move sufficiently close for van der Waals forces to predominate over electrostatic repulsion. However, the destabilizing influence of Brownian motion is insignificant.

Figure 3.

Forces acting on colloids

Coagulation of Colloidal Suspensions Coagulation is a chemical treatment process used to destabilize colloidal particles. Traditionally, metals salts such as aluminum sulfate (alum), ferric sulfate, ferric chloride, and ferrous sulfate have been utilized as coagulants. In recent years, polymers (long-chain organic compounds) have been used in conjunction with, or in lieu of, metal salts to enhance the coagulation process. Metal salts added to water will hydrolyze into complex metal hydroxides of the form Meq(OH)p. The actual hydroxide formed is dependent upon water composition, pH, and coagulant dosage. Figure 4 illustrates a typical equilibrium diagram for both iron and aluminum hydroxides in water.

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Coagulants typically destabilize colloids by a combination of five mechanisms: compression of the double layer, counter ion adsorption and charge neutralization, interparticle bridging, enmeshment in a precipitate, and heterocoagulation.

Compression of the Double Layer (DLVO Theory) As the concentration of counter ions in solution increases, the counter ions cause the net charge in the diffused layer to neutralize and result in the compression of this layer. This compression affects the thickness of the entire double layer and so allows colloids to come closer together. The phenomenon is illustrated in Figure 5.

Figure 4.

Equilibrium solubility diagram of aluminum and ferric hydroxide in water

Figure 5.

Forces acting on a colloid after compression of the double layer.

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Counter ions adsorption and charge neutralization Counter ions from coagulants can also be adsorbed onto the surface of the colloidal particles. In this way, the repulsive charges on the surface of the particles may be fully neutralized by the charges carried by the counterions. Enmeshment in a precipitate The dosage of metal salts used in coagulation is usually slightly in excess of the amount required for reduction of the zeta potential. The excess metal salts hydrolyze into the form Meq(OH)p. These hydroxides are extremely insoluble in water. As the hydroxide precipitate forms and accumulates, the small colloidal particles are entrapped or enmeshed in the hydroxide floc structure (“enmeshment in a precipitate”; “sweep-floc coagulation”). Interparticle bridging If a synthetic organic polymer is utilized, the interparticle bridging begins with adsorption of the polymer onto specific sites on the surface of colloidal and/or coagulant particles. The resulting structure grows into a single particle several times larger than any of its individual constituents (Figure 6). In order for this bridging to occur, the segments of a polymer chain must be adsorbed onto sites on more than one particle. An excess dosage of polymer may cause restabilization due to surface saturation or sterical stabilization (Figure 7). Figure 6.

Agglomerated solids using polymers

Heterocoagulation The surface charge on the surface of some naturally occurring particles may not be uniform. Oppositely charged sites may exist on the surface of the same particle. The coagulation of these colloidal particles can therefore occur via simple electrostatic interaction between these oppositely charged sites (“heterocoagulation”).

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Figure 7.

Destabilization and restabilization of colloids by polymers (a) Initial adsorption at optimum polymer dosage. (b) Floc formation. (c) Secondary adsorption. (d) Initial adsorption with excess polymer dosage.

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Enhanced Coagulation In recent years, the coagulation process has also been broadly utilized to remove, not only turbidity, but also other undesirable organic and inorganic contaminants from the raw water. These objectives can be achieved by use of an enhanced coagulation process, in which an elevated coagulant dosage is usually required (“Enhanced Surface Water Treatment Rule and the Disinfection and Disinfection By-products Rule (D/DBPR) – 1994”). Most recently, the application of enhanced coagulation are no longer limited to TOC removal but also applied to many other impurities, such as color, arsenic, and other heavy metals.

Dissolved Solids and Chemical Precipitation The design of coagulation process involves: (1) selection of proper coagulant chemicals and their dosages and (2) design of rapid-mix and flocculation basins.

Definitions: Coagulation is the addition and rapid mixing of a coagulant, the resulting destabilization of colloidal and fine suspended solids, and the initial aggregation of the destabilized particles (“chemical conditioning”). Particulates in source water that contribute to color and turbidity are mainly clays, silts, viruses, bacteria, fulvic and humic acids, minerals (including asbestos, silicates, silica, and radioactive particles), and organic particulates. Flocculation is the slow stirring to aggregate the destabilized particles and form a rapidsettling floc (“physical conditioning”). Chemical precipitation, which is closely related to chemical coagulation, consists of the precipitation of dissolved minerals or solids in water by chemically or physically modifying the solution.

Dissolved Solids and Chemical Precipitation Characteristics of ionic compounds In water treatment, the precipitation process is used for softening (removal of hardness caused by calcium and magnesium) and for removal of iron and manganese.

Aqueous Chemistry of Iron and Aluminum Salts Coagulant reactions are carried out by addition of a coagulant, usually a metal salt, to water. Commonly used coagulants are ferric sulfate [Fe2(SO4)3], ferric chloride [FeCl3], and alum [aluminum sulfate, Al2(SO4)3*14 H2O]. The overall reactions of these coagulants in water are described below:

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Ferric sulfate: Fe2(SO4)3 + 3 Ca(HCO3)2 2 Fe(OH)3(s) + 3 CaSO4 + 6 CO2 Ferric chloride: 2 FeCl3 + 3 Ca(HCO3)2 2 Fe(OH)3(s) + 3 CaCl3 + 6 CO2 Alum: Al2(SO4)3 * 14 H2O + 3 Ca(HCO3)2 (2 Al(OH)3)(s) + 3 CaSO4 + 6 CO2 + 14 H2O The following observations can be made on these reactions: •

1 mg of ferric sulfate will produce approximately 0.54 mg of insoluble Fe(OH)3 precipitates and will consume approximately 0.75 mg of alkalinity (expressed of CaCO3).

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1 mg of ferric chloride will produce approximately 0.66 mg of insoluble Fe(OH)3 precipitates and will consume approximately 0.92 mg of alkalinity (expressed of CaCO3).

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1 mg of alum will produce approximately 0.26 mg of insoluble Al(OH)3 precipitates and will consume approximately 0.51 mg of alkalinity (expressed of CaCO3).

In conventional coagulation, the Fe3+ dose is in the range of 2 to 4 mg/L; the Fe3+ dose may reach as high as 11 mg/L in enhanced coagulation. The metal-ion content in a liquid commercial coagulant is typically in the range of 10-15% by liquid weight. The physical and chemical properties of commonly used coagulants are summarized in Table 2.

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Table 2.

Commonly used coagulants and limes in water treatment processes

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Rapid Mix Coagulation and precipitation processes both require the addition of chemicals to the water stream. The process of dispersing chemicals is known as rapid mix or flash mix. Types of Mixers Rapid-mixing units can be classified according to the method of agitation (mechanical or static) and type of flow pattern (plug-flow or complete mix) (Figure 8). Mechanically agitated mixers: • impeller radial-flow impellers or turbine impellers, Figure 8 (a) and (b); axial-flow pitched-blade impellers (Figure 8 (c) • propeller (Figure 8 (d) and (e)) Figure 8.

Radial- and axial-flow impellers

Static mixers create turbulence by the use of hydraulic jumps, baffles, turbulent flow in a pipeline or channel, or contractions or enlargements in a pipeline. Table 3 compares the advantages and disadvantages of mechanical and static mixers. Table 3.

Comparison of Mechanical and Static Mixers

Mixer Type Mechanical mixers

Static mixers

Advantages • • • • •

Agitation independent of flow rate Agitation is adjustable High flexibility in operation Little or no maintenance Very reliable

Disadvantages • • • • •

Additional equipment required for maintenance Reliability subject to equipment failure Agitation dependent on flow rate High head loss Less flexibility in operation

Agitation Requirements In water treatment, the degree of agitation in a mixing unit is measured by velocity gradient. For mixing equipment, the value of the velocity gradient is given by equation 1.

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(1) where: G = velocity gradient, 1/s (G = 700 to 1,000 1/s) P = power imparted to the water, N-m/s or Watt, W (lb-ft/s) V = volume of the basin, m3 (ft3) μ = absolute viscosity of the fluid, N-s/m2 (lb-s/ft2)

The motor power of the mixer is the power to drive the speed reduction gears. The power imparted to the water by a mixer is calculated from equation 2. P=2
nT

(2)

where n = impeller speed, revolutions per second (rps) T = impeller shaft torque, N-m (lb-ft) Power expressions for laminar-flow range (Reynolds number NR < 10), equation 3 P = Npμn2d3

(3)

where Np = power number of impeller (Table 4) d = impeller diameter, m (ft)  = mass density of fluid, kg/m3  = water density, N/m3 (lb/ft3) g = acceleration due to gravity, m/s2 (ft/s2) μ = dynamic viscosity of water, N-s/m2 (lb-s/ft2) Power expressions for turbulent-flow range (Reynolds number NR > 10,000), equation 4 P = Npn3d5

(4)

The Reynolds number for rapid mixers is given in equation 5. (5)

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The design value of the velocity gradient G is dependent on the detention time in the mixing unit, the coagulant dosage rate, and the geometry of the mixing unit. Velocity gradients normally range from 100 to 1000/s. In general, a short duration of high intensity mixing often gives the best results. The velocity gradient for a mixing basin utilizing flow-induced turbulence can be calculated from equation (6). (6) where hL = total head loss through the mixer, m t = detention time, s In U.S. units, simplified for 4 ºC, equation 7 (7) where h’L = total head loss through the mixer, ft t’ = detention time, min

Table 4.

Power Numbers of Various Rapid-Mix Impellers Power Number, Np

Radial flow Straight blade turbine 4 blade (w/d = 0.15) 4 blade (w/d = 0.2) Disc turbine 4 blade (w/d = 0.25) 6 blade (w/d = 0.25)

2.6 3.3 5.1 6.2

Axial flow Propeller 1:1 pitch Propeller 1.5:1 pitch 45º Pitched blade 4 blade (w/d = 0.15) 4 blade (w/d = 0.2)

0.3 0.7 1.36 1.94

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The detention time in rapid mixers (equation 8) should provide sufficient time for complete homogenization of the chemicals with the water and also provide sufficient time for the floc to reach particle-size equilibrium. Particle-size equilibrium refers to the condition where no additional rapid mixing will result in any further turbidity removal by settling alone. Typical detention time for rapid mixers ranges from 10 s to 5 min. Shorter detention times require higher velocity gradients to achieve effective mixing. Conversely, longer detention times permit lower velocity gradients. Optimum design values are best determined experimentally. (8) where t = average detention time, min Q = flow rate, m3/min (ft3/min) V =volume of the reactor, m3 (ft3)

Geometry of Rapid-Mix Basin Rapid mixers utilizing mechanical mixers are usually square and have a depth-to-width ratio of approximately 2. Mixing units with vertical flow patterns utilizing radial-flow mixers tend to minimize short-circuiting effects (Figure 9). Round or cylindrical mixing chambers should be avoided for mechanical mixers.

Figure 9. Flow pattern in radial flow mixed unit

Figure 10. Mixing utilizing a hydraulic jump

A channel with fully turbulent flow of sufficient length to yield the desired detention time, followed by a hydraulic jump, has been used successfully (Figure 10). Also, pipe reducers and increasers with a sufficient length of pipe develop fully turbulent flow condition and may give the desired detention time. In all cases, the coagulant chemical should be added to the water stream immediately prior to the point of greatest turbulence.

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Figure 10.

Details of rapid-mix basins: Overall layout and plan view of rapid-mix basin

Figure 11.

Details of rapid-mix basins: A-A and B-B

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