Studies On The Treatment Of Coal And Brewery Wastewater Using Adsorption And Coagulation– Flocculation Techniques

These are broadly divided as anionic, cationic and non- ionic categories (Brostow et al, 2009).

1). Non-ionic polyelctrolytes : These include polyacrylamide (PAM) and poly (ethylene oxide) (PEO).

2). Cationic polyelectrolytes: These are derived by introducing quaternary ammonium groups onto the polymer backbone and phosphonium groups are used to a limiting extent (Brostow et al, 2009). The most commonly used cationic polyelectrolytes are poly(diallyl dimethylammoniun chloride) (poly DADMAC).

3). Anionic Polyelectrolytes: Here, mainly two types of polymers are used:
a).Polymers containing carboxy functional groups. A representative is poly cacrylic acid) and its derivatives.
b). Polymers containing sulfonic acid groups. A representative is poly (Styrene sulfonic acid) (PSSA).

2.6.4 MECHANISM OF COAGULATION xlii Coagulation process is associated with different mechanisms that brings about the destabilization of charged stable water medium. The various mechanisms are discussed below:
a). Double- Layer Compression: The mechanism of double–layer compression relies on compressing the diffuse layer surrounding a colloid. This is accomplished by increasing the ionic strength of the solution through the addition of an indifferent electrolyte. The added electrolyte increases the charge density in the diffuse layer. The diffuse layer is compressed towards the particle surface reducing the thickness of the layer. This ensures that the Zeta potential, Zp significantly decreased to encourage aggregation (Reynolds and Richards, 1996;Menkiti, 2007).
b). Adsorption and Charge Neutralization: This mechanism holds when a coagulant with opposite charge to that of colloids is brought in contact with the colloids such that the charged colloids adsorb the coagulant and thus the colloids are neutralized to initiate the aggregation of these colloids (Reynolds and Richards, 1996). The effect of the adsorption is that it leads to a reduction of Zeta potential, Zp to a level where the colloids are destabilized (Sanks, 1979; O. Melia, 1978).
c). Enmeshment by Precipitate (Sweep-floc Coagulation): This is common with coagulants that have the ability to form precipitate. Typical examples are Fecl3 and lime. The precipitate physically entrap the suspended colloidal particles as they settle. Also this mechanism holds when the colloidal particle themselves serve as nuclei for the formation of the precipitate, which is the hub for the formation of floc that gives rise to sweep-floc coagulation (Bagwel et al, 2001; Sanks, 1979; Swift and Friedlander, 2000).

2.6.5 MECHANISM OF FLOCCULATION With large solvent volumes pervaded by the macromolecular chains, solid particulates are pushed outside the solvated domains (Brostow et al, 2009). This is the main mechanism of action of the flocculating agents. The solid particles aggregate in much smaller regions now available to them. Clearly higher molar mass of the polymer results in larger or more solvated domains. The solvent molecules inside the domains (See figure 1) are protected even more than those which by solvation are attached to macromolecular chains outside (Brostow et al, 2009).

Figure 2: A section of a macromolecular chain in forming of a domain pervaded by a chain in the system of liquid + suspended solid particles. Liquid molecules are either solvated by the chain (shaded area) or else are located inside a domain. When a flocculants is applied and such domains then formed, the solid particulate matter remains outside of the domains (Brostow et al, 2009).

2.6.6 COAGULATION TRANSPORT MECHANISM Coagulation transport mechanism is classified based on the driving force of the process.

a). Perikinetic flocculation/ Coagulation: This is the aggregation of particles caused by random thermal propelled motion (Brownian diffusion). This accounts proposed by Smoluchowski’s theory. The driving force particle movement is thermal energy of the fluid (Han and Lawler, 1992).

2.6.7 FACTORS INFLUENCING COAGULATION

a). Effect of pH: The pH range in which coagulation occurs may be the most important factor in proper coagulation.The vast majority of coagulation problems are related to improper pH levels. Coagulation should be conducted in the optimum pH zones. When this is not done, lower coagulation efficiency results, generally giving rise to waste of chemicals and lowered water quality (WST, 2005).
b). Effect of salt species: Natural waters contain various levels of cations, and anions such as calcium, sodium, magnesium, sulphate , chloride, phosphates and others. Some of these ions may affect the efficiency of coagulation.This is due to the fact that changes in the concentration and species of these ions affect the pH of the water medium. The resulting changes in the pH in turn affect the coagulation process.
c). Effect of Temperature: Low water temperature causes low turbidity removal efficiency and poor effluent quality. Kang et al (1995) indicated that low temperatures had a pronounced detrimental effect flocculation Kinetics slowing down the rate of flocculation. xlvi As water temperature approach freezing temperature, almost all chemical reactions occur more slowly. It can be more difficult therefore to evenly disperse the coagulants into the water. As a result, the coagulation process becomes less efficient, and higher coagulant dosages are generally used to compensate for the effects.
d). Velocity Gradient: High velocity gradient provides more opportunities for collision, but the shear forces from too high a velocity gradients can break up larger floc and will limit the maximum floc size (Yan .J., 2005). The velocity gradient in full scale flocculation basin can be created by a variety of mechanism, including baffle chambers, rotating paddles, reciprocating blade and turbine-type mixers.
e). Effect of dosage: Dosage is a vital factor in coagulation efficiency. For effective coagulation, the dosage must be optimum. Normally, with increasing dosage, turbidities decrease to minimum values as complete destabilization occurs (Reynold and Richards, 1996). This process is dominated by adsorption and charge neutralization mechanism.

The optimum dosage often (but not always) correspond to zeta potential which is near zero. It is important to note that above the optimum dosage, restabilization can take place due to charge reversal on the colloids. It is important therefore that the coagulant dosage should be proportional to the quantity of colloids present (WSSA, 1992).

2.6.8 FACTORS INFLUENCING THE SPEED OF FLOCCULATION Ando et al (1935) concluded the following results after experiments:

a) The higher the LF- units of diphtheria-toxin or anatoxin, the smaller the Kf-value, as has been asserted by many observers.
b) If diphtheria toxin or anatoxin is freed from accompanying substances by dialysis or purification by the calcium phosphate methods, its Kf-value becomes smaller.
c). The Kf-value of diphtheria-toxin or anatoxin largely depends on its pH. Irrespective of the kind of culture media in which toxin was produced, it was found that:

i). The Kf of the purified and its original toxin is least in the untreated solution and increases with the amount of acid or alkali added.
ii). When purified toxin was mixed with the buffer solution of different pH, the kf was found to be the smallest in the mixture with the buffer of pH 6.
iii). Both original and purified anatoxinhad the smallest Kf-value in the most acid-smallest kf-value in the most acid range within which flocculation did occur.
iv). Anatoxin prepared from semi-synthetic broth toxin the kf of which was found , showed flocculation when it was dialysed, acidified or purified

2.6.9 ZETA POTENTIAL AND COLLOIDAL STABILITY.
Zeta potential is the electrokinetic potential across the diffuse layer of the ions surrounding a charged particle which is largely responsible for colloidal stability.
The magnitude of Zeta potential (Zp) is usually used to indicate colloidal particle stability. As the coagulation reaction and destabilization are occurring, the Zp at the surface colloided particle is found to be reducing. The higher the Zp, the greater are the repulsion forces between the colloidal particles and, therefore the more stable is the colloidal suspension . Low Zp indicates relatively unstable system ie. the particles tend to aggregate (Reynolds and Richards, 1996). The equation for

Zp = (4μ)
Where μ = absolute viscosity of the solution (NS/m2)
= electrophoretic velocity
D= Dielectric constant of the solution