COAG FLOCCULATION AND ADSORPTION OF PHARMACEUTICAL AND VEGETABLE OIL WASTE WATER USING NATURAL MATERIALS
1.1BACKGROUND OF THE STUDY
In the last few decades, industrial wastewater and effluent treatment gained much importance. This is not unconnected with increased concern about the environmental quality. The presence of contaminants in some water bodies used for domestic and industrial processes and coupled with the strict national and international regulations on water pollution makes it imperative that industrial wastewater should be treated prior to discharge to water bodies and environs. High concentrations of contaminants like heavy metals, organic compounds, sulphates, etc, in surface water are generally associated with industrial effluents. Some of these contaminants are of special concern because they are non biodegradable, and therefore persist in the environment. Heightened awareness of the deleterious effects of industrial effluents on the environment and particularly, water bodies has resulted in an intensive research effort to identify efficient methods of effluent treatment so as to minimize the impacts. Generally, the techniques employed for the removal of pollutants include, precipitation, coagulation and flocculation, lime softening, sorption/adsorption, ion exchange and reverse osmosis or electro dialysis. Although all these techniques afford moderate to efficient industrial pollutants removal, coagulation - flocculation and adsorption or sorption on solid substrate materials (adsorbents) are preferred because of their high efficiency, easy handling, and cost-effectiveness, as well as the availability of different coagulants and adsorbents (Prasad, et al, 2000). The two techniques (Coag-flocculation and adsorption) will be employed simultaneously as non of them can singly achieve the required purity.
Coag-flocculation techniques are very important in wastewater treatment
operations. It is employed to separate suspended solids from water. Finely dispersed solids (colloids) contained in wastewater are stabilized by electric charges on their surface, causing them to repel each other. These charges prevent the particles from colliding to form large masses called flocs. Coagulation is the destabilization of
colloids by neutralizing the forces that keep them apart. These colloids may include organic and inorganic particles (O‘Melia et al, 1978). Several factors influence the operations between the colloids, such as attractive and repulsive forces respectively. These forces may affect the colloids in different ways depending on variation of the operation conditions like (pH, temperature, salt concentration, etc) surrounding them. The rate at which two particles approach each other depends on their static and dynamic properties (Suidan, 1988).
The most frequently occurring forces between colloids are; Van der Waal‘s forces, electrostatic forces and forces due to adsorbed macromolecules. In addition, specific forces may act in specific cases. For instance, magnetically or chemical bond may be found between the colloids (O‘Melia et al, 1978). Some of these forces are active at long range such that their impacts are felt over several tens of nanometers. It is on this phenomenon that the fundamental principle of colloids stability is based. Chemical bonds are short range and therefore can only come into effect in the absence of long-range repulsion.
The flocculation of particles in a liquid depends on the rate of collision between particles, caused by their relative motion. This relative motion may be caused by Brownian force resulting in fluid movement, giving rise to velocity gradient or by particle motion due to an external force (e.g gravity). The rate of flocculation is determined by the collision frequency induced by the relative motion. When this collision is caused by Brownian force/movement, it is referred to as perikinetic flocculation. But, if it is by velocity gradient it is called orthokinetic flocculation. If there is no surface repulsion between the particles, then every collision leads to aggregation and the process is called rapid flocculation. If a significant repulsion exists, then only a fraction of the collision results in aggregation and in this case it is called slow flocculation.
Coagulation includes two separate and sequential stages, a collision stage followed by attachment stage (WST, 2005). The collision stage could be a product of Brownian motion, fluid shear or differential sedimentation. Brownian motion, only affecting the movement of particles (< l µ m) is the random motion of particles caused by the thermal energy surrounding liquids. Fluids shear, either laminar or turbulent, is caused by velocity gradient that occur in all real flow fluids. Differential
sedimentation is produced at a rate associated with the gravity and buoyancy forces. In this process aggregated dense and large particles can settle faster than smaller or less dense ones. (Lentech, et al, 2005; Thomas, et al, 1999).
Adsorption is an important step in many industrial processes mainly in products purification and effluents treatment. Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid or dissolved solid to a surface as a result of some attractive forces between the adsorbing surface and the substance adsorbed. This occurs only when the substance is in contact with the adsorbing surface. The substance thus adsorbed on the surface is called the adsorbate and the substance on which is adsorbed is known as adsorbent.
The extent of adsorption is largely dependent on the characteristics of the adsorbent such as surface area, particle size, porosity, residual electric charges on the adsorbent and adsorbate, capacity for aggregation, and degrees of dissociation, solubilization, and ionization. Also, the nature of transport medium which may include concentration, temperature, pH, presence of foreign ions etc, greatly influence the process of adsorption.
Based on the prevailing forces, adsorption processes can be classified as either physical (Van der Waal‘s adsorption) or chemisorption (activated adsorption). In physical adsorption, the individuality of the adsorbate and the adsorbent are preserved. In chemisorption, there is transfer or sharing of electron or breakage of the adsorbate into atoms or radicals which are bound separately.
The possibilities of using natural coag-flocculants and adsorbents in a view to reduce/remove organic/inorganic loads from pharmaceutical and refined vegetable oil wastewaters have been studied. This will serve as a link to improved ecological and environmental sustainability.