Coagulation-Flocculation
The choice of removal method for FOG from wastewater depends on oil form such
as free or emulsified present. If oil is present in free form, the methods mentioned
above can be used for its removal. However, if oil in wastewater is in emulsified
form, these emulsions must be broken down by physicochemical methods
[41]. Chemical treatment of an emulsion is usually directed toward the destabilization of the dispersed oil droplets or the destruction of emulsifying agents present in a
first stage followed by the removal of the separated oil [37]. Coagulation and
flocculation are used for this purpose.
Coagulation and flocculation are often used interchangeably, but they refer to two
distinct processes. Coagulation refers the process by which colloidal particles and
very fine solid suspensions are destabilized so that they can begin to agglomerate if
the conditions are appropriate. On the other hand, flocculation is the process by
which destabilized particles actually conglomerate into larger aggregates so that they
can be separated from the wastewater.
Most solids suspended in water possess a negative charge and, since they have the
same type of surface charge, repel each other when they come close together.
Therefore, they will remain in suspension rather than clump together and settle out
of the water. Coagulation and flocculation processes are used to remove the
suspended solids portion from the water. The suspended particles vary considerably
in source, composition charge, particle size, shape, and density. Correct application
of coagulation and flocculation processes and selection of the coagulants depend
upon understanding the interaction between these factors. The small particles are
stabilized (kept in suspension) by the action of physical forces on the particles
themselves [28].
Coagulation is commonly achieved by adding different types of chemicals
(coagulants) to the wastewater to promote destabilization of the colloid dispersion
and agglomeration of the resulting individual colloidal particles. The addition of
some common coagulants to a wastewater not only produces coagulation of colloids
but also typically results in the precipitation of soluble compounds, such as phosphates, that can be present in the wastewater. In addition, coagulation can also
produce the removal of particles larger than colloidal particles due to the entrapment
of such particles in the flocs formed during coagulation.
The choice of chemicals used for coagulation and flocculation depends upon the
nature of the suspended solid to be removed, the raw water conditions, the facility
design, and the cost of the amount of chemical necessary to produce the required
effluent quality, as well as the intended disposal route for the sludges. Final selection
of the coagulant should be made by jar testing and plant-scale evaluation. Common
coagulant chemicals used in wastewater treatment are aluminum salts, ferric sulfate,
ferric chloride, ferrous sulfate, sodium aluminate, and lime (Ca(OH) 2 ) or quicklime
(CaO). The first four will lower the alkalinity and pH of the solution while the
sodium aluminate and lime will add alkalinity and raise the pH [42, 43]. Also,
polymers (cationic polymers, anionic and non-ionic polymers) which are longchained, high-molecular-weight, organic chemicals are widely used, especially as
8 Vegetable Oil Refining Wastes
343
The choice of removal method for FOG from wastewater depends on oil form such
as free or emulsified present. If oil is present in free form, the methods mentioned
above can be used for its removal. However, if oil in wastewater is in emulsified
form, these emulsions must be broken down by physicochemical methods
[41]. Chemical treatment of an emulsion is usually directed toward the destabilization of the dispersed oil droplets or the destruction of emulsifying agents present in a
first stage followed by the removal of the separated oil [37]. Coagulation and
flocculation are used for this purpose.
Coagulation and flocculation are often used interchangeably, but they refer to two
distinct processes. Coagulation refers the process by which colloidal particles and
very fine solid suspensions are destabilized so that they can begin to agglomerate if
the conditions are appropriate. On the other hand, flocculation is the process by
which destabilized particles actually conglomerate into larger aggregates so that they
can be separated from the wastewater.
Most solids suspended in water possess a negative charge and, since they have the
same type of surface charge, repel each other when they come close together.
Therefore, they will remain in suspension rather than clump together and settle out
of the water. Coagulation and flocculation processes are used to remove the
suspended solids portion from the water. The suspended particles vary considerably
in source, composition charge, particle size, shape, and density. Correct application
of coagulation and flocculation processes and selection of the coagulants depend
upon understanding the interaction between these factors. The small particles are
stabilized (kept in suspension) by the action of physical forces on the particles
themselves [28].
Coagulation is commonly achieved by adding different types of chemicals
(coagulants) to the wastewater to promote destabilization of the colloid dispersion
and agglomeration of the resulting individual colloidal particles. The addition of
some common coagulants to a wastewater not only produces coagulation of colloids
but also typically results in the precipitation of soluble compounds, such as phosphates, that can be present in the wastewater. In addition, coagulation can also
produce the removal of particles larger than colloidal particles due to the entrapment
of such particles in the flocs formed during coagulation.
The choice of chemicals used for coagulation and flocculation depends upon the
nature of the suspended solid to be removed, the raw water conditions, the facility
design, and the cost of the amount of chemical necessary to produce the required
effluent quality, as well as the intended disposal route for the sludges. Final selection
of the coagulant should be made by jar testing and plant-scale evaluation. Common
coagulant chemicals used in wastewater treatment are aluminum salts, ferric sulfate,
ferric chloride, ferrous sulfate, sodium aluminate, and lime (Ca(OH) 2 ) or quicklime
(CaO). The first four will lower the alkalinity and pH of the solution while the
sodium aluminate and lime will add alkalinity and raise the pH [42, 43]. Also,
polymers (cationic polymers, anionic and non-ionic polymers) which are longchained, high-molecular-weight, organic chemicals are widely used, especially as
8 Vegetable Oil Refining Wastes
343