4.4.2 Chemical Treatment
Wastewaters that have undergone physical treatment become ready for chemical
treatment. In such wastewaters, pollutants are mostly suspended and colloidal
organic and inorganic substances and to a lesser degree some oil particles. Most of
the suspended particles 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. The
amount of oil present in the wastewater makes this situation even worse.
Chemical treatment methods applied to the system involve the addition of iron
(III) chloride (FeCl 3 ), anionic polyelectrolyte chemicals, and limes to the wastewater, so that they stick together and form heavy clusters (flocs) in a polymerized
hydrated form. The precipitation of these flocs provides removal of them from
wastewater. Thus, the COD of the wastewater is targeted to be at the lowest level
before biological treatment.
In WWTP of Kirlangic Oil Factory, wastewaters are taken into the first rapid-mix
chamber, wherein FeCl 3 purchased as a 40% solution is dosed as 10% solution.
Thus, colloidal substances present in wastewater interact with FeCl 3 and form
positively charged Fe(OH) 3 . Then, the wastewater is taken to second rapid-mix
chamber where neutralization is done by lime dosing to form more stable flocs,
wherein the pH of the wastewater is brought to the range of 7–8 by the addition of
5% lime slurry. After rapid mix, wastewater containing stable flocs is taken into
slow-mixing tank. In this tank, the microflocs are brought into contact with each
other through the process of slow mixing. Collisions of the microfloc particles cause
them to bond to produce larger, visible flocs called pinflocs. The floc size continues
to build up through additional collisions and interaction with inorganic polymers
formed by the coagulant or with organic polymers, 0.002% of the anionic polyelectrolyte, added. Macroflocs are formed. After this process, wastewater is directed to
the chemical precipitation basin where formed macroflocs are settled down at the
bottom of the basin. Also, foams formed on the surface of the basin during chemical
precipitation are skimmed by scrapers and sent to the equalization basin. The effluent
free from flocs is taken into CTP tank for final pH adjustment. There, the pH of the
wastewater is brought to the suitable range for biological treatment (pH 7–9).
As known, iron(III) chloride has an acidic character. The studies also showed that
it drops pH to about two units. Therefore, the necessity of pH adjustment range to
ensure the efficiency of treatment requires using of lime in high amounts which
causes increased levels of sludge formation. Due to the good quality of the wastewater produced in soap-stock unit and the long retention time of the wastewater in
the equalization basin at the first stage of treatment that resulted in a clear wastewater
free from most of the colloidal substances, the use of lime in chemical treatment is
deemed appropriate both economically and in terms of creating less sludge. Therefore, in the present system, only the lime has been used as a coagulant.
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365
Wastewaters that have undergone physical treatment become ready for chemical
treatment. In such wastewaters, pollutants are mostly suspended and colloidal
organic and inorganic substances and to a lesser degree some oil particles. Most of
the suspended particles 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. The
amount of oil present in the wastewater makes this situation even worse.
Chemical treatment methods applied to the system involve the addition of iron
(III) chloride (FeCl 3 ), anionic polyelectrolyte chemicals, and limes to the wastewater, so that they stick together and form heavy clusters (flocs) in a polymerized
hydrated form. The precipitation of these flocs provides removal of them from
wastewater. Thus, the COD of the wastewater is targeted to be at the lowest level
before biological treatment.
In WWTP of Kirlangic Oil Factory, wastewaters are taken into the first rapid-mix
chamber, wherein FeCl 3 purchased as a 40% solution is dosed as 10% solution.
Thus, colloidal substances present in wastewater interact with FeCl 3 and form
positively charged Fe(OH) 3 . Then, the wastewater is taken to second rapid-mix
chamber where neutralization is done by lime dosing to form more stable flocs,
wherein the pH of the wastewater is brought to the range of 7–8 by the addition of
5% lime slurry. After rapid mix, wastewater containing stable flocs is taken into
slow-mixing tank. In this tank, the microflocs are brought into contact with each
other through the process of slow mixing. Collisions of the microfloc particles cause
them to bond to produce larger, visible flocs called pinflocs. The floc size continues
to build up through additional collisions and interaction with inorganic polymers
formed by the coagulant or with organic polymers, 0.002% of the anionic polyelectrolyte, added. Macroflocs are formed. After this process, wastewater is directed to
the chemical precipitation basin where formed macroflocs are settled down at the
bottom of the basin. Also, foams formed on the surface of the basin during chemical
precipitation are skimmed by scrapers and sent to the equalization basin. The effluent
free from flocs is taken into CTP tank for final pH adjustment. There, the pH of the
wastewater is brought to the suitable range for biological treatment (pH 7–9).
As known, iron(III) chloride has an acidic character. The studies also showed that
it drops pH to about two units. Therefore, the necessity of pH adjustment range to
ensure the efficiency of treatment requires using of lime in high amounts which
causes increased levels of sludge formation. Due to the good quality of the wastewater produced in soap-stock unit and the long retention time of the wastewater in
the equalization basin at the first stage of treatment that resulted in a clear wastewater
free from most of the colloidal substances, the use of lime in chemical treatment is
deemed appropriate both economically and in terms of creating less sludge. Therefore, in the present system, only the lime has been used as a coagulant.
8 Vegetable Oil Refining Wastes
365