In water treatment coagulation is the destabilization of colloidal particles by neutralizing their charge with an added chemical, called a coagulant.
In dirty water there are - in order of decreasing size - three groups of substances: suspended solids, colloidal particles and soluble particles. Colloidal or suspended particles are less than 1 micron in diameter, have a negative charge and are stable in water. They don't settle. By adding coagulants such as trivalent iron or trivalent alumium the negative charge of colloidal particles is neutralized by the positive charge of the metal ions of iron or aluminum. This charge neutralization will ensure that the colloidal particles no longer repel each other but can clump together. The agglomerated particles will setlle or float and can easily been removed from the water. By adding flocculants this process can be accelerated.
Besides inorganic coagulants such as iron and aluminum salts, also organic coagulants can be used. The most popular coagulants are FeCl3 (iron trichloride) and PAC (Poly Aluminum Chloride)
By the means of dosing pumps these liquid chemicals can be dosed in the desired concentration.
The dosage of chemicals should best be done correctly. This is necessary for optimal process conditions, for environmental purposes and from an economic point of view as well.
Using a combination of a flow meter on the treated water stream and possibly on the coagulant flow, an optimal dose can be obtained.
The dosing pumps are activated via a 4-20 mA signal or pulse contacts depending on the wastewater flow.
The operator can still manually adjust the dosage per m³ of wastewater as its composition can vary in function of time.
By the means of dosing pumps liquid chemicals are added according to the desired concentration. The most common dosing pumps are:
The stirred flocculator is an open tank with an agitator that delivers the necessary mixing energy. In a classical physico-chemical treatment, there are 2 or 3 mixing chambers.
The flocculator is used for pH correction, coagulation and flocculation. Depending on the application, the rotation speed and the type of agitator are determined. The volume of the flocculator detemines the residence time and a visual inspection gives an immediate idea of the floc forming.
The mixing energy in a pipe flocculator is provided with the feeding pump and through the many bends and strictures.
The contact time is very short and visual inspection is difficult.
This type of flocculator is mainly chosen where easy physical-chemical reaction is suspected.
The flow meter is an important tool in the control of the dosing and ensures that a correct dosage of chemicals takes place.
The electromagnetic flowmeter is the most common as the flow meter measurement takes place in an open tube without "obstacles" and is thus almost insensitive to pollution.
