Coagulation-Flocculation

(Adapted from MRWA 2003)

Following coagulation, flocculation, a gentle mixing stage, increases the particle size from submicroscopic microfloc to visible suspended particles.

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. The floc size continues to build through additional collisions and interaction with inorganic polymers formed by the coagulant or with organic polymers added. Macroflocs are formed. High molecular weight polymers, called coagulant aids, may be added during this step to help bridge, bind, and strengthen the floc, add weight, and increase settling rate. Once the floc has reached its optimum size and strength, the water is ready for the separation process (sedimentation, floatation or filtration). Design contact times for flocculation range from 15 or 20 minutes to an hour or more.

(Adapted from SNF FLOERGER 2003)

In water treatment, coagulation and flocculation are practically always applied subsequently before a physical separation. TheCoagulation-Flocculationprocess consists of the following steps:

• Coagulation-flocculation: The use of chemical reagents to destabilise and increase the size of the particles; mixing; increasing of flog size,
• A physical separation of the solids from the liquid phase. This separation is usually achieved by sedimentation (decantation), flotation or filtration.

The common reagents are: mineral and/or organic coagulants (typically iron and aluminium salt, organic polymers), flocculation additives (activated silica, talcum, activated carbon…), anionic or cationic flocculants andpHcontrol reagents such as acids or bases. Certain heavy metal chelating agents can also be added during the coagulation step.

(Adapted from TRIPATHY and DE 2006)

The jar test is used to identify the most adapted mix of chemical compounds and concentrations for coagulation-flocculation. It is a batch test consisting of using several identical jars containing the same volume and concentration of feed, which are charged simultaneously with six different doses of a potentially effective coagulant. The six jars can be stirred simultaneously at known speeds. The treated feed samples are mixed rapidly and then slowly and then allowed to settle. These three stages are an approximation of the sequences based on the large-scale plants of rapid mix, coagulation flocculation and settling basins. At the end of the settling period, test samples are drawn from the jars and turbidity of supernatant liquid is measured. A plot of turbidity against coagulant dose gives an indication of the optimum dosage (i.e. the minimum amount required to give acceptable clarification). The criteria thus obtained from a bench jar test are the quality of resultant floc and the clarity of the supernatant liquid after settling. The design of the full-scale plant process is then done based on the bench-scale selection of chemicals and their concentrations.

Unfortunately, the jar test suffers from a number of disadvantages, despite its widespread application. It is a batch test, which can be very time-consuming. And the results obtained from a series of jar tests might not correspond to the results obtained on a full-scale plant.

(Adapted from WHO 1996)

The operation of coagulators, flocculators and clarifiers requires trained operators. Maintenance work should be undertaken regularly. The key aspects of operation and maintenance of coagulators, flocculators and clarifiers are:

• Chemical stock: There should be a good stock (at least sufficient for one month of operation).
• Dosing control: Correct dosing of coagulant chemicals is very important for efficient and effective removal of suspended solids. Samples of raw water should be taken regularly, and tested with a range of coagulant concentrations to determine the optimum dose rate of coagulant. The results should be used to adjust the coagulant dose.
• Rapid mixing of the water and coagulant chemicals at the point where the chemicals are added is essential.
• Flocculation should be achieved by gentle mixing so as to maximise the number of collisions between suspended particles and flocs, without breaking the flocs up through rapid mixing.
• Plant layout: The flocculator and clarifiers should be located close to one another and water should flow slowly between them so as to not break up the flocs.

During the course of coagulation-flocculation treatment, a substantial amount of sludge coming from the settling process is generated. This sludge can be reused as fertiliser for agriculture when no toxic compounds are present. In the presence of toxic sludge the solid waste has to be treated or disposed of in an environmentally proper manner.