The flocculation/clarification process is applied when there is a high degree of turbidity in the water or when solids must be separated from liquids. Flocculation/clarification is highly effective at reducing turbidity and removing color, solids and colloidal material from water and wastewater when used together with chemical feed, sludge treatment and filtration of clarified elements.
Flocculation/clarification consists of four distinctive processes – coagulation, flash mixing, flocculation, and clarification. The variables that affect how these processes are carried out are water velocity, time, and the pH. Sufficient time and velocity are necessary to maximize the probability that particles will come together. The pH level is an important determinant of how thoroughly colloids are removed.
During the coagulation process, chemical coagulants are added to water to destabilize colloidal and finely divided materials and to cause them to begin aggregating. The most commonly employed metal coagulants fall into two groups – aluminum-based, such as aluminum sulfate, aluminum chloride and sodium aluminate; and iron-based, such as ferric sulfate, ferrous sulfate, ferric chloride and ferric chloride sulfate. Other chemicals sometimes used as coagulants in the water treatment process are magnesium carbonate and hydrated lime, among others. Aluminum and iron coagulants work by forming highly adsorptive multi-charged polynuclear complexes. The pH of the system can be manipulated to control the characteristics of the complexes and their effectiveness.
After coagulant chemicals are introduced, the water is mixed quickly and forcefully by the flash mixer so that the chemicals are evenly distributed throughout the water. This step is very important to create the conditions for efficient, effective water treatment. Flash mixing must last at least 30 seconds, or else the chemicals will not be properly distributed, but it typically lasts for less than 60 seconds. When water is flash-mixed for a longer period, the mixer blades will tend to chop or shear the aggregating material back into small particles. Coagulation actual begins during flash mixing as the coagulants neutralize the electrical charge of the fine particles. This stops the repulsion of like-charged particles and allows the particles to begin bonding and forming larger clumps.
After flash mixing, flocculation begins through a slower, gentler mixing that brings the fine particles produced during the coagulation step into contact with each other. The flocculation phase usually goes on for 30-45 minutes in a flocculation basin that may have multiple compartments. Each compartment has a different mixing speed, and these speeds randomly decrease as water flows from the top of the basin to its bottom. This approach allows increasingly large clumps of matter to form without being broken apart by the mixing blades.
At the end of the mixing/flocculation process, most of the turbidity and particulate matter in the water should be formed into a material called floc. Floc consists of relatively large clumps of impurities and bacteria bound together in clusters of about 0.1 to 3 mm in size. Smaller floc or larger floc does not settle as well, and a larger floc is more likely to break apart in the flocculation basin.
Clarification is the last of the four steps in the process. Clarifiers consist of tanks or basins which hold water or wastewater for a period sufficient to allow the floc and other suspended materials to settle to the bottom. The clarification process makes the water clear by removing all kinds of particles, sediments, oil, natural organic matter and color.