Coagulation, Flocculation and Clarifier

Groundwater and surface water contain two kinds of solids:

• Total Suspended Solids (TSS), particle size 0.01-10 microns
• Total Dissolved Solids (TDS), particle size of <0.01 microns.

TSS generally float in water because they are negatively charged and repel each other, making it difficult to coalesce and settle. Therefore, to reduce the TSS content in general, the filtering stage is carried out in the form of coagulation - flocculation - sedimentation - filtration.

Flocculation is the process of adding injection coagulant.

Flocculation is the process of adding injection or dosing flocculant (flocculant / polymer) to make small flocs merge into large flocs, with the help of a slow mixer and retention time of 15-20 minutes.

Clarifier is the process of settling/sedimentation of large flocs with the principle of gravity. The type of clarifier varies, but the most common are Circular Clarifier and Rectangular Lamella Clarifier.

Circular Clarifier is usually used if the capacity and land owned is large enough, while Lamella Clarifier is usually chosen if the land is limited.

Then how to find out the land area?

Then how do you know the land area required for a clarifier? Clarifier dimensions depend on the hydraulic loading rate (HLR), which is the water flowrate per settling area in the clarifier. Typical HLR for a clarifier is 0.5-1 m³/(m².hour). For example, if the river water flowrate is 100 m³/hour with HLR of 1 m³/(m².hour), then the required clarifier area = flowrate/HLR = (100 m³/hour) / (1 m³/m².hour) = 100 m². If the clarifier is circular, then the diameter of the clarifier can be calculated by the circular area formula, which is 11.2 m, or if the clarifier is rectangular, it can use the ratio P : L = 4 : 1, or about P 20m: L 5m. Pretty big right?

What if the area we have is not that big? It can be reduced by using a lamella clarifier. The lamella clarifier uses the total projected area of a row of plate or tube settlers mounted at an angle of 55-60° with a distance of 2-5 cm between settlers. Thus, the footprint of the clarifier is reduced because large flocs will hit the settler and speed up the settling time.

For the Clarifier example above, if we use a plate settler measuring P 2.4mx L 1.2m and an inclination angle of 60° then the projected area per plate = (2.4 x cos 60°) x 1.2 = 1.44 m². The number of plates required = 100 m2 / 1.44 m2 = 70 plates. If the distance between plates is 5 cm, then we need an area length of (70 x 5 cm) + (2 x 1.2m) = 5.9m. The total area required is ± P 6m: L 1.5m, much smaller than the area calculation without lamella settler above.

A good coagulation-flocculation-sedimentation design does not only take into account the physical dimensions of the equipment. The selection of the right type and dosage of coagulant and flocculant chemicals is also very important for the success of the TSS reduction process from the water source. Therefore, please contact our engineering team for further discussion. Thus our article this time, hopefully useful. See you in the next article about Filtration Media