Groundwater and surface water contain two kinds of solids:
- Suspended solids or Total Suspended Solids (TSS), particle size 0.01 – 10 microns
- Total Dissolved Solids (TDS), particle size < 0.01 micron.
TSS generally floats in water because they are negatively charged and repel each other, making it difficult to combine and settle. Therefore, to reduce the TSS content in general, a filtering step is carried out in the form of coagulation – flocculation – sedimentation – filtration.
Coagulation is the process of adding injection or dosing of a positively charged coagulant to neutralize the negatively charged TSS. This process is assisted by using a rapid mixer with a retention time of 1-3 minutes. After neutral, the TSS will stick together and form small flocs.
Flocculation is the process of adding injection or dosing of flocculant (flocculant / polymer) to make small flocs combine into large flocs, with the help of a slow mixer and a retention time of 15-20 minutes.
Clarifier is the process of deposition/sedimentation of these large flocs using the principle of gravity. There are various types of clarifiers, but the most common are the Circular Clarifier and the Rectangular Lamella Clarifier. Circular Clarifier is usually used if the capacity and land owned is large enough, while Lamella Clarifier is usually chosen if land is limited.
Then how to calculate the area of land needed for the Clarifier? The dimensions of the clarifier depend on the hydraulic loading rate (HLR), which is the water flow rate per deposition area in the clarifier. Typical HLR for the clarifier is 0.5-1 m3/(m2.hr). For example, if the flow rate of river water is 100 m3/hour with HLR 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 using the formula for the area of a circle, which is 11.2 m, or if the clarifier is rectangular, it can use a ratio of P : L = 4 : 1, or about P 20m : L 5m. Big enough isn’t it?
So 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 that are mounted at an angle of 55-60° with a distance between settlers of 2-5 cm. Thus, the footprint of the clarifier is reduced because large flocs will hit the settler and accelerate the settling time.
For the Clarifier example above, if we use a plate settler with a size of P 2.4 mx W 1.2 m and a tilt angle of 60°, the projected area per plate = (2.4 x cos 60°) x 1.2 = 1.44 m2. Number of plates needed = 100 m2 / 1.44 m2 = 70 plates. If the distance between the plates is 5 cm, then we need an area length of (70 x 5 cm) + (2 x 1.2m) = 5.9 m. The total area required is ± P 6m : L 1.5m, much smaller than the area calculated without the lamella settler above.
A good coagulation-flocculation-sedimentation design does not only take into account the physical dimensions of the equipment. Selection of the right type and dose of coagulant and flocculant chemicals is also very important for the success of the TSS reduction process from water sources. Therefore, please contact our engineering team for further discussion.
Well, that’s all our article this time, we hope you find it useful. See you in the next article about Media Filtration.