Many impurities in water are in the form of suspended particles which have a density greater than that of water. They remain in suspension due to the turbulent motion of water as it flows. However, if the conditions are made more quiescent in nature, these particles will settle out under the influence of gravity. This is the principle behind clarification processes.

Particles may settle in one of four distinctively different ways depending on concentration and the relative tendency of the particles to agglomerate while they settle. At low solid concentrations, typically less than 500– 1000 mg l−1, settlement occurs without interference from neighboring particles (Discrete particle settling). As the concentration increases the influence of surrounding particles increases the settling rate. As the particle concentration increases further the process changes from clarification to hindered settling and thickening.

Discrete particle settling(Type-1 settling)

Horizontal clarifiers are based on either rectangular or circular configurations and were at one time the standard clarification process around the world. However, they are rarely used now in the UK and Europe for new works. They are still used in North America due to their ease of covering and construction. In rectangular tanks the water to be treated flows in at one end and exits from the other. Typical surface overflow rates are in the range 1–2mhr−1 for coagulated water (typically closer to 1 m hr−1) with retention time of 2–3 hr. Circular clarifiers operate from a central feed well with the flow moving outwards to the peripheral weirs such that the velocity progressively decreases.

( Tube settlers)

The principal problem with horizontal clarifiers is the space they take up in generating the required area. One method of improving this is to insert a series of plates or tubes at an inclined angle to effectively increase the area for settlement, known as lamella plates.

Design of Tube or Plate Settlers

Advantage of High-rate systems over traditional systems Settling rates are observed to be an order of magnitude higher than in traditional systems with a flocculation process which is 2–8 times quicker. The combined benefits mean that the clarification process with about a 60–80% reduction in area compared to a traditional settler.