Clarification is a process whereby liquid is separated from solid matter. Solids settle due to the pull of gravity.
Clarification
The clarification stage is usually preceded by coagulation and flocculation, during which colloids are destabilized in order that they gather together into flocs of a large enough size to settle. A clarifier (a hydraulic process) is then used to encourage these very light flocs to settle in a tank, from which they can be easily removed.
Clarification takes place in a state of continuous flux. There are several types of flux :
- horizontal flux (generally used in parallelepiped clarifiers),
- vertical flux (generally used in cylindroconical clarifiers with the settling zone in the cylindrical section),
- tilting flux in a countercurrent or co-current lamellar clarifier.
Horizontal flow clarifiers
Horizontal flow clarifiers are the oldest type of clarifier. They are used in major water and wastewater treatment plants around the world, including the water treatment plants at Choisy-le-Roi, Neuilly-sur-Marne and Méry-sur-Oise (together these plants provide water for the majority of Paris' suburbs).
The horizontal flow clarifier consists of a parallelepiped tank in the form of a channel (this is where the name channel clarifier comes from). The flocculated water enters through one end and the clarified water leaves through the other end after it has flowed horizontally through the tank.
Horizontal flow clarifiers are bulky and require a lot of space. However stage clarifiers can be used to reduce the amount of space required. This means that instead of using one single clarifier to treat the Q flow, the Q/n flow is treated using n staged clarifiers.
Lamellar Clarifiers
A clarifier's capacity can be improved by increasing the settling surface. In order to do this, tilted surfaces (plates) are added to the clarifier.
Theoretically, the distance between the plates can be reduced ad infinitum in order to increase the clarifier's capacity.
However, a minimum distance is required in order to ensure the accumulation of sludge. Lamellar modules may either be flat or V-belt plates, which are joined together to form channels. The spaces between each lamella are in effect primary clarifiers.
Ballasted floc clarifiers
According to Stokes' law, particles will settle far more quickly the heavier they are.
The idea behind a ballasted floc clarifier is to weigh down the flocs considerably by attaching microsand grains to them (the size of the grains range from 10 to 100 µm). These are added in large quantities to the raw water. It has been accepted for some time that the presence of this granular load completely changes the coagulation/flocculation process.
Instead of helping to form flocs which swell in 15 to 20 minutes to become 1 to 2 mm large and then slowly settle, the mass of the microsand passes very quickly through the water taking with it all the suspended and colloidal matter. It forms an amalgam of sand + flocs.
Each grain of sand becomes in itself a floc. It is necessary to use an interim binder (polymer) in order to attach the flocs to the grains. The now activated microsand has a twofold effect :
- it acts as ballast for the coagulated matter thereby considerably accelerating sedimentation,
- it also acts as a coagulation initiator, whereby the coagulated matter coalesces around each grain of sand.
Around 3 minutes should suffice in order to obtain a microsand deposit around which the aluminum hydroxide will gather (or iron hydroxide depending on the coagulant used) and that is already linked to the matter that is to be removed from the water.
The microsand grains have a large surface area and therefore provide a major reactivity zone, which makes it possible to overcome problems arising from the flocculation of cold or polluted water.