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Pollutants can occur in water in various forms, e.g. colloidal, suspended and dissolved. This type of pollution cannot be removed as such by DAF/DGF or by settling. A physical-chemical pre-treatment is needed to render them into a separable form. This can be achieved by means of coagulation and/or flocculation.

Coagulation/Flocculation-process

In this process two successive steps can be distinguished:

1. Peri-kinetic flocculation or coagulation
Coagulation is induced by the dosing of a coagulant agent which has an electrical charge opposite to that of the pollutant particles. Subsequently the very small particles conjoin as a result of the Brownian motion, forming micro flocs. This is a rapid process taking less than a second.

jar_test2. Ortho-kinetic flocculation
This is a comparatively slow process which takes place in a flocculator. A flocculant agent is dosed and agitation of the water, generating velocity gradients is effected. The velocity gradients bring the small flocs together so that larger flocs are formed. In the initial stage of flocculator the velocity gradients, or G-values, should be as high as possible for the purpose of increasing the collisions between the microflocs, promoting floc growth. As the flocculation progresses, the G-values must gradually be decreased in order to prevent floc disruption caused by undesired excessive shearing forces. On the other hand, the G-values should not become too low as this would lead to build up of week flocs. The type of chemicals to be used and their optimum dosing rates can be determined by means of a laboratory test: the jar test. This test is performed in a series of glass beakers fitted with stirrers driven by variable speed motors. Apart from type of chemicals and dosing rates, the test (s) can also yield the parameters upon which the flocculator design is based.

Figure 1: the jar test

Coagulation/flocculation - commonly used flocculators

In conventional water treatment plants flocculation is commonly achieved in one or more continuously stirred tank reactors or in long channels fitted with stirring devices. In those flocculator types disturbing short-circuiting and back mixing effects occur. As a consequence a portion of water leaving the flocculator has been treated for an unnecessary long time. In addition, the turbulent flow patterns in this reactor cause a great variety of velocity gradients, e.g. eddy turbulence at the stirrer tips.
This combination of varying residence times and uncontrolled G-values is a major factor contributing to a disturbed floc growth. Floc structure, shape and size will differ widely, impeding subsequent separation by DAF/DGF or settling. To compensate for these poor characteristics, the retention time in conventional type flocculators is three to five times longer than those indicated by the jar test.

Coagulation/flocculation - Pielkenrood Engineering flocculators

Pielkenrood Engineering flocculators are pipe flocculators and plate flocculators, which are both based on the principles of the “tube-plug flow reactor”.

pipe_flocculator_uk_01Pipe flocculator
The pipe flocculator consists of a calculated length of pipe which is expanded in diameter along its length in order to facilitate the slower ortho-kinetic floc building up. The pipe is divided into certain lengths that are connected by 180° elbows. The result is a “coiled” pipe flocculator that can be suspended on a steel support frame. The flocculator pipe length and diameters can be exactly determined on the basis of jar test results. Chemicals can be injected into the flocculator precisely at those points where they are most effective. The energy required for the flocculation process is not derived from stirring devices but by well defined factors such as fluid velocity and pipe friction under plug flow conditions.
Figure 2: pipe flocculator installed as pre-treatment for a Dissolved Air Flotation.



plate_flocculator_01Plate flocculator
The plate flocculator consists of a number of vertical corrugated plates arranged in two or more compartments. Part of the plates are fixed, while the other plates can be moved in a vertical direction. The water flows between the plates from compartment to compartment in an alternately upward and downward direction. The first flocculator compartment is the smallest one with each successive compartment being larger. This means that the flow velocity of the water decreases in each compartment, resulting in the so-called tapered velocity flocculation.

Figure 3: typical plate flocculator


plate_flocculator_02By raising or lowering the adjustable plates a corrugation phase shift can be effected. The phase shift causes a higher flow resistance between the plates, thus creating increased G-values.The advantage of the plate flocculator is that G-values can be adjusted so that the flocculation process can be optimised to the maximum possible extent. The plate flocculators are mainly used in combination with our plate settlers in process or drinking water plants.

Figure 4: phase shift





afbeeldingPielkenrood flocculator advantages
The Pielkenrood Engineering Coiled Pipe Flocculator and Plate Flocculator give the following advantages:

    Optimal and uniform floc growth by well defined and controlled velocity gradients;
    Narrow residence time distribution;
    Narrow G-values distribution;
    Pre-calculated velocity gradients can be realised;
    Reduced chemical consumption due to well defined flocculation conditions;
    Coiled pipe configuration results in a compact flocculator;
    No moving parts in the coiled pipe flocculator, resulting in low maintenance cost;
    Chemical resistant materials;
    Easy installation and operation.

Figure 5: plate flocculator for a drinking water plant.



The Pielkenrood Engineering coiled pipe flocculators and plate flocculators are ideally suited for combination with DAF/DGF units and Cross Flow plate settlers. 

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