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WHY OPTIMIZING THE FILTRATION PROCESS

In an ideal filtration process, the filtrate can drain off freely. In order to achieve the ideal filtration, it is important to optimise the filtration process so that the filtration pressure initially increases slowly and then rises significantly when the filter cake is compressed in the chambers of the filter press. At the same time, the throughput and flow rate should be kept at a high level as long and constantly as possible and only be decreased when the layer to be passed through grows. The suboptimal filtration process and the accompanying optimisation of the same are due to the following factors:

    • Changes in the production properties
    • Further development of your production plants
    • Rising energy and personnel costs
    • Correction of errors in the design

 

OBJECTIVE

 

The general goal in optimising the filtration process is to increase the throughput capacity in the filter press, to increase the filling level of the chamber filter press, to improve cake treatment, e.g. cake washing in membrane presses, and to generally increase the degree of drying in filter presses. At this point, we develop process technology problem solutions for our customers and then assist them up to optimisation in the daily operation.

 

DECISIVE FACTORS FOR THE FILTRATION PROCESS

 

The course of a filtration process is influenced by many factors. The following list contains only the most important of these:

    • Suspension
    • resulting filter cake structure
    • when using filter aids

        • dosing
        • dosing technology
        • handling
    • filter cloths
    • pumps and peripherals

In practice, it is not possible to change these factors arbitrarily. In waste water treatment plants, it is often possible to treat the suspension by thickening, for example, or to positively influence the filtration by adding filter aids. In product filtration, the possibilities of influencing the filtration are often very limited. The suspension cannot be changed and therefore the use of filter aids is often not possible. Ultimately, only the selection of the filter cloths in combination with the pump (type, performance, control) can change the filtration process. In this context, the degree of filling can be improved and the existing potential of the filter press can be exploited by automatically adapting the output rate to the requirements of the process phases by recording the feed pressure, output rate as well as the output amount and the speed control via frequency converters.

 

LAYER STRUCTURE OF THE FILTER CAKE

 

In general, with regard to a solid filter cake result, it is important that no barrier layer is formed during the cake-forming filtration which hinders a uniform cake build-up. The suspension to be filtered must therefore be made to form a filter cake that is capable of being drained. The adjustment of the fabric to the suspension reduces the initial filtration resistance of the fabric, which results in an improved cake structure and better drainage. The addition of flocculants or supporting agents can also enhance the result. However, if the composition of the suspension must not be changed, the result can only be optimised by adjusting the filter cloth accordingly.

Filter cake build-up with layering-praline

The friction of the flowing liquid causes a drag force on the particles. In the following, this is transferred to the underlying particles in the framework. The drag force therefore adds up in the direction of flow and produces the so-called solid and framework pressure in relation to the filter surface.

Comparison of solid-liquid pressure in the filter cake

During cake-forming filtration, the following can be noted:

    • The higher the filter cake, the higher the liquid pressure.
    • The lower the filter cake, the higher the solids pressure.

 

OPTIMISATION OF THE FILTRATION BEHAVIOUR THROUGH THE USE OF FILTER AIDS (E.G. FLOCCULANTS)

 

In order to improve the filter capability of various suspensions, filter aids in particular (e.g. flocculants) are used. These cause, for example, flocculation of the solids. This results in the same filtration properties as with mineral particles.

Use of filter aids in solid-liquid filtration with filter presses

OPTIMISATION OF THE FILTRATION BEHAVIOR BY CONDITIONING WITH POLYMERS

 

Solid-liquid separation is of major importance not only in waste water treatment, but also in many other technical processes. In order to increase the effectiveness of the separation, flocculation aids are used. Very fine particles can thus be quickly converted into large-volume flakes and flake lattices and separated better from the liquid phase. With regard to their chemical structure, the products differ in three areas:

    • cationic flocculants
    • non-ionic flocculants
    • anionic flocculants

The polymers are high-molecular organic flocculants with different degrees of cross-linking. They have been specially developed for the physical-chemical separation of suspensions. Applications for flocculation aids include processes for sludge dewatering and fermentation residue treatment in sewage treatment plants and biogas plants. Further areas of application are:

    • sedimentation
    • Flotation process
    • Filtration
    • Sludge treatment
    • Sludge thickening

 

MODE OF OPERATION OF POLYMERS

 

A prerequisite for optimum flocculation of the solids is the adsorption of the polymers onto the particles to be flocculated. The flocculants have the property of dissociating into ions in aqueous solution. They are therefore also called polyelectrolytes. The bond between the chain molecules and the suspended particles is based on electrostatic forces and hydrogen bridge formation. For the selection of a suitable polymer, appropriate laboratory trials are carried out before practical use. The application of the products requires the advice of one of our technicians in order to correctly design all conditions for storage, production and dosing of solutions.