Pre-coat filter
The continuous pre-coat filter works by constantly adding filter material to the product being filtered, forming layers that trap unwanted particles.
The main elements of the pre-coat filter include a bell, filter plates, a dosing pump and a mixing tank.
There are different types of pre-coating: horizontal bell, vertical bell, alluvial plates and candle; each with its own advantages and disadvantages.
The precoat, essential for filtration, ensures immediate clarification and makes it easier to break up the filter cake. Regular alluviation improves filtration cycles by delaying clogging and maintaining the porosity of the cake.
The main elements of the pre-coat filter include a bell, filter plates, a dosing pump and a mixing tank.
There are different types of pre-coating: horizontal bell, vertical bell, alluvial plates and candle; each with its own advantages and disadvantages.
The precoat, essential for filtration, ensures immediate clarification and makes it easier to break up the filter cake. Regular alluviation improves filtration cycles by delaying clogging and maintaining the porosity of the cake.
Alluvial filtration
In continuous alluvial filtration, the filtering material is continuously supplied mixed with the product to be filtered. The filter medium forms the filter, in one or more layers, which retains the undesirable particles.- Bell
- Filter tray
- Cake discharge opening
- Motorised system for rotating the trays
- Filter tray washing ramp
- External residual filtration element (depending on model)
- Dosing pump for the filtration additive
- Additive mixing tank, with agitator
- Main feed pump
- Flow rate indicator
- Bypass
Main elements
The bell
There are two types of bell: vertical and horizontal.The filter plates
In vertical bells, the trays are either horizontal or vertical.Tray construction
The trays are made up of:- a wide-mesh metal frame to support the filtration fabric and drain the filtrate.
- a stainless steel wire or reps fabric. The fabric pore diameters currently encountered are generally between 50 and 120 µm.
Plateaux horizontaux
Maillage reps
The mixing tank
It is fitted with an agitator.The feed pump
It determines the flow of wine to be filtered.The dosing pump
It ensures that the wine is alluviated.The by-pass
It ensures a partial return of the filtered wine with the wine to be filtered.The different alluvial filters
Horizontal bell and vertical plate filters
Although they are still widely used, their production has virtually ceased.| BENEFITS | DRAWBACKS |
|---|---|
| - Both sides of the trays are used | - High water consumption during cleaning |
| - Compact height | - Significant loss of wine |
| - Automatic filter cleaning | - Wastewater filtration not possible |
| - Easy removal of the filter elements | - Water-based gluing is not possible |
| - Often attractively priced | - No dry extraction of the filter cake (polluting waste) |
| - It is not possible to change the quality of the wine | |
| - Leakage problem in the filter column | |
| - Very delicate handling of the filter cake |
Vertical bell-type and horizontal tray-type filters
These are the most common sediment filters| BENEFITS | DRAWBACKS |
|---|---|
| - The cake has very good stability; filtration can be stopped without damaging the pre-layer | - Only one side of the tray is used - Removing the wire elements is rather awkward |
| - Ability to switch between different grades of wine without blending (making full use of the pre-coating) | - Depending on the make, problems with the seal on the column |
| - Dry extraction of the cake | - A more expensive investment |
| - Can be glued using water-based adhesive | |
| - Wastewater filtration possible | |
| - Low water consumption during washing | |
| - Long cycle time | |
| - Automation capability | |
| - Compact footprint |
Hollow-frame, backwashable plate filters
They are based on the conventional plate filter.Hollow frames are inserted between the filter elements, fitted with suitable support plates.
| BENEFITS | DRAWBACKS |
|---|---|
| - Very large filter surfaces | - Careful handling of the cake |
| - No mechanical moving parts subject to wear and tear | - Cannot be glued with water-based adhesives |
| - Versatility (use of plates or additives) | - No residual filtration |
| - Easy to maintain (sturdy materials) | - Time-consuming and laborious cleaning (labour-intensive) |
| - Long cycle time | - Time-consuming implementation |
| - Very high filtration costs | |
| - Significant filtration residues |
Candle filters
They are rarely used in oenology. These are vertical bell-type filters with vertical filter media. This media takes the form of ‘tubes' consisting either of stainless steel washers stacked on a shaft, or of a coiled stainless steel wire.| BENEFITS | DRAWBACKS |
|---|---|
| - Very large filter areas | - The cake is very delicate to handle |
| - No moving filter elements | - No water-based adhesives |
| - No residual filtration | |
| - High water consumption during washing | |
| - Problems with the maintenance of filter media | |
| - It is not possible to change the quality of the wine | |
| - Very high price |
Primer coat
This is a crucial stage that determines the effectiveness of all subsequent filtration.The objectives of pre-coating are :
- To ensure immediate clarification
- To facilitate the breaking up of the filter cake
Procedure:
It can be carried out in one, two or even three stages, depending on the objectives set but, above all, on the condition of the product to be filtered.The basic rules are:
- In all cases, the filter aid, applied directly to the filter medium, must have a permeability of at least 1 darcie. Never use fine kieselguhr (pink diatomaceous earth) directly, as it adheres poorly and some very fine particles may pass through the filter medium during filtration.
- Use a clear liquid (water or pre-filtered wine) to prevent the filter from clogging too quickly and to ensure a sufficiently long filtration cycle.
- The flow rate through the filter should always be as high as possible (1.5 to 2 times the filtration flow rate) to ensure an even distribution of the filter aid across all the trays.
- Generally, the minimum quantity of filter media for the pre-coat will be 1 kg/m² (up to 2 kg/m² may be used).
- In the case of a pre-coat consisting of several layers, avoid mixing the filter media. A structure comprising successive layers is always preferable, offering satisfactory repeatability.Furthermore, never place a high-permeability diatomite after a finer diatomite, as this will form a glaze layer inside the filter cake, leading to rapid and completely irreversible clogging.
- When filling the filter, leave the air vent open to prevent degradation of the filter cake, as CO₂ disrupts the formation of the pre-coat.
- Under normal conditions, once the pre-coat has formed, the pressure drop is 0.3 to 0.5 bar.
Filtration aid
Before implementing alluvial filtration , it is necessary to select the filter aid(s) best suited to the type of product to be clarified and the desired objectives.Generally speaking, diatoms can be classified into three categories:- Rapid or coarse filtration
- Medium or clarifying
- Fine or finishing
Note: the categorisation of perlites follows the same criteria. Cellulose and its derivatives are merely technical additives used in the composition of the pre-layer.Note: the categorisation of perlites follows the same criteria. Cellulose and its derivatives are merely technical additives in the composition of the pre-layer.
Alluvial deposition
Throughout the filtration process, the regular addition of kieselguhr makes it possible to:
- the quantity of particles to be filtered (load)It is necessary to determine the type and quantity of additive that will result in a slow and steady increase in pressure at the filter inlet. Experience makes these choices easier.
- increase the number of filtration cycles by delaying clogging
- to reduce the pressure differential
- to maintain the porosity of the filter cake as much as possible
- Backwashing is carried out using the additive that was used to form the final part of the pre-coat. Never backfill with a diatom that is ‘faster' than the one used to complete the pre-coat. This would cause a glazing layer to form inside the cake, as would happen with a poor pre-coat.
- The optimum dosage depends on:
- the quantity of particles to be filtered (load)It is necessary to determine the type and quantity of additive that will result in a slow and steady increase in pressure at the filter inlet. Experience makes these choices easier.
Horizontal trays holding the filter cake
| Types of wine | Average rates of alluvial deposition |
|---|---|
| NEW WINES Early filtration of products with a high solids content and very poor filterability |
150 à 170 g/hl |
| UNFILTERED WINES Unfined, racked |
110 à 130 g/hl |
| FINE WINES | 80 à 100 g/hl |
| PRE-FILTERED WINES + FINING | 60 à 80 g/hl |
Filtration pipe
In this filtration process, the filter medium is the filter itself, operating continuously.The filtration cycle takes place in two stages:
Formation of the pre-coat (bonding)
A filtration layer is formed on the filter medium using a filtration additive, which may be kieselguhr (diatoms), perlite or even various fibres (cellulose, polyethylene, etc.).
A filtration layer is formed on the filter medium using a filtration additive, which may be kieselguhr (diatoms), perlite or even various fibres (cellulose, polyethylene, etc.).
Alluvial filtration
Throughout the filtration process, a mixture of the wine to be filtered and a filtering aid is introduced by alluvial deposition. This results in a series of filtering layers comprising the filtering aid and the retained impurities.
Throughout the filtration process, a mixture of the wine to be filtered and a filtering aid is introduced by alluvial deposition. This results in a series of filtering layers comprising the filtering aid and the retained impurities.
Residual filtration
Horizontal plate filters are fitted with residual filtration devices, the purpose of which is to clarify and drain off, at the end of the cycle, all the wine contained within the filter.
There are two types of system:
Horizontal plate filters are fitted with residual filtration devices, the purpose of which is to clarify and drain off, at the end of the cycle, all the wine contained within the filter.
There are two types of system:
- The external auxiliary filter
- The trays integrated into the base of the filtering ‘pack'
Discharging involves removing the filter cake at the end of the filtration process or after clogging.
Depending on the type of filter, disassembly is carried out as follows:
In the latter case, two configurations are possible:
Depending on the type of filter, disassembly is carried out as follows:
- by washing with water for models with vertical trays
- in a paste-like form or otherwise for models with horizontal trays
In the latter case, two configurations are possible:
- automatically by setting the filter elements in motion
- manually by tilting the trays
Cleaning
Washing is carried out using pressurised water supplied by the main feed pump. The unit may be fitted with an internal washing boom, which may or may not be movable.
The waste from the cleaning process is paste-like in consistency.Note: Periodic chemical cleaning is necessary to prevent clogging and limescale build-up on the filter elements.
Washing is carried out using pressurised water supplied by the main feed pump. The unit may be fitted with an internal washing boom, which may or may not be movable.
The waste from the cleaning process is paste-like in consistency.Note: Periodic chemical cleaning is necessary to prevent clogging and limescale build-up on the filter elements.
Choosing the equipment
Determining the most suitable filter area for a given production unit requires taking into account a whole range of technical, economic and practical factors. To do this, a case-by-case assessment should be carried out in consultation with a professional.Resolving faults
The pressure ΔP is rising too quickly- The additive is unsuitable
- Use a coarser diatomaceous earth - Insufficient sedimentation
- Increase the additive concentration in the mixing tank - Increase the backwash flow rate
- The main pump's flow rate is too high
- Reduce the flow rate of the feed pump
- Increase the bypass volume
- The additive is unsuitable
- Use a finer diatomaceous earth - The sedimentation rate is higher than required
- Reduce the additive concentration
- Reduce the sand feed rate - The main pump's flow rate is too low
- Increase the feed pump flow rate
- The slurry additive is finer than that used in the last pre-coat, causing clogging
- The solids concentration is increasing (e.g. tank sediment)
- Reduce the impurity load by dilution: Part of the filtered wine is returned to the main pump's suction via the bypass. This is often necessary when filtering new wines.
- Reduce the inlet flow rate into the bell.
- Increase sedimentation by increasing the flow rate of the dosing pump.
- Increase the concentration of filter aid in the sedimentation tank.
- Operate the air vent frequently, especially in the case of young wines with a high CO₂ content
- Avoid pressure surges by operating the valves carefully.
