Types of milk filter

Types of milk filter

Nearly all the raw milk produced on farms is filtered by disposable milk filters. Permanent stainless steel milk filters do exist but are very seldom used on farms. But regardless of whether a milk filter is disposable or permanent, the basic principles of filtration are common to both.

Disposable milk filter

Disposable milk filters are constructed from nonwoven fabrics and are designed for single use (one milking session) only.

Permanent milk filters Permanent milk filters are designed to be placed directly in the pipeline system (in-line milk filter). They use very similar filtration technology to disposable milk filters. But where a disposable milk filter is made of nonwoven fabric, permanent milk filters may be made of fine stainless steel or plastic screens (mesh). Whichever material is used, it must be temperature and chemical resistant. A range of screens (meshes) can be combined to achieve the desired filtration required for different milking systems.

As with disposable milk filters, the filtration method is based on the principles of mechanical filtration – the driving force being the pressure difference between the two sides of the filter basket when the milk is pumped through it. The milk passes through an inlet to the filter basket and finally through an outlet. Any unwanted particles, such as manure or insects, are retained by the mesh of the filter basket.

Unlike disposable milk filters, permanent milk filters are designed to run continuously. They therefore need regular thorough cleaning to remove any trapped sediment and maintain the filter’s sanitary conditions. Cleaning is carried out manually, by removing the filter basket and washing it. All the materials in a permanent milk filter (the mesh and the housing) must comply fully with EU and FDA regulations, as well as USDA requirements.

Blue milk filter sleeve	Milk filter sock with a good flow rate

Milk filters are available in different sizes

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Milk filter shapes

Disposable milk filters are designed for single use. Three types of milk filter are available:

• Socks

• Sleeves

• Discs

Milk filter sock

A milk filter sock is a folded sheet of milk filter fabric that is joined at the seam along its longitudinal edges and one pair of end edges, to form a tubular sock that is open at one end.

Milk filter sleeve

A milk filter sleeve is also made from a folded sheet of milk filter fabric, but it is only seamed along its longitudinal edges to form a filter tube that is open at both ends.

Milk filter disc

Flat sheets, or so called disc milk filters, are either round or rectangular sheets and are used mainly in bucket or pipeline milking equipment. Milk filter discs do not normally provide the high filtration capacity offered by sleeve or sock filters and are therefore rarely used in industrial scale milking.

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Milk flow direction

In milk filter units that use milk filter socks or sleeves, the milk stream can either flow from the outside to the inside of the milk filter or vice versa. In an outside-to-inside (outside-in) unit, the milk flows inwards and the filter cake builds up on the outside of the milk filter. In an inside-to-outside (inside-out) unit the milk flows outwards and the filter cake is retained inside the milk filter sock.

An outside-in, dead-end milk filter provides a real built-in advantage in that it can help in the identification of mastitis problems within the herd. By inspecting the filter cake that has accu-mulated during the milking session, white flecks of mastitic milk or clots may be observed, indicating clinical mastitis. Coloured milk filters enhance the identification process as it is
much easier to see the white flecks against a coloured background.

Outside-to-inside milk filter unit

Milk filter size

In modern milking plants, milk is usually forced through the in-line milk filter by pump, and it is crucial to establish the correct size of milk filter that is appropriate for the milk flow and the filter housing.

A milk filter that is too small for the task has insufficient surface area for good, efficient filtration. As the filter cake builds up on the milk filter during a milking session, the flow rate through it gradually reduces. This could lead to milk by-passing the filter, or in extreme cases the filter could burst. This could result in unfiltered milk contaminating the bulk milk or damaging the plate cooler. If the milk filter’s capacity and flow rate is not in harmony with the milk pump capacity it will not withstand the pressure of the milk flow and will burst.

On the other hand, a filter that is too large is not cost efficient, as the farmer pays for filter capacity and filtration properties that cannot be capitalised on. There is also a common misconception that an over-sized milk filter can be used for a longer time, but this is not the case.

Professional milk filters are available in various sizes to cope with different requirements, and it is essential to select a filter of the correct size. Milk filter sleeves and socks are more efficient than milk filter discs as they have a large surface area and their design maximises milk flow. This is why these types of filters are used most often in modern milking plants today.

In order to achieve the high capacity and filtration conditions that exist in large scale milking plants, several regular milk filter units can be combined to operate in parallel.

Professional milk filters are available in different, highly specialised fabrics and different sizes. It is essential to select the right filter to solve different tasks in the most appro-priate way. For example, an automatiac milking systems (AMS) where cows are milked automatically one after the other, places entirely different demands upon a milk filter than an 80-stand rotary system (please see Table 4 : Differences of milk filtration in automatic milk systems and conventional parlours).

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