Sterile Water Filtration

With the rapid emergence of bottled water plants for the supply of both still water and flavoured carbonated water, it is important not to forget its quality aspects and that is, aseptic filtration of the bottled product.

Why filter?

- To remove particulate matter especially those unsightly and so often seen particles in suspension.
- To remove harmful or disease causing organisms for example, pathogenic bacteria, faecal coliform, parasitic cysts, yeast, moulds etc.
- The removal of disease causing organisms is especially important, especially if the product is to remain on the shop shelf for a period of time and in a qualitative state.

What is allowed in the treatment of bottled water. According to the SABS spec:

- The product must be filtered with at least a pre-filter followed by a micro-filter (membrane) of maximum 0.45µm or less. (0.2µm is recommended for still water).
- There shall be no odours emanating from the product.
- Shelf life must be at least 3 months in prescribed conditions.
- Coliform bacteria and faecal coliform bacteria shall be absent in a 100ml sample.
- There shall be fewer than 100 organisms per total plate count per 1 ml.
- No moulds shall be present.

Filter materials:

- Filter housings with possible exception of the borehole filter must be manufactured from stainless steel. This allows smooth crevice free surfaces, high pressure applications and most importantly, allows hot sterilisation without deformation with both hot water and steam.
- Filter cartridge materials vary but generally are polypropylene for the polishing and pre-filters and ideally Polyethersulfone for the final sterile membrane cartridge. The sterile vent filters are hydrophobic PTFE impregnated borosilicate.

How to filter:

The diagrammatic sketch illustrates a typical plant layout with respect to filtration.
Of particular note, are the pre-filters (10µm) after borehole, the sterile tank venting filters (0.01µm), the polishing filter (5µm) and finally the absolute rated pre-filter (1µm - 0.45µm) followed by the membrane filter (0.2µm).
To put the above sizes in perspective, the human eye can see approximately 40µm and the human hair is approximately 70µm.

How fast to filter:

This generally depends on the filter types used but ideally ought not exceed between 7 and 14 litres per minute per 10" equivalent cartridge length.

How big are the filter housings:

This depends entirely on the flow rate and the potential dirt loading. Housings vary to accommodate cartridges from 10" to 40" in length and in combinations of 1, 3, 4, 6, 7, 9 and 10 cartridges per housing.

Cartridge connections:

Double open ended cartridges are acceptable for the initial pre and polishing filters whilst the membrane cartridge and its pre-filter cartridge ought be fitted with bayonet type connections.

How to clean:

This differs from bottler to bottler and their available resources. Some use hot detergents and others cold detergents and with similarly varying means or methods of application. Others also make use of a hot water rinse at +80ºC and or steam at approximately 121ºC for searching sterilisation.
Whatever the method employed, there must be an assured and effective means of cleaning and sterilisation. Failure of this will result in bacteriological infection of the bottled product.
A simplified layout of a system which includes backwashing of the membranes’s pre-filter cartridge at times of blockage or high )P as well as sanitation of the filter system, its lines and including the filler is as typically illustrated in the following figure;
The above system for detergent cleaning, utilises a venturi dosing system and doses at approximately 2% concentration. Many other variations are possible.

The above information is intended as a brief and general guideline for bottlers. For more detailed information, please do not hesitate in contacting us.