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.