| Diatomaceous earth filters
have been used for many decades in a variety of industries such as the
drinks and pharmaceutical sectors. The process depends upon a filter cake of
diatoms, Kieselgur
or
expanded perlite on series of internal candles. The filter cake is normally
only a few milli meters thick. so it is important that the cake is applied
properly otherwise there will be a short circuiting of the media.
When the differential pressure reaches a certain
point, the filters will need to be back-washed. Only a small amount of
water is required for the application to discharge all of the filter
cake along with the solids removed by the cake. This procedure will be
required once or twice every week in a public pool. In order to
try and prolong the time interval between back-washes the filter media can
be injected into the filter to build up a filter and reduce blinding.
Depending upon loadings this could reduce the back-wash frequency to once
every 2 weeks.
Diatomaceous earth is no longer recommended because
the small particle high free silica component is extremely dangerous.
Perlite is a much safer alternative to diatomaceous earth, however the
very fine particles still represent a potentially serious health hazard.
Performance issue
Perlite can give a very good performance in
terms of solids removal, certainly filtration down to a less than 5 microns,
so the performance at removing the oocysts of Cryptosporidium can be very
good. Indeed the diatomaceous earth filter was first developed by the
US Army as a light weight filter for the mechanical filtration of water.
The system was first adopted for the treatment of municipal drinking
water in 1947, however since then there have only been around 200
systems installed worldwide for drinking water, compared to 100 of thousands
of sand filters.
While the filters have a low capital cost and high
flowrate per unit area, the process is not generally sustainable for
drinking water or swimming pool water treatment . There are health and
safety issues with the dust and cost implications in changing the filter
cake. If the filter cake is discharge to sewer it will be classified
as an industrial discharge and there will be high costs associated with the
discharged solids loading, either to sewer or to specialist disposal sites.
The technology has not been adopted for general water
treatment because you can not use a coagulant/flocculent chemicals with
perlite filters. This is turn means that the filters will not remove
dissolved organics or sub-micron particles from the water, the water will
therefore take on a gray appearance.
Swimming pools use chlorine, the chlorine will react with the
dissolved and colloidal organics to form noxious chlorine reaction products.
Perlite system also consume a great deal more chlorine, water quality will
not be as good as sand or AFM filters and atmosphere will contain
higher levels of carcinogenic THM`s (Triaholomethane). The high level
of organics in the water also promotes the growth of Pseudomonas bacterial
species leading to bacterial issues such as MRSA and Legionella.
Perlite filters are not accepted un German DIN
standards and in the opinion of Dryden Aqua, they should not be used for
swimming pools of any size and loading. |
