Pool Water Treatment: physical, chemical and biological facts
Filter performance /
Filtration speed
By reducing the
filtrationspeed from
30 m/hr to 15 m/hr
the performance is
increasing by a factor
4.
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90
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60
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40
30
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10
0
1 5 10 15 20 25 30 35 40 45 50
Filtration speed m/hr
Biological: biology of sand filters
Sand filters are mechanical as well as biological fil-
ters. One m 3 of sand has a surface area of 3‘000 m 2 .
Individual planktonic bacteria floating in the water
are oxidised by low concentrations of chlorine in
less than 30 seconds. Nevertheless, we will still find
bacteria growing in any swimming pool. How is
that possible? When bacteria colonise a surface
in contact with water, they immediately produce
an alginate coat that protects them from chlorine.
Every grain of sand in a sand filter will be colonised
by heterotrophic bacteria (bacteria that feed on
organic matter) after a few days. This bac-teria layer
is called biofilm and makes the sand sticky. On tiles
the biofilm is identified as a slippery surface often
incorrectly referred to as body fat.
Heterotrophic bacteria grow at an incredible speed.
Under favourable conditions they will double in bio-
mass every 15 - 30 minutes. They are the first to
colonise a sand filter and every wetted surface in
4
DRYDEN AQUA
After about 6 - 12 months autotrophic bacteria
colonise the biofilm as well. These bacteria grow
much slower than heterotrophic bacteria (double
approximately every 10 days).
How bacteria survive in a swimming pool?
Sand is a good filtration media and sand filters can
give a very good performance when operated pro-
perly in a good quality filter, such as a filter desig-
ned in accordance with the German DIN standard.
The performance of a sand filter with sand or AFM ®
is a function of the filtration velocity. The slower the
water passes through the filter, the better the per-
formance. However the relationship is not linear:
I.e. if the filtration speed is reduced from 30 m/hr
to 15 m/hr the filtration performance improves by a
factor of 4. For this reason we recommend a filtra-
tion velocity of 15 - 22 m/hr.
According to German DIN standard the maximum
filtration speed is 30 m/h. We recommend lower fil-
tration speeds particularly for new installations. If it
is not possible to re-design the system, a frequency
invertor should be used to slow down the pumps
at night by 50 % from 30 m/hr to 15 m/hr. This will
improve the system performance and save money;
indeed the capital pay-back is less than 12 months.
the pool circuit. The heterotrophic bacteria develop
a biofilm which is then colonised by a diverse range
of other species of bacteria, viruses and protozoa.
The growth of the biofilm is not affected by the
concentration of chlorine but by the availability of
food. If there is no food bacteria can’t multiply.
Bacteria
on
Physical: filtration speed – the lower
the better
Bacteria cling to surfaces (walls, floor, piping systems
and especially in the filter media)
They produce organic matter which in turn serves as
a food source for the heterotrophic bacteria. As a
consequence the biofilm develops even faster and
becomes more stable. The alginate acts like a glue
and sticks the grains together leading to the for-
mation of channels and lumps in the filter bed. The
filter function is considerably reduced which increa-
ses the chlorine demand and the formation of toxic
disinfection by-products. The chlorine demand of
the sand filter will now be in excess of the chlorine
demand attributed to the bathers. Even if there are
no bathers there will still be a chlorine demand and
disinfection by-products will be produced.
Sand filter
Anoxic zones in
the filter bed
Channelling in
the filter bed