| Seawater
Filtration Dryden Aqua are marine biologists specialising in
seawater filtration systems for the aquaculture industry over the last 25
years. It is absolutely essential to have the best possible filtration when
trying to cultivate delicate marine fish. It is also essential that
the systems are a low cost and reliable, because system failure, even for a
few minutes could result in loss of millions of juvenile fish.
Dryden Aqua in association with the European Commission under the Life
Environment initiative have completed extensive research for the development
of AFM ( Activated Filter Media). AFM replaces the sand in your media
bed filters. Simply by changing the sand and replacing it with AFM media,
you will double the performance of your filters, and present a much better
water quality to the membranes. AFM is more expensive than sand ( by around
a factor of 4 times), however huge savings will be made in energy costs,
cartridge replacement and in protection of the membranes. AFM has a
high negative zeta potential, and water filtered by AFM also appears to
reduce the energy requirements on the membrane pumps. |
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Why you should use AFM |
Replace the sand in your filters with
an engineered filtration media specifically designed for the application |
- Life cycle cost savings that will give a rapid return in capital which
could be as short as a few months
- AFM provides the membranes and cartridge filters with much better
protection than would be possible with sand
- AFM filter can cope with shock loads and high organics
- AFM filtration is repeatable and predictable, ideal for automation
- AFM does not support bacterial growth therefore water channelling
through the filter bed is eliminated
- AFM filter do not destabilise like sand and dump a high level of
solids and colloids back into the product water
- AFM filter media stabilises very quickly after a back-wash
- AFM carries a negative Zeta potential charge so it works
synergistically with positive charged low zeta potential flocculants such
as PAC, Ferric, polyelectrolytes and NoPhos. AFM will tend to increase the
zeta potential of the water and increase membrane permeability by dropping
surface tension.
- AFM does not contain free silica
There is no down-side from using AFM except the higher initial capital
cost, however there will be a return in capital savings measured in months |
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Biofouling and channelling |
| Conventionally, pre-treatment will involve the use of Media
Bed Filters, normally using sand filters, followed by cartridge
filtration stage down to 1 to 5 microns. Sand filters work well for
the application, however the performance of the filters tends to deteriorate
over a period of around 6 months due to biofouling of the media.
It is virtually impossible to prevent biofouling of sand in the media bed
filters. Bacteria will start to colonise the sand immediately the system
goes on-line. The growth rate of heterotrophic bacteria is exponential
with a doubling in biomass every 20 minutes in water over 25 deg C.
Bacteria excrete a monosaccharide alginate as a means of bonding onto the
sand. Over a period of a few months the alginate gradually accumulates
and becomes more resistant to back-wash abrasion. In effect the
alginate starts to glue the sand grains together which leads to worm hole
channels through the filter bed. Channels can start to develop in the
sand bed after 1 to 6 months and filtration performance deteriorates |
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Mechanical filtration & back-washing |
Sand filtration is used for the mechanical filtration of water. As the
water passes down through the sand bed, solid particles are trapped in the
spaces between the sand grains. The surface of the sand will be coated
by a layer of bacteria and the alginates excreted by the bacteria. The
alginate acts like a glue and solids present in the water will tend to stick
to the alginate coat. This is one of the mechanisms by which bacteria
trap their food. The problem is that the bond between the alginate layer on
the sand and solids removed by the filter is very strong, so trying to
back-wash all of the solids out of a sand filter is very difficult.
The bacteria will also react and adapt to any given condition, so
irrespective of the degree of agitation or fluidisation during back-wash
the bacteria will simply evolve and adapt to their environment.
AFM will also remove solids mechanically from the water, however AFM is
manufactured to have surface active properties that prevent the media from
supporting the growth of bacteria. The electron microscope photographs
give a dimension of 60 microns in width. The photo of sand shows the surface
to be cluttered with bacteria and debris, however the AFM is perfectly
clean. AFM back-washes much more effectively
than sand, because there is no alginate glue preventing the release of the
solids. The graph below presents a back-wash profile of a sand filter
and AFM filter operated under identical conditions. The data was
collected by Lyonnaise des Eaux France.
The graph shows the back-wash profile of two AFM filters and two sand
filters. The data was collected after the filters had been in commission for
8 weeks. The graphs clearly show that AFM back-wash profile was
a smooth sine curve, indeed the curve is predictable and exactly the
same over months and years. The area under the curve was 25% more than the
sand filter, which means that 25% more solids were removed by the AFM
filters in comparison to the best filter sand available in the UK.
( Leighton Buzzard deposit). The data was collected before the sand filter
started to exhibit worm hole channels, once the sand filters started to
channel the difference in performance was in excess of 80%. Operation
criteria were, 10 cubm/hr/sqm run phase, back wash was air scour at 90
cubm/hr/sqm followed by counter-current water wash at 45 cubm/hr/sqm
for 10 minutes .
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Physio-chemical filtration |
| Sand is an aluminosilcate mineral, AFM
is an amorphous aluminosilicate, however they both exhibit completely
different properties. While the chemical composition is almost
identical the difference relates to the shape and structure. For example
activated carbon, graphite and diamonds have the same chemical composition,
but they are certainly not the same.
Zeolites such as clinoptilolite is also an aluminosilcate like sand,
however the shape of the crystal lattice structure imparts molecular sieve
ion exchange properties. Zeolites need to be regenerated with a caustic
brine solution once a week which is a major inconvenience and expense.
Activated carbon has to be replaced when it reaches capacity. At Dryden Aqua
we wanted to engineer a product that would have surface active properties
and adsorption, but not molecular sieve filtration or absorption
because both of these properties require a regeneration phase.
AFM is an activated alumino silicate amorphous glass in which we have we
have manufactured to have a high surface area, high Zeta potential and high
catalytic oxidation potential. AFM does not require a regenerator
phase, nor does it impart any chemicals to the water, indeed the performance
of AFM tends to improve with age.
Adsorption and Zeta Potential.
The negative charge on the surface of AFM attracts +ve charged micron and
sub-micron particles as well as dipolar organics molecules. AFM is
therefore removing much smaller particles from the water than would be
possible for a sand filter. AFM is also acting like activated carbon
by removing dissolved organics from the water. The capacity of AFM for
organics is much lower than activated carbon, however because the
organics and solids are only held on to the surface by a weak charge,
every time the AFM media is fluidised during a back-wash, all of
the adsorbed materials are eluted in the back-wash water. AFM therefore
never becomes exhausted.
The surface of AFM also has catalytic properties and will dissociate a
small proportion of dissolved oxygen into very short lived free radicals.
The the radicals will crack carbon double bonds, and oxidise heavy metals.
Also the combination of the catalytic properties and zeta potential stern
layer both contribute to prevent the surface of AFM from becoming biofouled.
A more detailed explanation is available
Click here
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Operating parameters |
| AFM will remove between 25% and 80% more solids
from the water than sand, in addition AFM will remove a proportion of the
micron and sub micron particles as well as dissolved organics and heavy
metals such as iron and manganese. With sand the alginate bacterial
coat restricts the passage of water through the filter bed. The
pressure differential over a sand filter will normally be 15% to 25% greater
than an equivalent AFM filter bed, this translates to an equivalent saving
in pump energy requirements. The filtered water is also used as a source
of back-wash water, with AFM the back wash duration can be as short as 3
minutes as opposed to 10 minutes for sand. Also the back-wash
frequency is normally 50% to 75% lower than sand.
AFM will always perform better than sand at any given flowrate. AFM
has been used in filters up to a flowrate of 80 cubm/hr/squm, however
the performance of any media bed filter, be it sand or AFM varies inversely
proportionally to the flow of water through the filter, so it is
always best to operate at as slow a flowrate as possible, however this
increase the size and the cost of the system, a compromise is therefore
required, and this depends on the seawater quality and the objectives of the
operator.
The normal operating flowrate will be between 15 and 25 cubm/hr/sqm,
however flowrate below 15 cubm/hr/sqm are recommend to reduce the surface
shear forces and increase the adsorption capacity of AFM between back-wash
cycles. Therefore in situations where there is likely to be pollution
incidents, iron or heavy metal issues, hydrocarbon contamination and heavy
loads of solids due to turbulence and storm conditions, we would recommend
flows below 15 cubm/hr/sqm
Back-wash flowrate is the same as sand, it is important to fluidise
the bed by at least 15% at to achieve this task a flow of 45 cubm/hr/sqm is
required. Prior to a back-wash it is useful to employ and air scour at
95 cubm/hr//sqm |
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