Mechanical filters.
There are many ideas as
to the importance of the first stage of filtration, but it is the most
important stage to get right for a number of reasons:
The idea of the
mechanical filter is to remove particles from suspension and in turn reduce the
load on the biological section of the filter.
- If large quantities of waste get through to the biological section of the
filter it will clog the filter media and reduce the filters capacity to break
down waste products.
- The more organic waste there is the more oxygen is required to break it
down to harmless products. More oxygen demand means less for the fish in times
of hardship, such as in hot weather.
- The quicker the large waste matter is removed the less work the biological
filter has to do and the lower the nitrate level will be, therefore leading to
less food for the algae, and less algae growth.
- If waste is kept to a minimum the size of the biological filter can be
smaller as it will have less work to do.
There are numerous methods for mechanical filtration, the most commonly
used are a screen or media to remove or trap particles. The other method is to
use a vortex.
The basic principle of the settlement tank is to slow the
water down so that the suspended particles fall to the base of the tank, from
where they can be removed with ease.
In order to break up and slow the
flow of water there has been a number of medias created to do this.
- Filter brushes are the most commonly used and is in fact the one
used incorrectly in most installations. The brush should hang in the flow so
that the water has to pass through the side, not from beneath or through the
end.
The water level should be below the top of the brush to stop the water
by passing the settlement system, by flowing straight over the top of the
brushes.
The brush systems' biggest down fall is that it is very messy and
time consuming to clean.
- Bio-Block is another media that is now available to the pond world.
A simple media created by the joining of net tubes into ‘blocks’
which can be used in the filter either whole or cut to fit. This media has many
advantages over others, the most important is the easy maintenance and it is
almost impossible to clog ( good if you go on holiday). With flow rates of 7500
to 10000 litres per hour per m2 of surface area.
The longer it is
established the better it works as the bacteria build up a filter web between
the net elements and trap the finer particles.
Easy to clean without removal
makes this an excellent media for all ponds.
- The vortex is one of the most under used systems available, but many
that are sold are poorly designed. The flow rate is very
important with a vortex.
The water enters the vortex at a 90º
tangent to the side this causes the water to spin, creating a vortex ( see
picture below for basic design ). At the correct flow the waste works its way
to the centre of the tank and falls into a collecting chamber. If the flow is
to fast the vortex will pick the waste up ( like a tornado) and carry it on to
the next stage of filtration, this is not the desired effect.
The exit
of the vortex can also effect the efficiency of the unit. There are two trains
of thought for this, the first is a centrally mounted pipe that is set just
below the water level and collects the water from the centre of the water
column(Exit 'A'), and the second (Exit 'B')is an outer trough that collects the
surface water as it over flows into it from around the edge.
The ideal
flow rate is between 12,000 to 17,500 litres per hour per square metre of
surface area, this is for tanks over 1m deep. Shallow tanks less than 1m
generally don't work very well unless the flow rate is slowed right down to
suit, but this tends to make them some what redundant as brushes tend to work
better.
Example:
If a vortex tank was 0.9m
diameter (90cm) the surface area would be 0.9 x 0.9=0.81m2 so the average flow
required would be 0.81 x 15,000= 12,100 litres per hour. As with any system the
flow rate will differ from one pond set up to another so some amount of trial
and error will be needed to get the best results. Although slower flow rates
generally work better.
- Baffle tanks. An older system that works very well, requires very
little maintenance, but needs a large tank to work as the flow rates are very
slow. The baffles are set at angles from the vertical plain of 32.5º to
45º depending on the flow rate, this needs to be done very accurately. As
the water enters the tank it works its way up to the surface, which ever way it
flows it will travel the same distance and therefore the same speed depositing
the waste particles one the way.If you have the space this is a very good
system, but is difficult to set up and get started.
- Pressurised sand filters.
Sand filters where designed to remove
low volumes of small particles in relatively clean environments, so the garden
pond is not an ideal situation for them. They are expensive to purchase and run
as they require a large pump with a high pressure rating to clean them. They
require regular of cleaning as they clog very quickly and in some situations
can cause gas bubble disease ( not a disease but a condition similar to the
buoyancy in effect.)
The principle of the filter is to force the dirty
water through a sand bed and as the water passes through the particles are left
behind and the more the filter clogs the better it works, but the flow rate
slows down as well. The multi port valve is a single valve that allows the
control of the filter from one easy selection from the settings, it allows,
normal filtration,, back washing for cleaning, rinse to stop waste entering the
pond and recirculation. This allows work to be carried out to the filter
without stopping the pump, although it is always recommended to switch the pump
while working with water.
The back wash cycle requires a large pump,
typically a 1hp, to back wash a 60cm filter. There is 100kg of sand, in a 60cm
filter, to lift during the back wash process as the clean water is forced to
the bottom of the filter and then up through the sand and out to
waste.
Leave them to swimming pools where the work
well.
- Drum Filters.
The hi-tec way and the best method for particle
removal is the drum filter. They are the choice of the fish farmer as they
offer the best of all worlds. The drum filter has many advantages over all
other systems, they are:
- Drum filters can filter down to 20 microns with ease. At this
level they can remove algae that causes green water, free swimming parasites
and almost all waste particles leaving clear water to pass to the biological
filter.
- Once dirty they clean themselves. This is an enormous advantage
in this busy world, time is important to people and the less time spent
cleaning filters the better.
- The waste is washed away so nothing is left to do.
- Compact. The efficiency of the unit means they can be very small
and therefore don't waste space.
- Due to the quick removal of waste the nitrate levels remain lower
for longer and less water changes are needed.
The main drawback to drum filters is the cost, but the benefits of
size, can often be recouped in the rest of the system as the requirements are
low, no tanks or complex pipe work has it is built in.
Drum filters
biggest benefits are the lack of maintenance and efficiency, nothing comes
close to these filters, hence their popularity in fish farms all over the
world.
