Sand Filters and Fluidised Bed Filters For Goldfish and Koi Ponds
The following is an extract from the Aquaculture Dictionary.
This extract is actually discussing the commercial use in aquaculture of sand
filters but the principles remain for your consideration and comment. By the way
algae can be as small as 4 microns and this is why a sand filter does not remove
algae. Here is the extract:
"Used for the mechanical filtration of solids, not to be confused with fluidised
sand filters, which are used for biological filtration. Although the size of
particle that the sand filters remove is dependant on factors such as the size
of the sand particles, the depth of the bed of sand and the flow rate through
the bed, sand filters are usually regarded to filter water to a nominal 10
Sand filters are designed in two distinct ways:
The first is a simple box structure that operates with a low pressure across
the filter. The water flow-rate to cross sectional area ratio of such filters is
low, and the filters tend to rapidly block in the first few centimetres, with
the rest of the filter staying clean. Such filters are only of use in
applications where the use of pressure filters is impossible or the water is
generally clean and there are only a few particles that need removing. Such an
example may be a ground water supply which is thought to be contaminated with
pathogens through seepage into the spring / borehole. Large filters of this
design are difficult to clean effectively, usually resulting in the bed being
periodically dug out and replaced by fresh sand.
The second, pressurised sand filters are in common use in many aquaculture applications.
They consist of an enclosed vessel that is typically half to two thirds full
with sand. Water is pumped into the top of the filter under a pressure of
approximately 1-2 bar and is forced through the sand to a water-collecting
device at the bottom that allows the water through, but not the sand particles.
The flow is then reversed to back flush the filters.
Pressurised sand filters
are expensive to use for high flows due to the cost of pumping the water through
them. They are however used extensively in hatcheries and also some
recirculation systems, where they are either plumbed in for all the water or as
a side stream, where only a percentage of the water flows through. Their
limitations in recirculation systems is that, in addition to the operational
costs, they use a lot of water for back flushing (a typical sand filter in a
recirculation system will require back flushing 4-6 times a day for 5 minutes
The water flow rate whilst back flushing is the same as the flow rate
when filtering). This is exacerbated by the fact that sand filters in
re-circulated water will also act as biological filters, and a layer of heterotrophic and nitrification bacteria will build up on the sand, causing
channelling and increase back washing frequency. The back wash process is
insufficient to eliminate all the bacteria that soon multiply and block the
filter again. A way round this is to add ozone or other disinfectant chemicals
to the water when back flushing (the advantage of ozone here is that any
residual amounts after back flushing will quickly be neutralised by the organic
compounds in the water). Now that self-cleaning mechanical screen filters are
available with screens of less than 10 microns, the use of sand filters is
becoming less common. "
Fluidised Bed (Bead) Biofilters
Based upon chemical engineering principles this type of filter should be one of
the best types of biofilter.
However, they are probably overkill in most small to medium garden ponds.
This type of filter should be used for larger garden ponds and koi ponds
requiring extra special filtration.
If you refer back to the coffee cup description of biofiltration then the bead
filter is a continuously stirred coffee cup. What could be better? Not much.
In a bead filter (shorter more descriptive name for fluidised bed filter) beads
are actually suspended in a turbulent mass of water. The beads are being
thrashed about and are always in contact with fresh water & oxygen & ammonia. The
bacteria sitting on the bead surfaces thus convert ammonia to nitrite then to
nitrate quickly indeed.
In some bead filters there are up to 600,000 beads per cubic foot of space
this is high surface area, just like we discussed earlier.
Rule of thumb sizing for a bead filter is as follows
1 cubic foot of beads can convert the ammonia released from 1 lb of 35% protein
fish food daily. This can be translated as 1 cubic foot of beads will look after
a pond containing 100 lbs of fish.
Solids accumulate on the bio film of the beads and are removed by backwashing
the bead filter.
A word of warning about this type of filter . Very small or broken beads can
actually block what are called laterals inside the filter. It must be possible
to remove these laterals for cleaning out if necessary.
If you have reached the stage where you believe you want to understand the whole
process of pond filtration then go through these articles in the order