Fluidised Bed Filters
or how to build a compact, low maintenance fish room
One perspective from Tony Wilkins - an enthusiastic user
Presented at the Cichlid Society of New South Wales (Australia) meeting on July 3,
1999
Aquarticles
One of the problems often associated with keeping fish is the "ratchet
effect". You know, one tank becomes two, two become three, three become thirteen. It
was at that point that Sabine and I added up the time we were spending on maintenance and
said "there must be a way to spend more time on the fish and less on the
filters". Cleaning each filter had become a full time job.
We reasoned that using only one filter for several tanks might work. The first step was
to create a "test rack" of tanks. Five tanks in a row funning off a sump style
filter made out of a converted two foot tank. This five tank test rack proved one thing
for us.... that cleaning one filter is less work than cleaning five - pretty obvious,
hey?! It also proved another thing, mounting five tanks and a sump filter on top of a row
of filing cabinets is guaranteed to cause problems when the irreplaceable files in the
filing cabinets end up wet. We needed a fish room.
Well, with the stunning success of the "test rack" (and having cleaned up the
wet files from the filing cabinet as best we could) we started to design our long dreamt
of fish room. The plan grew into three racks of about 900 litres total capacity in each
rack, giving 27 tanks with about 2,700 litres capacity. Each rack would have its own sump
and filter system.
So far this looked good. But a measure up of the available space and a calculation of
the size of the mini-reef filters needed caused us to have to make a decision. The choice
was clear, either have less fish tanks to fit in the min-reef filters or design a filter
system that uses less space.
The quest
Since nobody ever has enough tanks we took the path of seeking to have a compact filter
system so as to fit in the maximum number of tanks. Whatever we used had to fit into a
space not much more than one or two feet wide and had to cost as little as possible.
On surfing the NET I started to find information like this:
"Due to its unique shape, the Fluidized Bed Filter is an extremely efficient
... high capacity "biological" filter. Aquatic organisms excrete toxic ammonia
as a metabolic waste product ... with additional ammonia produced as food and other
organic matter breaks down and decomposes. This toxic ammonia (NH3) is converted to
another toxic compound called nitrite (NO2) by the Nitrosomonas species of bacteria. The
nitrite is then converted to a relatively non toxic compound called nitrate (NO3) by the
Nitrobacter species of bacteria. This process is known as "nitrification". A
"biological" filter is a vessel where nitrification occurs and soluble waste is
removed. Beneficial bacteria attach to the media within the Lifegard Fluidized Bed Filter
... creating a thin film around the sand grains. Water is pumped up through the unit...
lifting the sand into a "fluidized bed". The beneficial bacteria attached to the
media utilize dissolved wastes (ammonia and nitrite), oxygen and other required nutrients
from the passing water, converting them to relatively harmless nitrate.
"The sand grains are in continual free fall through the water resulting in an
excellent transfer capability between the water and the bacterial film on the media. The
enormously high surface area combined with this excellent transfer capability creates the
perfect habitat for bacterial growth. In addition, the sand grains bump into each other
frequently knocking off excess debris and providing a self cleaning function which allows
new areas for bacterial growth. The Fluidized Bed Filter has been tested and proven to
supply the highest level of effluent water quality and will respond quickly and
efficiently to severe changes in ammonia levels caused by over feeding or the addition of
too many fish at one time. To obtain highest water quality ... a Fluidized Bed Filter
should be part of your filtration system."
Fluidised bed filters started to look better and better because of their compact size
and reasonable cost (about $145 for a unit capable of treating 1,000 litres of water).
The plan
Well what we decided was a "belt and braces" approach. A tank of 2 x 1 foot
modified to be a pre-filter, with a compartment for a biological medium (as a backup) and
a pump. The pump passed an adjustable flow of water through a sea-storm fluidised bed
filter. The idea was that we could adjust the flow of water through the fluidised bed to
the optimal rate - the reality was that the pump we chose was not really powerful enough
to provide the optimal rate of flow anyway.
The benefits
The filter system has worked a treat. No ammonia spikes even when several hundred fish
are moved from rack to rack challenging the biological filter's capacity to the maximum.
Some of the benefits of the fluidised bed system that we found are:
* time saving, just clean the pre-filters once every two weeks. Only three pre-filters
(one for each rack of tanks) makes it fast and easy.
* space saving, a ten centimetre diameter column only 50 cm high can do the job of
biologically cleaning a 1,000 litre tank system.
* cost effective, in fact dollar for dollar it would be hard to see a better biological
filter for your money. Perhaps the surprise in these units is their simplicity and I am
sure a handy person could construct one for only a few dollars with ground sintered glass
or fine sand as the medium.
* very low maintenance, because an effective pre-filter is used the fluidised bed is self
cleaning. Once a year I back-flush them but this is only necessary because I have allowed
my pre-filter to clog and unfiltered water can then flow directly through the fluidised
bed. A better design could eliminate this problem.
* self cleaning, by this I mean that the biological medium scraps itself clean continually
as the sand rubs past other grains of sand. This is unlike some other types of filters
that can build up areas of debris where biological efficiency is at best reduced or at
worst where anaerobic bacteria move in and contaminate the water.
* fast recovery after power failures. This is certainly my experience but I agree with
many others that this appears to be against common sense because a power failure in a
fluidised bed means no fresh oxygen flowing past the sand. One interesting comment on this
was found on the internet, on "Les's" homepage. I reproduce a part of it here as
I think it might be of interest:
"Les was negative in his comments about this type of filter a year ago and he
challenged the manufacturers claims that the fluid bed had a very high potential for
colonisation by aerobic bacteria. He thought that although the movement of the sand
released a lot more surface area with a potential for bacteria in practice, there could be
insufficient dissolved oxygen for this potential to be realised. On a recent visit to the
Tropical Marine Centre, Les noticed that they had installed large fluidised bed filters
alongside the trickle filter towers. The livestock manager explained that they were
installed because they have found that if there is a breakdown, due to power failure, the
trickle filters fare very badly and take a long time to recover due to the bacteria dying
off en masse. Fluidised bed filters recover quite quickly. This is surprising as one would
assume that the bacterial populations would survive much better than those in immersed
filter systems simply because the trickle filter bed had direct access to atmospheric
oxygen. Apparently there is little osmotic pressure to encourage oxygen to dissolve into
the thin layer of water coating the trickle filter media. Totally immersed fluidised beds
have an oxygen reserve present in the water. The author has installed his own small
fluidised bed filter of his own design in the sump of a Thiel Platinum trickle filter as a
piece of test equipment to handle any future power failures."
But watch out!
And this brings me to the only negative that I have recognised in fluidised bed
filters. They do not aid in gas exchange. Indeed they undoubtedly could cause problems as
the bacteria take out oxygen and add more carbon dioxide - the reverse of what the fish
need. So, to counter this I place airstones in tanks and in the filter sump.
And remember, all biological filters require either maturation of up to 40 days or
seeding to speed up maturation. Seeding can be done by introducing live nitrifying
bacteria into your filter, inoculating your filter with a small amount of detritus from a
mature aquarium, or by adding specific chemicals.
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