Biofilters

In addition to offering two standard biofilter models, Aqua Systems can also design and build Biofilters to suit a specific site or system

Which Biofilter? Biological filters generally fall into two categories, submerged, where the media is always underwater (it may either be stationary or free floating) and trickle, where the media is surrounded by air and water and is trickled through it. Aqua Systems now offer two types of biological filter: The Trickle Filter has benefits such as adding oxygen to the water, stripping CO2 and in addition to this can also be used - in conjunction with low power fans - to cool the water down through convective heat loss through evaporative heat loss (which works well only where humidity is low) Trickle filters do however require some on-going management and can - under some operating conditions - require back-flushing. The air fluidised filters are also effective at removing CO2 and will add sufficient oxygen for the bacteria in the filter. In addition to this, Fluidised Filters offer a much more steady state of biofiltration than trickle filters, and so require less management input. We therefore recommend that in applications where cooling is likely to be required, that Trickle Filters should be used, and in applications where no cooling is required, that Fluidised Filters are installed.

How does a Biofilter Work? Biofilters allow colonies of bacteria to form on the large surface areas of specialist media - which can be plastic shapes, sand, beads etc. - the types of bacteria on the media, develop in response to the quality of the water passing them by. The main two groups of bacteria that concern aquaculturists are the nitrifying bacteria and the heterothrophic bacteria. The former of these converts ammonia to nitrite and then to relatively harmless nitrate, and the latter oxidises organic matter (eg. BOD) in the water. These bacteria are naturally occurring in soil and water, and biofilters can be started with just a small amount of bacteria added to them from such sources. However, the start up time for biofilters can range from anything from 2 weeks to several months, dependant upon the water quality etc.. To achieve as quick a start up as possible, we recommend seeding with a bacteria culture and then maintaining an environment as close to the final operating environment as possible.

Starting a Biofilter When starting a biofilter, it is VERY important to monitor the ammonia and nitrite BEFORE you put fish in. Start dosing the filter with small amounts of ammonia chloride, to get the bacteria colonies going. Fish should only be introduced into the systems when the ammonia and nitrite levels have fallen. The graph below shows how the peak of nitrite will always follow the peak in ammonia. Nitrite is VERY toxic, so you must wait until this goes down before you put fish into the system. If you are unsure whether your system is ready, put a few fish in and see how they fare for a week to ten days. When you are ready to stock the system, do it in stages, spread over a couple of weeks, rather than suddenly load it all at once.

Fluidised Filters
Our Fluidised Biofilters use a small polyethylene media with a very high surface area. The density of the media, when covered in the biofilm is very close to that of water. This enables them to flow around the filter freely, and prevents them packing together should the water or air flow to the filter ever stop.
The design of the media serves to protect the biofilm from excessive erosion and abrasion caused by collisions with the other media elements and also the filter walls. One fluidised the filter has approximately 450,000 pieces of media per m3, with an effective surface area of >550m2/m3. The media elements are kept in suspension through a combination of water and air flow. Air flow is provided from a small compressor, which is more energy efficient than using a venturi. The ideal biological thickness is in the region of 300μm, if the media begins to get more than this amount it is noticeable by the gradual darkening of colour of the media, in this case the airflow is increased, which in turn increases the self cleaning capabilities of the filter.
The management input to fluidised filters is far less than other traditional flooded types of filters, due to the filter’s self-cleaning effect. This gives you more time to get on with the important business of growing your fish. As the filters are fully "submerged", they can either be sunk into the ground, or stand on the ground, when they can also provide the same function as a header tank. This means that filters are much more energy efficient than most others.
The standard fluidised filters (Model no. FFR-1) are designed for a feed input of 60kg of feed at 14 - 20oC and 80kg feed at 21 - 30oC (Consult Aqua Systems staff for applications where temperatures are <14oC or > 30 oC). These figures are for commercial diets such as those produced for intensive culture of salmonids, sea bass etc. For diets with lower protein levels the filters will cope with larger amounts of feed. If in doubt please contact Aqua Systems.

Aqua Systems engineers can aslo design larger fluidised biological filters for specific projects. The filters can then be built using either our standard components or using local materials, such as concrete and/or rendered blocks. For very large systems, the fluidised filters are usually modular, enabling part of the system to be shut down, should some maintenance be required.

Trickle Filters
Aqua Systems trickle biofilters are particularly suitable for applications where there may be some cooling required within the system. The filters use RMP media an inert, polypropylene media with a surface area of 400m2m-3. This high surface area, combined with the large open void spaces of the RMP media, make blocking and channeling an unlikely occurrence.
The filter bodies are manufactured to enable complete flooding for either disinfection or backflushing purposes. The standard trickle filters (Model no. TBF-1) are designed for a feed input of 60kg of feed at 14 - 20oC and 80kg feed at 21 - 30oC (Consult Aqua Systems staff for applications where temperatures are <14oC or > 30 oC.). These figures are for commercial diets such as those produced for intensive culture of salmonids, sparids, sea bass etc. For diets with lower protein levels the filters will cope with larger amounts of feed. If in doubt please contact Aqua Systems.

If enquiring about a custom biofilter for your system, we will require the following information to enable us to advise you correctly. Maximum / minimum system operating temperatures Dimensions of the building or area available for the biofilter Species and size of fish to be farmed Rough idea of the stocking and cropping plan Maximum feed to be added to the system per day Other treatment processes in the system