Bio filter calculator

We released this simple biofilter calculator over a year ago and since then we have modified it to make it simpler to understand and use.  As with any biological modeling we have to make some assumptions based on historical data and these we have included in this version of the calculator.  It now includes water temperature compensation which is especially important for cold water systems.  This is not a commercial filter design but will be a good reference point.

I recommend you read through the following information on how this biofilter calculates the size of your filter and the bio media required, but for those that just want to get into it you can download it HERE.

Measure Units

This version is in metric (SI) for all calculations.  We will update it to include imperial measures to save our international readers which use this system from converting the formulas to suit.  Google has quite a good calculator for converting units and can be found here.   This will help in the mean time while we convert it for you.

Fish Tank Volume

We have included some tank volume calculators of different shapes to help work out the volume of your fish tanks.  The volume of the fish tanks is not entirely needed for calculating bio filter sizes however it is important to know what fish density you may be running the system at.  This is more of a reference for your system design and will not impact on the filter size.  Input only the volume of your fish tanks to the level which they will be filled.  The volumes of pipes and sumps are not needed.

Number of Fish Tanks

Again not important for biofiltration calculations however it is important for working out your fish maximum density.  If you have multiple tanks of various sizes, add the total volume of the various tanks and leave this cell as 1.  This will make the fish density results incorrect.  So we assume you have fish tanks of the same size.

Number and Final Weight of Fish

The number and final weight of the fish you plan to grow is important to calculate the amount of fish feed you will be feeding each day for which bio filters are sized on.  It will also adjust your fish density.  The weight of the fish refers to the final weight of the fish when you plan to harvest them.

EG: if you plan to harvest your fish at 500 grams enter that number though you may want to design for 600 or 700 grams per fish unless you plan to harvest all of the fish at once.  Keep in mind the fish will continue to grow past the 500 grams if you keep feeding them.

A note on the fish density.  For none commercial growers aim for 25kg/m3.  You can go slightly higher but the system will become more difficult to manage at densities higher than that.

Protein and Feed Rates (the important stuff)

The percentage of protein in feed plays a significant role in designing and sizing biofilters.  The higher the protein, the more nitrogen is produced requiring more surface area, resulting in a larger biofilter.  The total ammonia nitrogen produced by the fish is a very loose or soft number because protein assimilation by the fish varies with environmental and biological conditions resulting in variations in nitrogen excreted.

These variations, along with the biofilters performance in various configurations/design/types we have to make quite a few assumptions about the capacity of the filter.  As follows:

• 80% of the feed is utilized by the fish.  Some fish feed wastes are accounted for though does vary depending on feed type and management
• 20% of the protein utilized by the fish is excreted.  Very much species and conditions dependant
• 100% of the excreted nitrogen is the total ammonia nitrogen
• 1 feed per 24 hours.   Spreading out the feed load over the 24 hours changes the biofiltration requirements so we assume you will feed your fish once per day which is the worst case for nitrogen production
• Wasted feed and excreted solids are removed.  Organic solids will contribute to the nitrogen production in the system.  We assume you are removing those solids.
• No further breakdown of organic nitrogen is included.  Relates to the removal of solids as quickly as possible.  If left to mineralize they will contribute to the nitrogen loading on the bio filter.
• Biofilter TAN efficiency is 50%.  Some filters can be 100% efficient however if you do not have adequate solids filtration the efficiency will be reduced.  We have assumed quite a low efficiency because this calculator does not account for different varieties of filters.
• Nitrogen removal rate is 0.45grams / M2 / day.  This is a very flexible number and be as high as 2grams and as low as 0.2grams /  M2 / day depending on filter type and design.  However we assume a lower which works for a basic moving bed bio reactor.
• Maximum Ammonia concentration is 1mg/L.  This may appear high but in most aquaculture systems they rarely run at 0 or the recommended 0.3mg/L.  This assumption affects the flow rate and retention time through the filter which we have not included in the results.

We can include each of these assumptions in a more complex calculator that permits you to change these assumptions to suit your particular situation which may be more suitable for commercial operations.  However that is not the scope of this biofilter sizing calculator.  Perhaps we will release a more complex version should there be enough demand, however those designing biofilters, like us have their own models to work with.

Bio Media and Filter Size (and temperature)

Temperature does effect the TAN removal rate of filters below 20 Celsius, so we have included a temperature compensation which adjusts the size of the biofilter for colder conditions.  Leave the temperature in this cell at 20 celsius if your system operates consistently above 20 as with higher temperatures the filter will become smaller.

Entering temperatures above 20 will result in the biofilter size showing “temp error”.   This is intentional.  There is no need to decrease the size of  your biofilter if your temps are above 20 which this calculator will do.

Unless you are trying to perfect a commercial system biofilter sizing to keep capital costs to a minimum there is no need to change from 20 celsius.  I remind you this is only a guide not suitable for commercial design.

The biomedia surface area is exactly that, the surface area (m2) of the media used per unit volume (m3).  This is often expressed as SSA m2/m3.  For example if you were to use 50mm gravel as your bio filter, provided it was of uniform size, you would have a surface area of about 75m2/m3 (very low).  If you are buying bio media such as kaldnes,  the manufacturer will indicate what the specific surface area (SSA) of the media is.  However, tread with caution as some media available on the market grossly over state the surface area specs on their product.  Get that wrong and your biofilter will end up too small to cope with the final feed loading.

We have included a biofilter tank sizing that is specifically for MBBR (moving bed filters) which assumes a 60% media volume.  We have found that higher than this, depending on the media type increases the hydraulic loading requirements on the filter to keep it fluidized/moving and induces shearing of the biofilm reducing capacity.

You can download the biofilter calculator here.

Feedback always welcome.

Regards
Paul