Biofilter Sizing Calculator

earthangroup-biofilter-calculator

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 find 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

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 find 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

About the author

Paul Van der Werf

Paul is the Operations Manager for a 4400m2 integrated aquaculture pilot project in the United Arab Emirates desert he designed and built. This is a commercial aquaponics pilot to evaluate integrated farming in arid climates.

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22 Comments on “Biofilter Sizing Calculator

  1. Interesting I am curious about your assumptions as I have been looking at the UVI system. How do they compare?

    Reply
    1. Hi Alan,

      I was not aware that UVI used a specified biofilter outside of the bird netting in thier solids filter and the surface area of the tanks and rafts. I doubt there is any comparison.

      Regards
      Paul

      Reply
      1. john

        what is the different outside temperatures I need for which of the tilipia, trout and catfish for harvesting?

        Reply
        1. Hi John,

          Check with your local fisheries in your region for that info. They are three very different species, one cold, one temperate and one tropical…

          Regards
          Paul

          Reply
  2. Pingback: Buy Filtration Biomedia for Aquaculture and Aquaponics | Earthan Group Pty Ltd

  3. Alex

    Hi Paul,

    Many useful information. thank you!
    Question. Is it possible to oversize a Bio-Filter. Is it good or bad to have too big bio filter?

    thanks

    Reply
    1. Hi Alex,

      Yes it is easy to oversize but not many downsides aside from cost and retention time can be problematic.

      Reply
  4. David

    Hi Paul,

    Thank you for the very good quality of your postings.
    As you wrote in a other post, frequent feedings during the day will reduce overload on bio filter since it will spread the load during a longer period. Therefore, can we reduce the volume of media according to the # of feeding?
    According to your experience, what is the minimum renewal duration in the biofilter for efficient process?

    Thanks!

    Reply
    1. Hi David and thank you.

      Generally, species and water temperature dependent, 1 to 4 hours between feeds. Within 24 hours of feeding 50% of the metabolism in the system has stopped.

      Regards
      Paul

      Reply
  5. Mark

    Hi,
    very nice post and definetely useful.
    Just a couple of questions: how can I estimate the air requirement for optimal movement of the moving bed biofilter?
    Is there a maximum flow rate through a moving bed biofilter? Is there a level over which the adesion of the bacterial biofilm is reduced or threathened?

    many thanks in advance!

    great job

    Reply
    1. Hamish

      That is a question id really like to know the answer too as well. How to size the air pump for moving bed bio filters.

      Reply
      1. Hi Hamish and Mark,

        A basic rule of thumb for the biofilter aeration is 120 LPM per 1000 litres of biofilter volume. Example, if you are using a 200 litre drum as your biofilter you will want a maximum of 24 litres per minute (120/5). You can adjust it down if you do not have very much biofilter media in the drum. If your filter vessel is filled 50% with media, then the 120lpm/1000ltr will work well.

        Regards
        Paul

        Reply
  6. Hi Paul
    really great web and blog , can I please go back to David’s question do I understand correctly that if I don’t feed my fish for 24 hours the bio filter has become 50 % less efficient? secondly I do tilapia and am stocking at 75 kg /1000l of water do you think the moving bed filter is the best bio filtration I have 52000 litres of tanks and +- 3800kg of fish temp 28 ‘ Celsius how would I size a moving bed filter for this volume of fish.
    regards Neil

    Reply
  7. Pingback: 10 Helpful Tips For Starting the Design of a Fish Farm - Earthan Group Pty Ltd

  8. Dr. Gary Rutledge

    Hi Paul,
    Could you check the link to download the bio-filter calculator?
    When I try it, it returns a 404-Page not found error.
    Thank You!

    Reply
  9. there is a very interesting pdf by James M. Ebeling, Ph.D. that explains most if not all:
    biofiltration-nitrification design overview.pdf

    Specific surface areas range from 4,000 to 45,000 m2/m3 for sand versus 100 to 800 m2/m3 for
    trickling biofilter media and 1050 m2/m3 for bead filter media.

    I have gathered some specific surfaces:
    fluidised sand bed:
    fine sand (D10=0.20-0.25 mm) : ~11.500 m²/m³
    coarse sand (D10 = 0.6-0.8 mm) : ~5.000 m²/m³
    rotating biological contactors and trickling filters: 100-300 m²/m³
    structured packing Accupac CF-3000: 102 m²/m³
    moving bed filters:
    Kaldness 500 m²/m³
    floating beads 3-5 mm (PP or PE): 1.150 – 1.450 m²/m³
    polystyrene micro-beads ~1 mm: 3.780 m²/m³
    polystyrene micro-beads ~1.5 mm: 2.520 m²/m³
    polystyrene micro-beads ~2 mm: 1.890 m²/m³
    polystyrene micro-beads ~3 mm: 1.260 m²/m³

    crushed rock 5 cm 75 m²/m³

    there must be a formula to calculate the average specific area of i.e gravel of different sizes
    which could be interesting to calculate the biofiltration capacity of a gravel growbed

    Soil Texture Classification:
    Soil separate equivalent diameter size (mm)
    gravel > 2 mm
    Sand 0.05 – 2 mm
    very coarse 1 – 2 mm
    coarse 0.5 – 1 mm
    medium 0.25 – 0.5 mm
    fine 0.1 – 0.25 mm
    very fine 0.05 – 0.1 mm
    Silt 0.002 – 0.05 mm
    Clay < 0.002 mm (< 2 micrometer)

    Reply
    1. There is plenty of info from the same author:

      The first major category of submerged biofilters employ a fixed, static packed bed of media that has no active management of either the biofilm or solids accumulation.

      Examples of fixed, static packed beds are submerged rock biofilters, plastic packed beds and shell filters. Submerged packed beds relay entirely upon endogenous respiration to control biofilm accumulation (Manthe et al., 1988). The water can flow either from the bottom up (upflow) or from the top down (downflow). Thus, the hydraulic retention time can be controlled by adjusting the water flow rate.

      Solids from the culture tank can accumulate within the submerged filter, along with cell mass from nitrifying and heterotrophic bacteria. This process can eventually block the void spaces, requiring some mechanism to flush solids from the filter for successful long term operation.

      To provide large void spaces to prevent clogging of the filters, the media used for submerged biofilters has been traditionally of large size, such as uniform crushed rock over 5 cm in diameter or plastic media over 2.5 cm in diameter.

      However, 5 cm diameter crushed rock would only have a specific surface area of 75 m2/m3 and a void fraction of only 40 to 50%. Random packed plastic media would also have a relatively low specific surface area of 100–200 m2/m3, but a much higher void fraction, greater than 95%.

      Drawbacks of this type of filter include problems of low dissolved oxygen and solids accumulation, resulting from heavy loading of organic matter (feed) and the difficulty of backflushing.

      Although this type of filter was promoted and used in aquaculture in the past, it has since been replaced in aquaculture due to the inherent high construction costs, biofouling problems, and operational expense. Packed submerged biofilters are still used in lightly loaded systems such as display aquaria and seafood-holding/display systems, where oyster shells are often used to help maintain calcium carbonate concentrations and other important trace minerals.

      Reply
  10. Renata

    Hallo Paul!
    Vielen Dank für das Tool.

    Ich verfolge Dein Projekt in Dubai seit Anfang an. Habe dabei schon viel gelernt.

    Liebe Grüsse aus Österreicht!
    Renata

    Reply
    1. Sie sind herzlich eingeladen

      Reply
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  12. Alejandro

    Dear Paul,

    I want to know if it´s possible to connect the bioreactor directly to a fish hatchery or it must be previous to a UV filter, could the bioreactor bacteria be harmful to the fry?
    My filter order is Pump->Beads pressurized filter->Uv system (180um/cm2)->Bioreactor (Kaldness K1)->fish tank

    This order is because the bioreactor I did build is not pressurized.

    Regards!

    Reply

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