Understanding Biological Surface Area in Aquaponics

Understanding Biological Surface Area in Aquaponics
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Growing with aquaponics can be a fantastic way to experience higher yields, better efficiency and healthier plants. One aspect of this type of production method that often gets overlooked is Biological Surface Area (BSA) in Aquaponics.

Understanding Biological Surface Area In Aquaponics

This post is to help you better understand the importance of biological and specific surface area to produce higher yields and fewer frustrating mistakes!

What is Biological Surface Area?

 

To start, Biological Surface Area (BSA) is the amount of surface area inside your system that microbes can live on. BSA is very important in aquaponic systems because these microbes are the engines of a healthy aquaponics system. They oxidize ammonia, assist in nitrification and mineralizes materials like iron in order to foster healthy plant growth and a healthy system overall.

Measuring Biological Surface Area

We typically measure BSA in the total number of square feet per system.

To fully grasp this measurement, we’ll also need to understand how much Specific Surface Area (SSA) is our system. SSA is measured as the number of square feet per cubic feet (ft2/ft3).

This is the amount of square feet there are inside of the volume of media you’re using. (EXAMPLE: If you were using crushed granite or river rock as your grow media, you would need to calculate the total surface area of each piece of granite/rock.)

Once we have calculated the Specific Surface Area, all we have to do is multiply the SSA by the VOLUME of the grow beds or ZipGrow towers to get the Biological Surface Area.

Why Understanding Biological Surface Area is Important

Figuring out how much BSA is in your system will help you to understand:

  • Whether or not your fish are understocked or overstocked and help you make the adjustments necessary for a efficient, effective growing system.

Biological Surface Area in Common Aquaponics Media Types

To give you an idea of how much BSA/SSA  is in various media types, I’ll turn it over to Dr. Nate Storey’s research on the matter.

From Storey, 2012:

Table 2.01  Specific surface area comparisons for different substrates.

Particle Size

Specific Surface Area

Media Type

inches

mm

ft2 ft-3

m-2m-3

Void Ratio (%)

Hydraulic Conductivity (m/d)

Medium Sand

0.12

3

270

886

40

1

Pea Gravel

0.57

14.5

85

280

28

104

Rock

1

25

21

69

40

105

Large Rock

4

102

12

39

48

106

Plastic biofilter media

1

25

85

280

90

107

Plastic biofilter media

2

50

48

157

93

108

Plastic biofilter media

3.5

89

38

125

95

108

ZipGrow Matrix media

N/A

N/A

290

960

91

107*

* estimated to be approximately that of small diameter plastic biofilter media

These studies are especially relevant to this research, and especially the design phase of tower development, during which the properties of the media used had to be closely defined.  Deciding on the media type was difficult and literature detailing the inverse relationship between particle size and Specific Surface Area (SSA in m2 m-3) was useful.  This is due to the relationship between percolation and SSA that is a feature of most aggregates.  As particle size gets smaller, specific surface area for that media type increases, that is to say, the surface area to volume ratio increases, i.e.:

 

- medium sand (3 mm diameter), SSA= 886 m2 m-3;

- pea gravel (14.5 mm diameter), SSA=280 m2 m-3;

- medium gravel (25 mm diameter), SSA=69 m2 m-3;

- large gravel (102 mm diameter), SSA=39 m2 m-3; (Crites, et al., 2006).

 

It should be noted that values in the literature can be somewhat contradictory depending on the source.  This is primarily due to differences in measurement and classification standards.  What these values will show however, regardless of technique, is that smaller particles are better suited for integration into systems where high SSA values are important.

 

Unfortunately, the reality is that these small particles trap solids much more efficiently and rapidly foul with accumulated biosolids, leading to anerobic conditions and lower dissolved oxygen (DO) concentrations that negate the benefits of small particle size.  This low hydraulic conductivity and small pore size (low void space/void fraction) makes small-particle media inappropriate for most biologically active systems with active cycling.  To avoid this problem, larger particle sizes are commonly used (17 mm crushed granite or ¾ inch crushed granite) having higher void ratios (and resulting high hydraulic conductivity) so that solids impact percolation less.  However, even though these crushed aggregates have significantly higher SSA than non-angular and non-crushed aggregates, SSA is still comparatively low, resulting in reduced overall system Biological Surface Area (BSA or total surface area of system measured in m2). 

As you can see, different medias have drastically different Biological Surface Areas. 

Calculating YOUR Biological Surface Area

Remember: as an ABSOLUTE minimum, your system needs at least:

2.5ft2 of BSA/gallon of water (at low stocking densities and low feeding rates)

 

For a healthier system, we would recommend:

10ft2/gallon of water OR 100ft2/pound of fish

 

ANOTHER EXAMPLE:

If you’re stocking fish at 1 pound per 10 gallons, for every pound of fish, you’ll need 25 ft2 of BSA – This will be the amount you’ll need for adequate waste and ammonia processing.

Does the Age of My System Matter?

Yes!Biological Surface Area in Aquaponics

Generally speaking, older systems are going to be much more efficient at processing waste (i.e. the microbial communities inhabiting older systems are much more established, stable and able to operate more effectively as a result).

 

Younger systems (see: newer/less mature systems),  you’ll need more BSA right away to help in the nitrification process.

** IMPORTANT:  If you haven’t properly cycled your system, it doesn’t matter how much biological surface area you have.**

Remember:

A truly healthy AP system requires as much Biological Surface Area as possible – BSA is the horsepower of your aquaponics system! 

 

ZipGrow Towers & High Specific Surface Area

 

If you noticed in the table above, ZipGrow Towers have a very high SSA, BSA and void ratio.

Lettuce in ZipGrow Towers
The reason for this is that WE DESIGNED THEM THIS WAY!

As you see in the table, our Towers and Matrix Media have 290 square feet(!) of specific surface area per cubic foot of our media.

Our media fibers provide a ton of surface area for our microbes to hang out on and keep our system healthy.

 

The high SSA, in combination with a void ratio of 91%, which allows water and solids to flow through our towers easily, creates a productive powerhouse in our aquaponics system. (Don’t forget the light weight and ease of transport/maintenance!)

Particle Size

Specific Surface Area

Media Type

inches

mm

ft2 ft-3

m-2m-3

Void Ratio (%)

Hydraulic Conductivity (m/d)

ZipGrow Matrix media

N/A

N/A

290

960

91

107*

 

Find more examples and equations in our video on Biological Surface Area in Aquaponics

Sources:

Table adapted from Crites et al. (2006) by Storey (2012) showing specific surface area (SSA) in square feet per cubic foot and square meters per cubic meter  for several different media types common to trickling biofilters and constructed wetlands.  Nonwoven fiber medium, such as ZipGrow Matrix Media supplements this table for comparison to traditional media types.  Values are those reported by the manufacturer and determined through testing and estimation based on similar media types.

Crites, R., E. Middlebrooks, and S. Reed.  2006.  Natural Wastewater Treatment Systems.  Taylor and Francis Group, Boca Raton, Florida, USA.

Storey, N.R. 2012. Vertical Aquaponic Crop Production Towers and Associated Produce Sales and Distribution Models: Design, Development, and Analysis. Ph.D. Dissertation, University of Wyoming.

27 Comments

  1. hi Nate.
    I have another question. If a given amount of surface area of media can filter the ammonia of 1 lb. of fish at 10 gallons of water per fish, why can’t it be done at 5 gallons per fish or lower? TIA

    Reply
    • Russell,
      It will still operate just fine at that stocking density. The problems arise from other issues-i.e. filtration, dissolved oxygen consumption, fish stress, and ammonia accumulation due to inadequate cycling times.

      Reply
  2. Can you ever have TOO much bio surface? I don’t think so but I just wanted to make sure. Thank you for your time.

    Reply
    • Hey Don, we don’t think so either! Although.. it may be possible, but there’s no way we could ever reach such a number!

      Reply
  3. Another great video/blog post! My apologies if this information is elsewhere but what is the media volume of the 3′ and 5′ ZipGrow Towers (or rather, what is the BSA/tower)? I’m trying to get a feel for the number of towers/gallon needed. Cheers and keep up the awesome work.

    Reply
    • I’d like to see this explained also. What is the BSA for each foot of tower?

      Reply
      • Hi DunMor Perma, it’s about 30 square feet of BSA per foot of ZipGrow tower.

        Reply
    • Hey Ted,

      Our 5 ZipGrow towers have 150 square feet of biological surface area in them while our three footers have about 80-90 square feet of BSA.

      Reply
  4. Thanks Nate- one other question if you dont mind:
    I read that each square meter of biologically active surface can metabolize nearly one gram of ammonia per day, dependent on temperature. If this is true then 1 lb of fish would only require a minimum BSA of 2, not 25? My calculations:

    1 pound of fish, will eat 0.015 lbs of feed (i.e. Tilapia eat 1.5% of their body weight per day) which will produce 0.00045lbs of ammonia per day (3% of the feed is converted to ammonia (or 10% of the protein in a 30% protein diet). 0.00045 lbs of ammonia = 0.2 grams of ammonia
    If 1 square meter of BSA can metabolize 1 gram of ammonia per day, then only 0.2 sq meters or 2 square ft of BSA is required to process 0.2 grams of ammonia that is generated from 1 lb of fish/day. Why is the minimum BSA for 1 lb of fish 25 with a maximum set at 100? Seems like this is overkill if you only need a BSA of 2 for every lb of fish.

    Reply
    • Would love to hear a comment, Nate ? Anyone?

      Reply
      • Sorry for the delay!

        Yes, your numbers are technically correct, but a BSA of 2 square feet for every pound of fish falls apart in the real world environment when you have fluctuations in temperature, pH, system chemistry and ecological successions. In your average environment, once all of the variables are calculated in, you don’t want to plant for best case scenario, but worst case, or minimally, standard conditions. Also, 3% conversion of protein to ammonia is very low. Depending on temperature,operator error, pH, fish stress, etc. much more is often converted to ammonia. If you go with the textbook calculations, you’ll have fish kill after fish kill. If you run with my recommended BSA requirements, your system will be much more stable. (From Dr. Nate)

        Reply
        • In addition, the amount of BSA required for bio-filtration capacity of media is also dependent on the fish solid waste load (not just ammonia) which requires 10 fold more SSA than ammonia for nitrifying bacteria to process it efficiently when you crunch the numbers! (see http://russellwatergardens.com/Styles/metabolization.php ).

          Reply
  5. Got to say I love the information from this site and Dr. Nate’s YouTube videos! Concerning BSA, I calculated the BSA for my current system and it is roughly 1.8 ft^2 per gallon of water. What types of things can I do to increase the level of BSA without growing the footprint of my system (currently size constraint)? My system is 285 gallons with a combination of (1) 3/4″ expanded shale media bed (half the size of the DWC) and (1) DWC bed. Can I add extra expanded shale/high SSA fibrous plastic media to the bottom of the DWC bed or fish tank??

    Keep up the great work!

    Reply
    • Hi Steve,
      Yes, you can always add growbeds or add high SSA material to your fish tank or sump. I wouldn’t put it in your DWC bed since most high SSA material can collect debris and solids, leading to lower DO, but if you put it in your sump you should be fine!

      Reply
  6. Dr Nate Storey

    I am from Venezuela and I have a question for you.

    You state that: “If you’re stocking fish at 1 pound per 10 gallons, for every pound of fish, you’ll need 25 ft2 of BSA – This will be the amount you’ll need for adequate “”waste and ammonia”” processing”

    Suppose that I get rid of the waste (feces and unprocessed food ) from the water before it enters in contact with the filter. I imagine that since the bacteria only has to deal with ammonia it would need less BSA. If it is so? How much BSA I would need to process only ammonia?

    I can use rice husk here in Venezuela. It is relatively cheap. According to my calculations rice should have a SSA of around 1300 m2 m-3 with a void ratio of 40% to 50%. The void ratio is not too high but I will get rid of the waste before enters the filter. Do you think in your opinion that rice will work under this conditions? As long as I keep it as free of waste as possible?

    Reply
  7. Sorry I made a mistake the SSA of the rice husk is around 650 m2 m-3 according to my calculus

    Reply
  8. Hi Mate,

    I have a question for you. What is the sf2/ft3 of expanded shale and expanded clay?

    Reply
  9. Given the better performance of it, is there a reason why I shouldn’t use the verticle media in a horizontal grow bed instead of stone media?

    Reply
    • Hey Jeff, you can totally adapt Matrix Media for your horizontal application. This would help tremendously with aquaponics specifically because of the insanely high biological surface area. You could actually probably design a really neat horizontal system with our media that would allow you to clean them easily, replacement as needed and reduce weight as well. Could be a really cool thing! Let us know how it goes?

      Reply
  10. Hi Chris. I’m using Hydroton as my grow medium. Do you have any SSA/void ratio/hydraulic conductivity data for expanded clay pebbles? Thanks.

    Reply
    • Hi Tom,

      Fortunately we primarily use Matrix Media and aren’t aware of the SSA/void ratio/hydraulic conductivity of Hydroton. I’d guess it would be similar to 3/4” crushed granite. You may reach out to it’s manufacturer for more detailed info.

      Sorry I’m not more help!

      Reply
  11. I’m still in the research and development stage. Thanks to everyone for your ideas and comments.

    Reply
  12. Just curious if disolved oxygen is increased by use of towers? (Waterfall principle)

    Reply
    • Hi Katrina,

      That will depend on the system. It can significantly increase the amount of DO in many systems, especially if they’re always dealing with low DO to begin with. Great question!

      Reply
  13. I have a question please. My medium is lava rock. It ranges from 3/4″ to 1-1/2″ with very odd shapes. My beds are 40″ x 48″, and 10″ deep. They take 8″ of water twice an hour. I have two beds per 175 gallon fish tank. It’s a chop system, so I have a sump tank. I’m asking what you think my fish population should be per gallon.

    Thanks! Love the videos!

    Reply
  14. So I watched the video and started looking for a company to buy bulk hydroton from because I’m planning on about 10,000 gallons of water. I cam across this company. http://www.growstone.com/hydroponic-substrate/ I wrote to them asking what the SSA of their product was and they tell me 18000 ft2/ft3 I ask again they say they are way superior to any other product. Pardon the pun but it seems a little fishy. Has anyone looked into this product before?

    Reply
    • Yeah. . . that’s pretty fishy. . . There are a few ways to measure SSA- some of them are great at giving the absolute, theoretical SSA of a material, but no one uses these measurements practically, as these values don’t hold up in the real world when systems are being engineered based on these values. Most folks tend towards more practical tests that tell you what SSA is in practice- once colonized with bacteria, fouled with waste etc. 18,000 ft.2/ft.3 is crazy. I’ve never heard of a material that comes close to this in real application. If you’re interested in doing live sales at all, Jody, do let us know.

      Reply

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