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.

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 on which microbes can live. 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 foot (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.

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

As you can see, different medias have drastically different biological surface areas. 

“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).

Calculating your BSA

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

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!

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 they were designed 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!)

This media and ZipGrow towers are available to anyone on our online store.

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.