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 ZipGrowTM Vertical Farming 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
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!
ZipGrowTM Towers & High Specific Surface Area
If you noticed in the table above, ZipGrowTM Towers have a VERY high SSA, BSA and void ratio.
The reason for this is that WE DESIGNED THEM THIS WAY!
As you see in the table, our Towers and Matrix MediaTM 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.
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