I originally calculated the number of grow bins needed using a 4'x8'x12" size but we never made any bins that deep- with the exception of one 10" deep bin the others are about 8" deep. However this still cannot be used in the calculations because the functional depth (the depth the water shouldn't go above) is 6" so the formula should be 4'x8'x6". Using this I worked the calculations:
- Convert all numbers to the same unit of measure (inches)- 4'x8'x6" = (4'x12") x (8'x12") x 6", or 48" x 96" x 6", or 27648 cubic inches.
- Convert cubic inches back into cubic feet- 27648 cuin / 1728 cuin (1 cubic foot) = 16 cuft
- Convert cubic feet to US dry gallons using website (http://www.asknumbers.com/VolumeConversion.aspx)- 16 cuft = 102.86 dry gallons (dgal)
- So each of our grow bins needs 102.86 dry gallons of medium like pea gravel
- We need 800 dgal and we have space for 514.30 dgal however the 514.30 number is using only 5 of 6 grow bins of the above mentioned size. We also have one bin that is functionally 4'x8'x8" = 21.33 cuft = 137.12 dgal.
- So 514.30 + 137.12 = 651.42 total dgal
- Therefore 800 dgal - 651.42 dgal = 148.58 dgal needed to meet the requirement for the 200 lbs of fish.
- One solution would be to build another grow bin functionally 4'x10'x8" = 26.67 cuft = 171.49 dgal.
The system I build for my microfarm will not need to be so large but will utilize the same formulas. I will probable build a ferro-cement sump tank as well as long ferro-cement grow bins. I was going to use plastic food grade barrels but have only found a couple and they have their own problems.
- They are... wobbly. They warp and sometimes aid in water spillage. They need cross bracing
- They have rounded bottoms making them a little difficult to calculate medium
- They are harder to mount on support tables
The fish tank needs some work too. I may put a drain in, or I might just jack it up and use a pump, or even manual siphon vacuum for cleaning the bottom. It just depends on... how I'm feeling that day I guess. I am going to try to utilize a overflow pipe that sucks solids from the bottom, but it may not work.
Lots of work to do.