Backyard Aquaponics
A place to discuss aquaponics

Hello people smarter than me (it wouldn't take much),
As part of the design and build of my new/first aquaponics system I am focussing on recycled materials which has meant that the moving bed bio reactor will be a 200L blue plastic drum, probably 70% filled with bottle tops (the HDPE plastic type that you get off drinks, milk etc).

Their will be one MBBR per 1000L fish tank.

Now I am well aware that as far as surface area for my bacteria to grow on I would be better off going with a commercial media but that sort of goes against the spirit of the design so instead I pose the question:

Do you believe it would be worth drilling holes in thousands of bottle lids for the small gain in surface area (and if so bonus points for suggestions on hole size and pattern) or do you think their would be insufficient benifit compared to the extra work involved?

Some further thoughts/math:

From what I can tell an average bottle cap top thickness is around 0.7mm
The smallest drill bit I have on hand is 1.6mm so I will use that for calculations.

1.6mm circle surface area = 0.79 x 2 (outside and inside) = 2.01mm2 (the surface area that would be removed by drilling)

1.6mm hole surface area = circumference x depth = 5.03 x 0.7 = 3.521mm2

So from my average maths it would seem that for every hole drilled with a 1.6mm bit i would gain around 1.5mm2 of surface area for each hole drilled in a cap thickness of 0.7mm.

Over time however i would imagine a 1.6mm hole would fill with bacteria, with the bacteria on the inside being deprived of nutrients/oxygen ending up with the same surface area as I began with before drilling. That is a pure assumption though and without someone with a similar experience it may well be that the movement of the media is enough to knock enough bacteria off to keep it clear.

It really depends on too many other variables to give you an answer. For example if you only have one small fish then the biofiltration needs are minimal and you probably don't even need an MBBR. Another thing we don't know is how many media beds you will have - since these provide filtration they take some of the load off the MBBR.

So my best guess is that you don't need to drill extra holes in the bottle caps. If you plan on having really high fish densities and no grow beds then I might be wrong. You can probably add another MBBR if you have to though and changing out water will also help.

Yeah i'm mostly looking at a generalised idea rather than a specific to my needs, although I will be DWC so grow bed wont be doing any filtration.

I did some more quick math with my aqua interested teen last night and I think it is safe to safe I wont be drilling holes at least in version one.

volume of 60% of 200L drum: 120000mL
rough volume of a bottle cap: 5mL
rough surface area of plain bottle cap as taken from (https://www.researchgate.net/publicatio ... ter_medium): 2900mm2

bottle caps for 60% fill of MBBR: 120000/5 = 24,000
surface area if caps unchanged: 24000*2900 = 69600000mm2 or 69.6m2

if 5 holes drilled in top, total holes to be drilled: 120,000
for an increase in total surface area: 120000x1.5 = 180000mm2 or 0.18m2

for 10 holes each it would be 240,000 holes for in increase of only 0.36m2

Needless to say i think i can talk myself out of drilling 240,000 holes for a less than 1% inprovement in surface area that may or may not remain effective.

With a DWC you'll get a lot of filtration from the organisms growing on the walls and on the plant roots (once they develop). I think you're right not to mess with drilling the bottle caps.

That sounds like a lot of drilling!