DIY Backyard Greenhouse Aquaponics Project

DIY Backyard Greenhouse Aquaponics Project – Part 1

I’ve wanted to do another aquaponics setup since I had my small AP system in town.

I talked about aquaponics basics in a previous article, so read that first to better understand this project.

To recap quickly, aquaponics combines keeping fish with hydroponic (soil-less) growing of plants.

For more info on the greenhouse itself, see how I turned a carport into a greenhouse.

Oh, and I may use the abbreviation “AP” to mean aquaponics in this series’ posts.

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Digging a hole?

Initially I started digging out the greenhouse floor to put in a geo-solar system with heat pipes.

This is a setup where the hot air from the ceiling is pumped to underground pipes, which heats the earth under the greenhouse. In winter the airflow removes heat from the ground and puts it into the greenhouse.

This allows you to extend your season, growing earlier in the spring and longer into fall/winter.

It equals being able to grow more food in the same space. This is a GOOD THING!

But as I got further along in hole-digging I decided to sink an aquaponics fish tank there instead.

I used to have a small 150-ish gallon system, but this time I wanted to go bigger.

And this time I had a greenhouse, so I should have more successful growth and be able to grow most of the winter.

Design considerations

My carport greenhouse is 10 feet wide, with sides that angle in from the bottom up.

DIY Backyard Greenhouse Aquaponics Project

The initial plan was to make 30″ wide grow beds on each side of the greenhouse. I would put the 36″ wide fish tank in the middle between the grow beds.

Why these dimensions? 30″ is easy for most people to reach to harvest from the grow beds. The 36″ fish tank was a somewhat arbitrary decision.

I wanted it as wide as possible without pushing into the aisle. I thought this would be 36″ or so. Plus it was how much fencing I had.

I thought this spacing should have left enough room for an aisle between the sunken fish tank and grow beds.

Of course, simple math (10*12-(30*2+36)=24/2=12) shows that gives us two 12″ aisles. I didn’t think it was good for easy access.

Another other consideration was grow bed construction cost, in money and time.

Growbed choices

I thought that using the barrels would also be faster to build. And since my time is limited this was a major factor in the decision to use barrels.

To be fair, comparing a grow bed of half-barrels to one of a rectangular-box-style EPDM grow bed is difficult.

The depth of barrels isn’t the same across its length, and in some ways it’s an inferior method of grow bed construction compared to an EPDM box style.

For the limited space in the greenhouse, you get more growing volume using the box style.

The box style also requires fewer plumbing fittings, since a bed only requires one each fill and drain fitting. In contrast each barrel needs a fill and drain.

And since you can make a box bed as long as you want, the savings from not buying fittings can really add up.

Plus, some plants really need the larger volume of the consistently 12″ deep EPDM grow beds. They just don’t do as well in barrels.

I’m not convinced that building with barrels is actually faster. I thought that because the barrels are stiff and support themselves, the need for framing would be lower. Therefore I assumed construction time would be less with barrels.

I also wouldn’t have to fully enclose the sides and bottom like the EPDM grow bed.

But you have to cut the barrels in half (straight!), and build a frame for each one, then connect them to the frame and each other. Then you have to plumb each one together with the rest.

It took me most of 2 afternoons to construct the grow bed, plus some additional tweaking hours.

The need for additional framing and enclosure is still true, but I don’t know if the construction time difference is enough to choose going with barrels again.

With the EPDM, you build the frame, then put in the underlayment and liner, securing them to the sides. Then there’s one bulkhead fitting and one supply line for the whole bed.

I’ll just need to build a EPDM grow bed and find out how fast it is! Maybe as a future project…


Cost calculations

BTW, I love using spreadsheets to do calculations. People sometimes lie to themselves, but spreadsheets don’t.

As I calculated it, the cost for building the grow beds using wood framing, plywood sheathing and EPDM pond liner would have been more than using HDPE barrels.

This depends quite a bit on materials and fittings cost, and (mostly) how much the EPDM costs.

I figured that it would cost $5.32 per sq ft (which is also cubic feet since they’re 12″ deep) to go with the EPDM grow beds, versus $2.27 per cu ft for the barrels.

That looks pretty good, right? Clear winner, barrels!

Except not.

Barrels are what I chose, but not primarily on cost. That was based more on time of construction.

I calculated that I needed to buy extra plywood to make the grow bed sides and bottom, since they’re not needed for the barrel build.

I have reclaimed stuff I can use and there are alternate construction methods than using plywood for the EPDM bed.

Taking out the plywood cost made it look a little better at $3.49/sf.

However, in the limited space of the greenhouse, growing space density (cubic ft per square ft) should also be considered.

I calculated this at 0.72cuft/sqft for the barrels and 1cuft/sqft for the EPDM grow bed.

Now who’s the clear winner? It depends…

What! You made me think about math for an “it depends” answer?!

It’s because the choice depend on your priorities.

If you want the maximum grow bed volume in the greenhouse, go with EPDM.

If you want to do it for lowest cost, go with the barrels.

Note: I didn’t do the above calculations until after the barrels were built. Of course I wish I’d had more money and time to work on the system. I might have done the EPDM box instead. 

But since a slightly less efficient completed system is better than a more efficient incomplete one, I chose to use barrels.

Unsurprisingly, I made the same choice in building the greenhouse.

Planning successPlanning the aquaponics system

So assuming 36″ grow beds (using barrels), I would have (10*12-(36*2+36)=12/2=6) a whopping 6″ aisle. Minus the frame around the fish tank and barrels. So basically just about 2 to 3 inches.

Yeah, ummm. No.

That’s not going to work as an aisle.

Instead, I’m going to need to stand on the fish tank (which is just above ground level) to access the barrel grow beds. So it needs a good solid cover that can be walked on, like 2×4 reinforced exterior plywood or metal grating.

If I do it this way, the size will work.

I also considered putting in a sump tank to prevent the fish tank level from going up and down too much. But since I want this system to actually be completed, it will be put off until later.

The idea is to put together a simple complete system that works. There won’t initially be a solids swirl filter, deep-water beds, NFT towers or pipes, or a sump tank.

These features may get added later. Many aquaponics people are DIY tinkerers, and I’m no exception.

The initial design is for a custom 480 gallon in-ground EPDM fish tank, a submersible pond pump, and 6 half-barrel media beds filled with crushed basalt, connected together with one bell siphon.

There will also be an air pump to keep the fish happy.

This size fish tank will let me expand and add grow beds in the future. Assuming a ratio of 2 gallons of grow bed (excluding media) to 1 gallon of fish tank, I can have about 130sf of 12″ EPDM grow beds.

This would cover most of the greenhouse except the center aisle and fish tank.

On the other hand, I could have 200sf of barrel beds which is larger than the footprint of the greenhouse!

See how much more dense the grow volume of the EPDM grow bed is?

Next steps

In the next post we’ll talk about budgeting and materials, and digging the fish tank hole, building the fish tank form, grow beds, lining the fish tank, and plumbing up the supply and return lines.

This post continues in Part 2.



OK, that’s all folks! Do you have any questions or comments about aquaponics, sustainable homesteading, permaculture design or anything else? Ask your question down below and let’s talk! You can also use the contact form, or email me at info at thepermaculture dot life.

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