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Scaling Up to a Larger Aquaponics System – Part 4

Previously in Part 1 of Scaling Up to a Larger Aquaponics System, I talked about how the aquaponics system was put together. In Part 2, I covered startup and operation. In Part 3, I talked about the results. Here in part 4 of this 4 part series, I will talk about some additions I made and my final conclusions.

Expansion with Raft Beds
After I saw the outstanding growth of the system, I decided to undertake an expansion in mid-summer. During my research, I had read about raft beds for growing lettuce. I thought this would be an interesting test to see how they compared to the flood and drain beds.

I decided to add just two raft beds. I built a structure similar to the first one but smaller, cut one barrel in half, and installed the beds. Since the sump tank was buried in the ground in line with the first six beds, the two new ones had to be built off to the side. As long as they were slightly below the others, the supply plumbing line would flow downhill to them. I had to make sure they were still above the sump tank so the discharge lines had enough height to flow down to the sump.

These beds are a little simpler because there is no bell siphon. Water flows in one end and out the other. The supply rate is controlled with a PVC valve just like the flood and drain beds. The level is controlled with an overflow line on the opposite end tied to pipes that flow downhill to the sump tank. The water level remains constant as the water flows out the overflow line.

At this point, the raft bed was a half barrel full of water circulating through it very slowly. The next step was to build the raft. I got a 4’x8’ sheet of 2” foam insulation from Home Depot, and cut a piece to fit the opening. The piece of foam floats on top of the water. The lettuce plants are grown in 2” plant cups with holes in them that I ordered online.

Raft Beds Showing Supply and Overflow Lines

Raft Beds Showing Supply and Overflow Lines

There is a trick to cutting holes in the foam to hold the cups. I constructed a cutting tool from 2” schedule 40 PVC about 6” long. Using a Dremel grinding tool, I sanded down the inside edge of the PVC until it was fairly sharp. The outside edge should be left intact and only the inside edge is sanded so the resulting hole is clean and smooth. The tool is used to cut the holes by pressing down and turning the tool slowly. The result is a nice clean hole through the foam that the 2” cup fits perfectly in.

Plant Cup and Homemade Cutting Tool

Plant Cup and Homemade Cutting Tool

The final piece of the raft bed is an air bubbler to provide oxygen to the roots. I got an aquarium air pump and two long air stones which were placed in the bottom of the raft bed. The air pump runs 24 hours a day. The roots need constant oxygenation since they are fully submerged all the time.

Air Pump Under the Beds

Air Pump Under the Beds

Adding Plants to the Raft Bed
To add plants to the raft beds, first I needed plants. I started some lettuce seeds a couple weeks earlier. By this time, they had germinated and grown enough to move to the cups. They don’t have to be very big. I don’t even bother thinning my seedlings when they’re going into raft bed cups. I gently remove them from the seed tray, soil and all. I rinse them until most of the soil is removed from the roots. Then I carefully separate them into individual plants. Each cup gets one plant.

To “plant” the seedlings, I feed the roots through one of the holes in the bottom of the cup. Then I fill the cup about half way with small rocks, while protecting the fragile young plant. Once there are enough rocks in the cup to support the plant, I don’t put any more. The rocks are about 1/8”-1/2” rocks I got from Home Depot in a 50 pound bag. The rocks serve no other purpose but to support the plant until it gets big enough to support itself. The cup is placed in one of the holes in the raft bed so the roots dangle in the water.

Raft Beds with Plants in Place

Raft Beds with Plants in Place


Close Up of Plants

Close Up of Plants

I have two raft beds with 18 holes in each. Once all 36 plants were in place, it was time to let them grow. Similar to the flood and drain beds, for the first couple weeks it seemed like nothing was happening. A few of the plants didn’t make it and were replaced with more seedlings. Then the plants seemed to start growing rapidly. Lifting up the raft revealed significant root growth underneath. It didn’t take too long for the plants to get big enough to start harvesting leaves.

This is so cool because it would never work to grow 36 lettuce plants in 12 square feet of traditional garden beds. But because the water and nutrients are being delivered directly to the roots, the plants can be placed very close together.

After a Few Weeks

After a Few Weeks


Very Robust Growth

Very Robust Growth

Even though water is abundant, I found that the lettuce plants were still more likely to bolt in the high summer temperatures. To minimize this, I added a 50% shade cloth over the top of them. Even with that, they still get plenty of sun. It helped some but they still bolted eventually.

One pitfall in my design was that I made the overflow lines in the raft beds very small. The roots of the lettuce plants got so big that they would occasionally find their way into the overflow pipe and cause a clog. This was enough to result in the bed flowing over the edges and causing loss of water. Loss of water can result in the sump tank eventually running dry. I learned to keep an eye on this every day to prevent problems.

Conclusions and Possible Future Improvements
After this year long experiment, I learned quite a bit. The system is great for some types of plants. A second system that could be dosed with supplemental phosphorous might product more fruit. A heated or indoor system could allow for year round fish production. One thought is to place the fish tank inside the garage or another structure that could be heated more efficiently, and running the plumbing to grow beds outside. Or maybe even an entirely indoor system with grow lights. That would mitigate problems I experience with lettuces going to seed in the summer heat.

Another possible improvement is that I would consider designing the system with a larger sump tank. The 55 gallon sump tank was at times not enough to handle the fluctuations in water levels due to the flood and drain cycles, and occasional leaks.

Once the system was established, it required very little maintenance. Of course the fish had to be fed several times a day to get them to grow fast. I had to add a few gallons of water every week. I tested the water daily early on and weekly later. Iron chelate was added every couple weeks. Rock phosphate was added to some of the vegetables as needed. I checked the sprayers in the fish tank when I fed the fish, and unclogged them with a piece of wire as needed.

There were two unexpected benefits. First, the absence of weeds was great. Occasionally a weed will show up in a bed but it is easily identified and removed. Second, the system could be worked standing up. The beds are all about typical countertop height, making them very comfortable to work on. Not having to kneel down on the ground was nice for a change.

Overall I was very happy with the results from the first year of operation. The system was productive enough that I planned to continue using it again the next year, although I considered possibly operating it with goldfish and focusing on the plant production. If you want to produce vegetables and fish in a very water-efficient manner, you should consider an aquaponics system.

Resources for this post:
Introduction to Aquaponics
My First System: A Small Scale Aquaponics System
Building the larger system: Part 1 of Scaling Up to a Larger Aquaponics System
Startup and Operation: Part 2 of Scaling Up to a Larger Aquaponics System
Results: Part 3 of Scaling Up to a Larger Aquaponics System

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