We strive for cycles of growth and nutrient use here at the institute. Modern farms are a one-way street for resources: fertilizers, seeds, and equipment come in, while the grown products, excess fertilizer, and money go out (for a discussion of this, have a listen to Low Tech Podcast no. 3). One cycle we’ve built goes kitchen scraps → chickens → compost → garden → vegetables → kitchen scraps, and repeat. Another is our aquaponics system: fish → fish poo → fertilizer → plants → clean water → fish, and so on.
Basic Principle
Fish eat food and excrete ammonia. Beneficial bacteria in the water turn the ammonia into nitrites, which are dangerous for fish. Another set of bacteria turn the nitrites into nitrates, which are harmless to fish but are beneficial to plants. Aquaponics systems create a closed loop between a fish tank and growing bed. The water cycles through the bed, and the plants clean the water before it heads back to the fish tank. The fish grow and can be eaten, as can the vegetables and greens grown in the bed. The only constant inputs to modern systems are electricity to run the water pumps and fish food. Startup inputs include the fish environment, grow bed with growing medium such as lava rocks, water pump, fish fry (young fish), and plants.
Our Proof-of-Concept System
The current system consists of a 45-gallon fish tank, 12 standard goldfish, a 4-cubic-foot grow bed filled with lava rocks and pea gravel, a 300-gallon-per-hour fountain pump, and a filter box (described in tomorrow’s DIY project).
I cycled water through the system starting in mid-August. No fish were present at this time, but we did feed a little food each day. When this food broke down, it created ammonia. That gave the bacteria a chance to get established and once I got a nitrite reading and, later, a nitrate reading, I added fish. Over time the ammonia and nitrite levels have dropped to low levels (<0.25 and 0.25 ppm this morning, respectively), while my nitrate levels have risen to over 80 ppm.
The pump kicks on for 30 minutes each hour. In that 30 minute, it pumps about 150 gallons through the grow bed. The grow bed is equipped with a bell siphon. This type of siphon kicks in once the bed is full of water. It drains the bed down to about 1/2 inch of water at which point the siphon is broken, allowing the bed to be refilled. This keeps the plants roots from being constantly underwater. As the water flows back into the tank, it passes through a manifold with two dozen small holes; as the water streams out of the holes into the tank, it aerates the water, providing oxygen to the fish.
I am growing basil and tomatoes at the moment, but will soon be adding lettuce. The plants grow well. I haven’t quantified the amount yet, but that will come with the next system.
I’ve lost a few goldfish, as is typical when establishing any fish tank. I lost a few more last week due to anchor worms brought in during a recent fish introduction. I’ve treated the tank with Parasite Guard (sodium chloride, trichlorfon, and diflubenzuron). I did some research and found that these chemicals are not harmful at the levels present in the water, let alone what is taken up by the plants (in fact, trichlorfon is used to treat intestinal parasites in humans). It isn’t something I would give to the fish if I planned to eat them, however, as they will accumulate a higher concentration of the chemicals as they live in the water over the six days it persists.
This system runs with external inputs: electricity for the pump, an LED grow light to boost the plants, and purchased fish food for our (inedible) goldfish. This is a learning process and I want to get comfortable with this small system before building our larger system, described next. In that system, I’ve sought to reduce external inputs.
Plans for Future, Large-Scale System
On the south side of the institute’s building or the south slope of a hill, we plan to build a large greenhouse, somewhere around 30 x 20 feet in size. In this greenhouse, we hope to build a large aquaponics system that will run off of wind power. We hope to create insect traps to catch much of the food for the fish. I’ve created a concept drawing, shown here.
The top of the figure shows the plan, or bird’s-eye, view of the system. Five grow beds total 180 cubic feet of growing space. Beneath the beds is a 3350-gallon tank, where I would like to grow either perch or trout. The tank is built into the ground to help keep the water to a constant temperature. Above the tank are two walkways, one to service two of the grow beds and another to access the water wheel.
The water wheel is an ancient design: a cogged wheel turns to lift a chain of buckets up out of the tank. The water is dumped into a collection tank, from which it flows into the first grow bed through a canal. The wheel will be powered by a windmill. When the wind isn’t blowing, the wheel will turn by means of a weight that will store the wind energy as it is lifted (not pictured).
Each bed is equipped with a bell siphon. Once full, it will empty all of the water into the next bed through a canal. As seen in the bottom of the figure, each bed is lower than its neighbor to allow gravity to do the work of moving the water along. Once the final bed is reached, the water empties back into the tank.
The goal of this system is to create a perpetual symbiotic relationship between the fish and the plants. We humans get to enjoy the fish and plants as they grow and reproduce in return for managing the system.
We hope to apply for grants from the USDA for the costs associated with the construction of this system.
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