Aquaponic systems rely on the nitrification cycle to supply nitrogen to plants, but if too much is available, then denitrification can be used to lower those nitrogen levels.

The systems range from the simple to the elaborate. At the simplest, water can be scooped out of a small aquarium and used to water a small houseplant. Some of the most complicated are large commercial aquaponic systems that harvest both edible fish and edible vegetables. In my backyard I’ve had a simple aquaponic system running for more than a decade with minimal fuss. The pump has been replaced twice, and the fish are (or are the descendants of) the original fish.

A Simple Backyard Aquaponic System

Water from a small fish pond is pumped to a series of half wine barrels that start at the top of a small rise and end at ground level. The elevation of each barrel is key since they gravity feed one into the other. Each half barrel contains lava rock and a pipe near the rim leading down to the bottom of a second half barrel, which is set lower than the first. The lava rock provides mechanical filtering as anchorage for the plants and a hospitable environment for beneficial microbial growth.

A third and fourth half barrel are situated lower, and lower yet, and the bottom barrel empties back into the pond. With this arrangement, water travels from the pond (via pump) to the bottom of the first half barrel, through the lava rock to the top, where it makes a turn to feed into the bottom of the lower barrel and then repeats for the remaining barrels until returning to the pond.

Impact of Nitrogen on the System

Nitrogen is added to the system in the form of fish food. The pond fish eat the food and convert it into more fish, fish excrement, and ammonia. The fish excrement is caught in the lava rocks and allowed to decompose in a process called mineralization, which converts it into plant-available nutrients.

If the pH isn’t too high, and the water is cool, the ammonia will capture a hydrogen atom from the water then form ammonium. Since concentrations of ammonia are more toxic to both fish and plants than ammonium is, it is important to keep the water from becoming too basic.

Nitrifying bacteria convert the ammonium first into nitrite and then into nitrate. Plants growing in the barrels take up the nitrates and convert them into more plant material. The lava stones collect any solid materials in the water, which decompose in place for additional plant nutrition. The water goes back into the pond cleaner than when it was pumped to the highest barrel. Aquaponic systems tend to work best under a mild load and the backyard system described above was designed more for ease of maintenance and aesthetic appeal than maximized productivity.

Don’t Overcrowd the Pond

The nitrogen concentration in the water must be high enough to feed the plants, while being low enough for fish to tolerate. That’s why leafy plants that do well with mild fertilization tend to be simpler to grow aquaponically than more nutrient-demanding plants. Nutrient levels can be elevated by increasing the number of fish for the same size pond. But the more crowded the pond is, the more likely nutrient levels, pH, fish health, or other factors can spin out of control. Crowded pond conditions can lead to an excess of nitrogen in the water.

Use Denitrification to Lower Nitrogen Levels

If the plants don’t get enough nitrates to maintain desired levels (and adding more plants isn’t an option), then denitrification can be used to lower nitrates. Denitrifying bacteria are a bit more finicky than their nitrifying counterparts. While nitrifying bacteria are aerobic (oxygen loving), denitrifying bacteria are anaerobic (oxygen hating), so they require water with a flow slow enough to stay stagnant. This can be accomplished with a sealed separate filter or sand bed with very little flow to prevent the fresh water from bringing too much oxygen in. The denitrifying bacteria also require a carbon source such as sucrose or ethanol.

Denitrifying bacteria first convert nitrate back to nitrite, which is a step backward from the nitrification process. Then they convert the nitrite to nitric oxide, then nitrous oxide, and finally into nitrogen gas. Since they remove nitrogen from the system, denitrifying filters are more common in “fish heavy” aquaponics than in “plant heavy” set-ups.

Aquaponic systems range from some of the simplest forms of gardening to some of the most advanced and they can be daunting to undertake for the uninitiated. Understanding the flow of nitrogen through the system can help with the design and use of aquaponic gardens.