February 12, 2014

Redefining Waste in Aquaponics

Laura Genello

Laura Genello

Guest Blogger

Johns Hopkins Center for a Livable Future

cilantro-dill-gutterOn the surface, the aquaponics system appears to be a replica of a natural ecosystem where the word “waste” is an artificial concept. The fish don’t excrete waste, they excrete nutrients, and the plants take up these nutrients, filtering the water for the fish. However, in our earlier post “When the Fish and Plants Don’t Get Along,” we discussed how solid accumulation on the bottom of the plant beds can lead to poor water quality. To prevent this, we employ clarifiers after the fish tanks and before our plant beds; these tanks are designed to slow the water enough to allow solids to settle. Every day we drain the sediment from the bottom of the clarifiers to remove a total of 12 gallons of muddy, solid-filled water from the system. Just because we don’t want these solids sitting beneath our plant rafts, doesn’t mean they are a waste product. In fact, the solids are loaded with valuable nutrients including high levels of Phosphorus. Here are three ways we put that material to use:

Compost Tea Foliar Spray: We strain the solids into a mesh bag (the water can be returned to the system), add a handful of worm castings, and set the bag in a bucket of aerated water overnight. We add a teaspoon of molasses to trigger bacteria growth, and in one to two days we have a solution rich in nutrients and beneficial microorganisms that can be sprayed on our plants, allowing them to absorb the nutrients through their leaves.

Expanding Our Growing Area with Gravel Beds: Recent visitors to the aquaponics project would have noticed the gravel bed and gutters filled with bright orange marigolds. Instead of using soil in these containers, we plant in gravel and irrigate the gravel with the clarifier effluent. The gravel filters the solids, which break down aerobically on surfaces of the stones and provide nutrients for the plants. This is a great way to expand our growing area and capture some of the nutrients in the solid waste.

Fertigating a Soil Garden: Just outside the greenhouse, we have a perennial herb garden in soil. In the hot summer months, the clarifier effluent helps us keep our perennial herb garden alive by providing both water and nutrients.  Now if only we could keep the deer away!

While aquaponics takes inspiration from nature, as an ecosystem it is greatly oversimplified, and many of the natural checks and balances that mitigate things like solid accumulation simply do not exist. Some aquaponic growers are trying to address this by designing hybrid systems that use a gravel media grow bed between the fish tanks and the raft beds to filter solid material.  Other growers stock prawns in their plant beds to eat the solids. For us, we can effectively salvage the solids from the waste stream with a little extra work.

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