Aquaponics, inspired by sewage treatment plants, delivers healthy fish and tastier plants

A current challenge to sustainable aquaculture is to increase the amount of farmed fish while reducing waste products that can lead to the accumulation of harmful fish sludge. New research aims to understand how this fish waste can be treated for use in aquaponics systems by removing excess carbon while preserving the mineral nutrients plants need to grow.

In this study in Frontiers in Plant Science, researchers from the Department of Marine Sciences at the University of Gothenburg, Sweden, demonstrate a novel and effective way to convert this fish sludge into fertilizer, thereby improving the nutrients for plants in hydroponic cultivation. The main author, Mr. Victor Lobanov, explains:

“Fish sludge is a waste product, made up of leftover food and fish droppings, that is normally broken down by bacteria in the water. In addition to physical damage to the fish gills, excess carbon in the solids leads to excessive bacterial growth – lack of oxygen in the water and impedes the fish’s breathing. We wanted to find out whether this waste can be used to fertilize plants in aquaponics systems by removing the excess carbon while preserving the minerals needed to grow plants. “

Fish waste as fertilizer

Researchers investigated a potential solution, inspired by sewage treatment plants found around the world, called improved biological phosphorus removal (EBPR). This process was adapted by the researchers in such a way that the risk of bacteria formation in the water was reduced, but the minerals from the fish waste were soluble in the water and thus bioavailable to the plants.

They found that the solids treatment system they developed was highly effective at bringing nutrients from the fish waste into the aquaponics system in the form of a liquid fertilizer as efficiently as a commercial nutrient solution. Although the fertilizer did not fully meet the plant’s needs due to a lack of some nutrients such as manganese, the researchers hope to optimize this system in future studies:

“Hopefully in the future we can scale the system more efficiently not only for lettuce as in this study, but also for other plants, with the right number of fish that match the size of the system and fish solids through the solids treatment system, we can also achieve a faster treatment rate make the entire process more efficient, “explains Mr. Lobanov.

Commercial fertilizer solutions often have very high levels of nitrogen, which stimulates the plants to swell and absorb large amounts of water and gives the appearance of improved growth, but often reduces the amount of minerals in the plant. Although the fertilizer produced by the solids treatment system contained less nitrogen than commercially available chemical fertilizers, the plants were not lacking in nutrients. This suggests that the high levels of nitrogen commonly used are beyond the needs of the plants. The authors hope that this finding will stimulate further research into the relationships between plant nutrients, health, and taste:

“Our work shows that this type of cultivation is not only more sustainable, but is also able to provide nutrients in a form that is easily accessible for plants. Farmers can use this system and possibly optimize it for their specific crops and production quantities. “Even supplement with additional nutrients if necessary.”

Lobanov VP, Combot D, Pelissier P, Labbé L, Joyce A. Improving plant health through nutrient remineralization in aquaponic systems. Front Plant Sci. 2021; 12. doi: 10.3389 / fpls.2021.683690

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