A student tries to make fertilizer from the waste produced in recirculating fish farms

7. 6. 2023
Experts from the Faculty of Agronomy have extensive freshwater fish farming right on the premises of MENDELU. They use the so-called recirculating aquaculture systems, which allow sustainable breeding by reusing the water in the tanks. Until now, the waste produced in some farms has been discharged directly into the sewage system, which Lukáš Harabiš, a student of fisheries, found unfortunate. So he is currently trying to turn the waste into fertiliser for plants.

In two experimental aquarium rooms at the Faculty of Agronomy, for example, rainbow trout or African catfish are kept. The water in the tanks is constantly circulated and purified thanks to pumps. Biological filtration removes ammonia from the water, while mechanical filtration separates large quantities of waste sludge and fish faeces, which are rich in nitrogen or phosphorus due to the composition of the food.

I like it when something can be saved or reused. That’s why I thought it was a shame that all the sludge is being dumped away and not used in any way. The waste is an excellent source of nutrients that are important for plant growth,” explained Harabiš, who is a PhD student at the Department of Fisheries and Hydrobiology at MENDEL.

The student’s aim is to convert the waste produced by aerobic digestion into a liquid fertiliser that could be used in hydroponic plant cultivation. “Aerobic digestion is essentially a process in which different groups of heterotrophic organisms break down fish feces into individual minerals,” Harabiš explained. According to the PhD student, the nutrients obtained in this way could find application in the production of lettuce, for example. “I would like to create a fertiliser that is comparable to commonly available chemical mineral fertilisers, which require minerals to be extracted. But that won’t be the solution forever, so I am offering a renewable alternative,” said the student.

Harabiš would like to test the quality of the product directly with colleagues at MENDELU who specialize in plant cultivation. “Of course, depending on the composition of the waste and how much nutrients we can mineralise, we will adjust the fertiliser. I expect that it will be necessary, for example, to dilute it so that we don’t burn the plant,” he explained. However, he expects it to be more of a supplementary fertiliser to ensure that the crop gets enough nitrogen, phosphorus and other minerals.

With his research, the PhD student is aiming at practice, and he is even already in contact with some breeders. “The simplicity of the process is important to us. Farmers could either process the waste themselves or offer it to fertiliser manufacturers,” Harabiš described. But anything is better than dumping the waste outside, he said. “There is generally a lot of phosphorus in surface water today. It comes from the farm, industry, and it also gets into the water from households, for example from washing gels and so on. In summer, it is a breeding ground for cyanobacteria in ponds. Any way of reducing phosphorus in nature therefore seems to me to be a good thing,” he reflected.

In general, the number of recirculating fish farms is increasing worldwide. According to Harabiš, the advantage compared to conventional fish farming is that it is easier to comply with hygiene conditions and the farming is more efficient. “You have the tanks somewhere under the shed, so you are less likely to introduce diseases into the farm, there is no way for predators to get in, you use very little water. Plus, if the population is going to continue to grow in the future, we need to feed people in some sensible way,” Harabiš said.

Contact for more information. Lukáš Harabiš, +420 733 280 506, lukas.harabis@mendelu.cz, Institute of Zoology, Fisheries, Hydrobiology and Apiculture AF MENDELU

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