Researchers discovered that digesting fish waste allows circular fish and vegetable farms (aquaponics) to produce biogas, which can then be fed back into the energy systems of the farms. This also provides the plants with excellent nutrition.
The findings were published in the journal 'Aquacultural Engineering
'Aquaponics, or circular, land-based, combined fish and vegetable farms, is growing in popularity. Aquaponics uses nutrient-rich water produced by fish (aquaculture) to fertilise plants (hydroponics) in a closed, soil-less system with the help of bacteria that naturally grow within the systems. These food production models mimic fertilisation in river and lake ecosystems. Until now, fish solid waste was a byproduct with no particular value.
However, a research project at the University of Gothenburg has used waste to create biogas, which can help meet the energy needs of aquaponic farms. Victor Lobanov's dissertation describes this.
Waste is broken down in an anaerobic environment
"By breaking down fish faecal matter in an anaerobic environment - known as digestion - we can obtain a concentrated gas mixture of 70 per cent methane that can be used as fuel. This can make aquaponics a source of energy," said Victor Lobanov, doctoral student of marine biology at the University of Gothenburg.
The study also shows that the nutrients released in the digestion of waste are more easily available for plants compared to synthetic nutrition solutions.
"Fish waste contains a lot of nutrients. These should also be usable in aquaponics to enable even more sustainable food production than today," said Victor Lobanov.
Another benefit is that carbon dioxide is produced when the biogas is used as fuel, which is a necessary supplement when plants are grown in an enclosed space, like a greenhouse.
For now, the digestion process has only been tested in a lab environment, but a pilot in a commercial aquaponics facility is starting this summer. It will give researchers insights into how well the method can handle perturbations to the system and what needs to be done to create a more robust digestion pipeline. Victor Lobanov's goal is to create modular digestion systems that can be integrated into existing aquaculture and aquaponic facilities.
There is significant interest from the industry, and the technology could also be used in other animal husbandry applications such as piggeries. The sludge left over after digestion is still extremely nutritious and can be used for traditional fertilisation of fields. In this new process, the residual sludge leftover and, crucially, its eutrophication potential, is reduced.
"In many countries, the quantity of fertiliser produced in livestock farming is a problem. It can only be spread on fields during certain times of the year and removing wastes from the farm is associated with extra costs during pumping and transportation. Digestion of the fish solids reduces the quantity of waste produced by farms while additionally producing energy and a great fertiliser for hydroponics," said Victor Lobanov.
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