Nitrogen (N) is present in various forms in aquaculture and aquaponic production systems. The most toxic form to fish is ammonia (NH3), which is diffused to the water by fish after feed digestion. A nitrifying biofilter can convert ammonia into less harmful nitrate (NO3-). However, in a recirculating aquaculture system (RAS) nitrate can accumulate to concentrations that have a negative impact on the health and growth of fish. The Business Unit Greenhouse Horticulture & Flower Bulbs from Wageningen University & Research investigated the performance of an innovative, low-tech denitrification reactor that can convert nitrate into nitrogen gas (N2) using fish sludge as an energy source for the reaction.
Denitrification takes place when facultative heterotrophic bacteria live in an environment with very low oxygen levels (hypoxic or even anoxic) and use nitrate instead of oxygen for respiration. To complete the conversion from nitrate to nitrogen gas the bacteria require carbon as a source of energy. Denitrification is frequently used as a processing step in wastewater treatment, but often carbon sources such as methanol or ethanol are added to the system, which are costly and potentially hazardous.
Denitrification reactor uses carbon in fish feces
Landing Aquaculture, a Dutch aquacultural engineering firm, developed a denitrification reactor that aims at simplicity and cost savings by utilizing the carbon content of fish sludge.
The reactor works by filling it with fish rearing water (rich in nitrate) and sludge (i.e. fish feces and uneaten feed) captured with mechanical filters from a RAS. As the denitrification kicks in, the water coming out of the reactor contains less nitrate than the incoming water. This allows the outgoing water of the reactor to be circulated back to the RAS which reduces overall water consumption. Measurements showed that the reactor can remove close to 500 mg of nitrate per liter of reactor water per day.
Do denitrification reactors produce biostimulants?
From a mere nitrogen management perspective, applying denitrification within an aquaponics system (i.e. the combined cultivation of fish and plants) is not as logical because plants remove nitrate from the fish rearing water and use it as fertilizer. However, a sludge-based denitrification reactor could still be interesting if it is established that the bacteria inside the reactor produce plant biostimulants. These are substances that are claimed to stimulate plant growth or -development and can even reduce abiotic stress such as drought. In 2022 new EU regulations will come into effect that must provide more transparency about the content of biostimulant products and their effects on plant production.
To have an indication of the substances that a denitrification reactor produces, water samples have been analyzed by Metabolomics, the Wageningen Omics Facility. In total, no fewer than 3000 compounds were detected in the fish water. Initial analysis indicates that more than 700 compounds were found in the water from the denitrification reactor that did not occur in other parts of the fish farming system. While there are indications that some of these substances could function as biostimulants, further research is needed to confirm this.
This research was funded by a VIDA Voucher provided by the EU project VIDA that is part of the INNOSUP Call of HORIZON 2020.