This PhD thesis focuses on enhancing the use of organic waste produced by the fish as an internal carbon source for on-farm denitrification, i.e., transforming the organic waste into a new resource following the “Residual Resource” approach.
In Denmark, the implementation of stricter environmental policies has forced the aquaculture sector to improve its practices and water treatment technologies, thus becoming progressively more environmentally sustainable and competitive. Even though the industry has managed to reduce the discharge of organic matter and phosphorous, the major challenge is now the removal of total nitrogen, where only 15-50% can be removed with the best available current technology.
From the nitrogenous compounds found in recirculating aquaculture system (RAS) effluent nitrate-N constitutes by far the major fraction of total nitrogen deriving from biological oxidation of ammonium, the major nitrogen compound excreted by the fish. Therefore, removing the content of nitrate-N from effluent water is an important step to take. In this respect, heterotrophic denitrification—a biological process where bacteria reduce nitrate into gas with the use of organic carbon as electron donor— is central.
The organic waste produced by the fish in RAS can be used as an internal carbon source for on-farm denitrification. In this way, two waste types (organic waste and nitrate) are treated simultaneously, reducing the associated costs for purchasing external carbon sources and the cost for disposing the organic waste.
Feed is indirectly the major source of the waste produced in RAS, with the digestibility of the feed ingredients along with the macro and micronutrient composition of the feed dictating the amounts and characteristics of the waste produced. Thus, coupling feeding with waste production allows influence on and an estimation of the masses of waste to be treated, including the availability of organic waste that may be used as a resource.