NITRATE REMOVAL IN MARINE RECIRCULATING SYSTEMS

Discharge of organic matter, dissolved inorganic nitrogen and phosphorus is a major cause of environmental pollution by recirculating aquaculture systems (RAS). While ammonia removal through incorporation of nitrifying biofilters is well established in RAS, active removal of nitrate, the end product of nitrification, is less common. Hence, in most RAS, nitrate levels are mainly dictated by the system's water exchange rate and to some extent by passive nitrate removal processes in the different system components.

Mainly due to stricter environmental regulations, an increased number of RAS are operated with incorporation of denitrification reactors in which nitrate is biological reduced to elemental nitrogen gas.  Fixed film reactors, in which heterotrophic denitrification is fuelled by addition of external carbon sources, are most often used for this purpose. Alternatively, uneaten feed and fish faeces may be used as endogenous carbon and energy sources for the denitrifying organisms.

This latter strategy was used in a zero discharge system which produces marine fish with no pollutant discharge and minimal use of valuable fresh, makeup water. In this particular RAS, denitrifying activity takes place in a digestion basin which is fed with organic-rich effluents from the fish basins. In the digestion basin, denitrification is part of an array of interactive anoxic/anaerobic biogeochemical processes which collectively cause an effective reduction of carbon, nitrogen, phosphorus and sulphide in the treatment water. Nitrate reduction in these basins was found to take place by either one of the following processes: heterotrophic denitrification, autotrophic denitrification on sulfide or dissimilatory nitrate reduction to ammonia (DNRA). The relative contribution of each of these processes was found to depend on the carbon, nitrogen and sulfide concentrations in the various parts of the basins. At ample concentrations of available organic matter, heterotrophic denitrification was the dominant nitrate removal process. DNRA was pronounced at relatively high levels of available carbon and low levels of nitrate (high C/N ratios) or at high sulphide concentrations. Autotrophic denitrification, with sulfide as electron donor, was evident both in absence and presence of heterotrophic denitrification. Sulfide, accumulating in the sludge as a result of sulphate reduction and desulfurization during organic matter decomposition, influenced the relative contribution of the various nitrate removal processes. Results presented are illustrative for the complex biogeochemical processes underlying nitrate removal in organic-rich, marine systems.