Modeling of TAN in recirculating aquaculture systems by AQUASIM

Modeling of total ammonium nitrogen (TAN) in recirculating aquaculture systems (RAS) contribute to identifying and quantifying the most important processes and their relative contribution to removal of TAN. AQUASIM is a flexible modular simulation system for water quality in natural and technical systems developed by EAWAG (Reichert, 1994). AQUASIM allows simulating complex biological, chemical and physical processes in standardized hydraulic systems. We used AQUASIM to model the steady state TAN concentrations in 12 experimental recirculating aquaculture systems (RAS) operated by DTU AQUA in Hirtshals, Denmark (Pedersen et al., 2009). Water from the fish rearing tank is treated in a sedimentation tank and subsequently by biological treatment in a submerged biofilter and in a trickling filter. Generally, the performance of the biological treatment was very well and average TAN concentrations in the RAS were in the range 0.1-0.4 mg TAN/L depending on the cumulative feed load. The average nitrite concentrations were a little higher than the average TAN concentrations, in the range 0.2-0.6 mg N/L. Our TAN model simulated TAN removal by the following processes: 1 & 2: Nitrification in the biofilm in the submerged biofilter and in the trickling filter, 3. nitrification by suspended nitrifyers (flocs) in all compartments of the RAS, and 4. TAN assimilation associated to biomass growth. The simulation model was able to describe the measured TAN concentration very well after least square optimization of the nitrification rate constants in the biofilm and in the suspended biomass. Thus, it was demonstrated that AQUASIM is a very useful simulation tool that can be applied to improving process understanding as well as to contributing to fish production optimization