Impact of water boundary layer diffusion on the nitrification rate of submerged biofilter elements from a recirculating aquaculture system Jonas Prehn a , Christopher K. Waul a , Lars-Flemming Pedersen b , Erik Arvin a, * a Technical University of Denmark, DTU Environment, Department of Environmental Engineering, Miljoevej, Building 113, 2800 Kgs. Lyngby, Denmark b Technical University of Denmark, DTU Aqua, Section for Aquaculture, The North, Sea Research Centre, P.O. Box 101, DK-9850 Hirtshals, Denmark article info Article history: Received 15 September 2011 Received in revised form 22 March 2012 Accepted 25 March 2012 Available online 1 April 2012 Keywords: Aquaculture Biofilter Nitrification Kinetics Mass transfer abstract Total ammonia nitrogen (TAN) removal by microbial nitrification is an essential process in recirculating aquaculture systems (RAS). In order to protect the aquatic environment and fish health, it is important to be able to predict the nitrification rates in RAS’s. The aim of this study was to determine the impact of hydraulic film diffusion on the nitrification rate in a submerged biofilter. Using an experimental batch reactor setup with recirculation, active nitrifying biofilter units from a RAS were exposed to a range of hydraulic flow velocities. Corresponding nitrification rates were measured following ammonium chloride, NH 4 Cl, spikes and the impact of hydraulic film diffusion was quantified. The nitrification performance of the tested biofilter could be significantly increased by increasing the hydraulic flow velocity in the filter. Area based first order nitrification rate constants ranged from 0.065 m d 1 to 0.192 m d 1 for flow velocities between 2.5 m h 1 and 40 m h 1 (18  C). This study documents that hydraulic film diffusion may have a significant impact on the nitrification rate in fixed film biofilters with geometry and hydraulic flows corresponding to our experimental RAS biofilters. The results may thus have practical implications in relation to the design, operational strategy of RAS biofilters and how to optimize TAN removal in fixed film biofilter systems. ª 2012 Elsevier Ltd. All rights reserved. 1. Introduction Recirculating aquaculture systems (RAS) employ water reuse to reduce water consumption, and have a significant reduced relative water consumption compared to the traditional cultivation of fish in flow through ponds. RAS technology provides the option of rearing fish at high densities under controlled conditions leading to a potentially reduced envi- ronmental impact from the fish production (Piedrahita, 2003). Efficient TAN and nitrite removal are essential issues in relation to the commercial fish production, as even low concentrations can be toxic to fish salmonids (Meade, 1985; Timmons and Losordo, 1994). A low effluent TAN concentra- tion from RAS is also required in order to protect organisms in surface water. TAN is typically removed from water in biofilters, where nitrifying bacteria oxidize TAN via nitrite to nitrate. In RAS, this process is mainly carried out in biofilters where the biomass is attached to a solid inert support medium, inter- nally in pore spaces or directly on the surface, forming a fixed * Corresponding author. Tel.: þ45 40628153. E-mail address: erar@env.dtu.dk (E. Arvin). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 3516 e3524 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2012.03.053