Fixed-biofilm reactors in aquacultural water recycle systems: effect of organic matter elimination on nitrification kinetics

Abstract

A special biofilm adaptation system and a biofilm monitoring system have been developed. By means of the adaptation system loaded with simulated fish tank effluent, artificial biofilm samples were available without having the necessity of stocking a certain fish biomass. The good resemblance of biofilm performance of these samples and of samples taken out of an operative trickling filter showed that the methods used are suitable tools for research on fixed-biofilm processes. The research is focused on the effect of simultaneous removal of organic matter on the zero-order biofilm nitrification rate and this effect is discussed in relation to the production of faecal organic matter and its 24 h fluctuation. The biofilm removed organic matter at rates directly proportional to the COD loading rate, showing efficiencies around 85%. Biofilm respiration experiments showed that only a small fraction ( c. 10%) of the COD removed has actually been oxidized simultaneously. The simultaneous oxygen consumption involved was only 0.065 gO 2 per g COD removed. As a result of the observed partial oxidation of the removed organic matter, the reduction of the oxygen penetration of the biofilm was restricted in degree. The corresponding reduction factor for the simultaneous nitrification process could be quantified: −0.015 g m −2 d −1 NH 4N nitrified per g m −2 d −1 COD removed. The reduction of the nitrification rate by simultaneous COD oxidation indicates that under the applied conditions the nitrification process has been limited by the transfer of oxygen into the biofilm layer.