| This study investigated the hydrodynamic characteristics and nitrification features of three types of biofilters: down-flow, upflow and horizontal moving bed biofilters. Measurements were taken in pilot-scale systems and in biofilters operating at salmon smolt hatcheries in St. George and Oak Bay, New Brunswick. Key variables that affect the performance of biofilters, such as bed expansion and residence time distribution (RTD), were studied. New models are proposed to predict the bed expansion and RTD of biofilters. They were validated using data reported in the literature and experimental results generated in this work. A pressure profile method is also proposed to determine the distributions of bioparticle size and density within fluidized sand biofilters. The bioparticle sizes calculated using this method are in good agreement with the measured values. It is shown that biofilm thickness is zero near the distributor plate and approaches a maximum value as height increases.;The horizontal moving bed biofilter was found to behave as a well-mixed system whereas the fluidized sand and down-flow biofilters behaved as plug flow systems. At the operating temperature of the hatcheries (11-14°C), the rate of nitrification was first order with respect to the TAN concentration in all three biofilters. The nitrification rate constant based on bed volume was 0.0020 s-1 and 0.0026 s-1 in the fluidized sand and Kaldnes media biofilters, respectively. Compared to the fluidized sand and down-flow biofilters, the horizontal moving bed biofilter has the extra advantages of reducing CO2 concentration, keeping a high oxygen level in the effluent and having a low pressure drop. |
