A pilot study was performed to evaluate the potential of an inert self-supported immersed fixed film media to upgrade aerated lagoons. Simultaneous organic matter removal and nitrification was assessed under different loading rates and temperatures (near 0 °C) using 12 laboratory-scale reactors operated in parallel. Test results showed that both the temperature and the load have an influence on organic matter effluent concentrations. Effluent quality seemed related to the observed biofilm thickness. Thicker biofilm is believed to have contributed to biomass detachment and increased particulate organic matter concentrations in the effluent. Simultaneous organic removal and nitrification was obtained at loads above 5 g CBOD/m·d. The highest nitrification rate at 0.4 °C was obtained for the smallest load, which showed a nitrification limitation close to freezing point.

Residence time distribution (RTD) of liquid phase in a two‐compartment (packed‐bed/packed‐bed or packed‐bed/packing‐free) bioreactor for wastewater treatment was evaluated via a complex liquid flow structure. RTD in packed‐bed compartments was modelled using a modified axial dispersion‐exchange model with a liquid stream which bypasses the dynamic liquid region. The liquid in the dynamic region is dispersed in axial and transverse directions, whereas the stagnant region exchanges mass with the dynamic region, the two regions being in direct contact. RTD in packing‐free compartment was modelled as a continuous stirred‐tank reactor with ideal or non‐ideal mixing zone. Time‐domain analysis coupled with the phenomenological RTD model was used to identify the model parameters. The proposed RTD model offers the opportunity to quantify parameters as dynamic liquid fraction, dynamic liquid flow rate fraction, number of transfer units between dynamic and stagnant liquid regions, stagnant liquid holdup, which otherwise would have been difficult to estimate using other methods and which can affect the bioreactor performance. The proposed RTD model describes adequately the hydrodynamics of the two‐compartment packed‐bed bioreactor while the simulations unveil likely tendency of the RTD curves subject to different sets of model parameters.

An experimental study dedicated to the characterization of the settleability of solids produced in immersed fixed media biofilm reactors has been carried out. The influence of operating temperature (0.1 to 16 °C) and surface organic loading rate (OLR) (0.4 to 10 g of soluble carbonaceous BOD per m of media per day) on settleable solids quantities, particle size distributions (PSD) as well as flocs morphology was evaluated. Results have shown that the OLR has no statistically significant influence on the settleability of the suspended solids. However, the operating temperature was identified as a factor that significantly influences the settling potential. The highest operating temperatures (14-16 °C) were related to the worst settling performances. On the other hand, the best settling performances were observed at intermediate operating temperatures (around 10 °C). The latter conditions were also associated with the largest fractions of large particles (>100 μm) in the effluent. Differences in PSD were found to be well correlated with settling performances. Part of the performance results variability which cannot be explained by differences in PSD can potentially be attributed to differences in flocs morphology (compactness).