Nitrifikation i biobädd - Adsorption av ammonium i pilotanläggningarna på Sjölundaverket
For model based control and optimization of plants using nitrifying trickling filters, the fast dynamics of the filters have to be modeled. Experiments have been carried out on a pilot-scale trickling filter at Sjölunda WWTP in Malmö, and a model has been developed to analyze the dynamics. The experiments were: (i) a pulse response experiment for calibration of the model to the plant, (ii) an experiment with a step increase in influent ammonium concentration, (iii) a step decrease in influent ammonium concentration, and (iv) an experiment where dissolved potassium chloride was added to the influent during half an hour. The flow and the influent total concentration of dissolved nitrogen were constant during the experiments. Transient changes in the difference between influent and effluent total concentration of nitrogen, in all of the experiments, indicate adsorption of ammonium when the ammonium bulk concentration increases and desorption when it decreases. These phenomena affect the dynamics such that the transients in the effluent ammonium concentration are slower than they would be otherwise. Simulations of the model, where the amount of ammonium adsorbed is assumed proportional to the ammonium concentration in the biofilm, agree with measured effluent concentrations of ammonium, nitrite, nitrate and alkalinity. However, at high ammonium loads the model overestimates the effluent nitrate concentration and underestimates the effluent alkalinity. This can be explained by denitrification in anoxic regions of the biofilm, which is not considered in the present model. A comparison between the response to the increase and the response to the decrease in ammonium concentration show that the transients were significantly slower when the influent ammonium concentration was increased. In the experiment where potassium chloride was added, the effluent ammonium concentration increased during a period after the addition started and decreased during a period after the addition was terminated. This indicates that there is an ion-exchange between ammonium and potassium ions. However, the changes in effluent concentration were too small in comparison to the variations in the measurements to estimate the amount of ammonium adsorbed.