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outflow, the inflow data was discarded. Instead, the NH +4 concentration in the inflow was assumed to be constant. The N 2-production was finally estimated as the difference between the NH +4 inflow and the total outflow of NO -2 , NO -3, and NH +4 out of the reactor. To be able to check if the estimates of the N 2-production were correlated with any of the measured on-line parameters, the on-line data had to be prepared. In the same manner as the conductivity had been prepared earlier, the outliers due to calibrations of the instruments were removed from the data and the data was averaged over the office-hours for the dates of interest. While reviewing the online data it was noted that many parameters were nearly constant over time. A number of linear least squares fits were made between the N 2-production estimates and different sets of the on-line parameters in order to get an estimator of the N 2-production based on the on-line parameters instead laboratory measurements. The correlation coefficient did not get much better than 0.6 which was considered too low to be of use. A few tries with squares and logarithms of the different on-line parameters did not improve the result considerably. Two factors explaining the low correlation was identified, the first was the lack of variation in the data. This lack of variation would allow noise and disturbances to dominate the variations, thus leading to low correlations. The second was that the theory indicated that the relation between some of the on-line parameters and the N 2-production was non-linear. Since the method based on using a linear least squares fit did not produce a good enough estimator, another approach had to be taken. At this point an ammonium-meter became available which made it possible to measure the NH +4 -concentration in the reactor on-line. If these measurements could be combined with estimates of the concentration of NO -2 and NO -3 it would be possible to estimate the N 2-production much in the same way as from the laboratory measurements. The on-line parameters were used to calculate linear least squares fits for both the NO -2- and the NO 3 -concentrations in the same manner as before. The correlation achieved for the NO 2concentration was low but a useful correlation was found between the pH and the NO -3 concentration (the correlation-coefficient was 0.7). By reviewing the available laboratory data, it was found that the NO -2-concentration had been very low and almost constant in comparison to the NH +4 - and NO -3 -concentrations during the entire time the reactor had been active. It was assumed that while the process was working, the NO -2concentration would remain low and could thus be neglected from the mass balance used to calculate the N 2-production. Thus, the final predictor of the N 2-production was based on the on-line ammonium measurements and the pH value of the outflow. 2[ N 2 ]=C −[ NH +4 - N ]k⋅pH


The predictor had a large number of assumptions tied to it but during the circumstances it was considered the only available option. The perhaps largest weakness of the estimator was that it would not detect a build-up of nitrite in the reactor due to the assumptions. Another problem was that the on-line ammonium meter introduced a time-delay of several hours due to the way the meter was built and placed in the facility. 3.2.3

System model

A part of the reason for developing the estimators was to use them to create a mathematical model of the system. This turned out to be impossible in the context of the project, mainly for two reasons. 20