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In Section F the ammonium-meter was once again showing a constant value. As a consequence, the only variation in the N 2-estimate was due to a changing pH. The pH was going up, causing the N 2estimate to increase. The controller “assumed” this was due to the most recent decrease in DO and thus continued to decrease the DO. The lowest DO level allowed was 0.8 mg l −1 and since the controller could not decrease it further it stayed constant for the remainder of the test. If the limit had not been in place the controller would eventually have turned the DO off completely. Summary Test 1 The first test was dominated by a malfunctioning ammonium-meter which caused the N 2-estimate to be seemingly independent of the DO-level in the reactor. This was unfortunate but it allowed a real-world analysis of the controller behaviour with independent variations in the N 2-estimate. This analysis confirmed that some of the behaviours identified in the simulations also occurred in a real implementation. 5.2.2

Test 2

Since the ammonium-meter malfunctioned during the first test, another test with almost the same controller settings as Test 1 was performed. The only difference was that  t was set to 3 hours instead of 2 (Table 4).This time, the ammonium-meter worked as intended. The first process-parameter examined was the DO(Figure 26). It did not vary in the way expected around an optimum (see simulations in figures 16 & 17) so the optimum was assumed to not have been found.

Figure 26 The DO level during the second test. In order to analyse the controller’s behaviour the pH and NH +4 measurements were reviewed (Fig. 27). Both the NH +4 and the pH seemed to vary in a very similar way, indicating that both these parameters might have been influenced by the variation in DO. 40