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Test 1

The first test with automatic control of the process was executed in late December 2010. The controller settings used during the test can be seen in table 4. The ammonium-meter broke after one day of testing. This is visible in Figure 23 which shows how the ammonium-meter suddenly started to record a constant NH +4 - N-level of 300 mg l −1. This was the maximum-level the ammonium-meter was configured to measure. After an investigation the fault turned out to be a broken membrane within the meter.

Figure 23 The ammonium-level according to the ammonium-meter. An ion selective membrane on the measuring-probe was penetrated after about one day of testing, causing the meter to report faulty values. Since the ammonium-meter malfunctioned during most of the test, the automatic controller did not work as intended. However, the results from the test were still considered interesting and thus included here. The pH-meter was still functioning and causing variation in the estimate of the N 2 production. In fact, the pH (Fig. 24 A) varied abnormally much compared to data gathered during previous operation of the process.