The time courses of the concentrati

The time courses of the concentrations of TN, NO3-N, NO2-N and NH4-N and the water temperature in Reactor 1 are shown in Figure 2. The water temperature was 10°C to 35 °C (mean,
23 °C) during the operational period. The TN concentration was not changed by the reactor. The ammonium concentration decreased and the nitrate concentration increased. No accumulation of NO2-N was observed during the operational period. The ammonia oxidation occurred after 23 days of operation. These results indicate that the start-up of Reactor 1 was comparatively short for the application of foam ceramics as a microbial supporting medium.
The relationship between the decreased NH4-N and increased NO3-N in Reactor 1 is shown in Figure S1 of the Supplementary Materials. The decreased NH4-N was associated with an increase in NO3-N, indicating that nitrification occurred steadily in Reactor 1. In addition, NO3-N accumulated without the occurrence of denitrification. We suspected that the influent of Reactor 1 contained almost no organic compounds as electron donors for denitrification. The relationship between the increased nitrate and decreased inorganic carbon in Reactor 1 is shown in Figure 3. Based on the assumption that the gross composition of Nitrosomonas and Nitrobacter can be represented by C5H7NO2, the nitrification can be described by the following equation [13]:
NH4+ + 1.83O2 + 1.98HCO3− → 0.98NO3− + 0.021C5H7NO2 + 1.041H2O + 1.88H2CO3 (1)
The pH values of the influent and effluent of Reactor 1 were almost the same: 7.0 ± 0.3 and
7.0 ± 0.4, respectively. We assumed that the neutral pH was stable because the effluent from the final sedimentation basin contains an adequate amount of carbonate, even if the nitrifying bacteria consumed carbonate (Equation (1)).