## 3 Results
### First Stage: Testing Ammonium Volatilisation
It is relevant to state that reported DO concentrations should be considered as an indication that the systems were aerated through the experimental period. However, for quantification purposes and the control of aerated and non-aerated periods, daily and in-situ measurements are recommended. Figure 4e shows that the COD concentrations decreased gradually over time, but only 27% removal was achieved in the case of treatment D. A removal of 65% was obtained when constant aeration was provided. The concentrations reported from treatment B and C showed that shorter aeration periods resulted in lower removal efficiency (63% and 58% respectively). For the specific case of BOD in Figure 4d, the applied model procedure was not able to estimate the statically significant effects of aeration. Due to fluctuation in the measurements, there was insufficient data to estimate the particular covariance structure [26]. Therefore, the effect of aeration in BOD is considered based on observations and interpreted with caution. The BOD measurements show high variability within the treatments. Only after the 10 th sampling day (one-month period), the BOD concentrations in treatments A, B,  
Figure 5: Evolution of ammonium-nitrogen, nitrate-nitrogen and total nitrogen in each IBC exposed to different aeration periods. (**a**) Treatment A (constant aeration), (**b**) Treatment B (intermittent aeration 4h AA:4h no-AA), (**c**) Treatment C (intermittent aeration 6h AA:4h no-AA), (**d**) Treatment D (no aeration), AA = aeration. and C followed a similar pattern and stabilised at approximately 8 mgO2/L. In treatment D, the BOD concentrations remained higher at approximately 33 mgO2/L. Similar to BOD, NO3-N (Figure 5 -) measurements showed great variability. In the case of NO3-N, the variation amongst the measurements could be attributed to the different analytical methods used. The test kits were used to determine TN, whereas, NO3-N and NH4+-N were determined by ion chromatography. The high dilution factors were needed to analyse the samples by ion chromatography due to high sensitivity and low detection limits of the equipment. Considering that this fact could have increased the error or variation in the analysis, the results were corroborated and some samples re-analysed using quick test kits. The inconsistent NO3-N values evidenced through the comparison with the TN concentration, and after testing similar methodologies, were considered as outliers and removed from the dataset. The overall tendency indicates that NO3-N concentrations decreased only in treatment D from 341 mg/L to 223 mg/L, indicating a removal of 35%. The estimated effects of this treatment supported that nitrate concentrations significantly decreased (\(p<0.05\)) in the non-earated container. The ammonium concentrations started to decrease significantly after a one-week period of the second stage of the experiment (Figure 5 -). A full NH4+-N removal was obtained in treatment A, B and C, whereas, a lower removal efficiency of 35% was achieved in treatment D. Looking at Figure 5 -, it can be stated that there is no clear distinction among the effects of aeration on TN concentrations between treatments. However, the 55% of TN removal achieved by treatment D was just slightly above the 50% removal at treatment A. By assessing the whole testing period through the model procedure estimate effects, the results corroborated that statistically significant effects on the reduction of TN concentrations were mostly recorded in the non-aerated container, and a few other occasions when intermittent aeration, as in treatment C, was applied. In Appendix B, Table 1 presents the mean values of the physicochemical, organic and nitrogen compounds recorded during the second stage of the experiment. Figure 11a-d represent the diagnostic plots of the mixed modelling procedure for COD, NH4+-N, NO3-N and TN, respectively. The plots indicated residuals are normally distributed.