Table 2 presents Pearson correlation coefficient analysis results using a two tailed test at 95% significance while Figure 4(a)–(e) presents the relationship between AE and input parameters (COD, airflow rate, temperature, OUR, and OTR/K_La). COD (−0.043**) produced a negative correlation relation to AE as shown in Table 2. This was because high concentrations of organic matter causes bubble coalescence in the biological treatment process. Bubble coalescence results in large bubbles that are faster, reducing contact time between air bubbles and wastewater. The less contact time between air bubbles and wastewater results in low AE, since the oxygen molecules carried by the air bubbles will not be able to penetrate wastewater thickness caused by high organic matter (Fan et al. 2014; Durán et al. 2016; Odize et al. 2017). Odize et al. (2017) reported that AE decreased by 0.47% with an increase in colloidal/particulate COD from 5 mg/L to 20 mg/L. Fan et al. (2014) reported a decrease of 1.744% in AE when the organic content increased from 2.840 g/L to 10.498 g/L in the biological activated sludge process. This can also be justified by the relationship between COD and OTR/K_La that produced a negative correlation of - 0.579**, which indicates that when COD increases, OTR/K_La decreases. The relationship between AE and COD shown in Figure 4(a) shows high AE at low COD concentration.