Variables Symbol Coded Level
–2 –1 0 1 2
KH2PO4 (g/L) X1 0.025 0.05 0.075 0.1 0.125
(NH4)2SO4 (g/L) X2 0.25 0.3 0.35 0.4 0.45
MnSO4 (g/L) X3 0.05 0.1 0.15 0.2 0.25
PHENOL (mg/L) X4 200 400 600 800 1000
Table 3: Levels of the variables tested in the Central Composite Design (CCD).