Variables
Symbol
Coded Level
–2
–1
0
1
2
KH
2
PO
4
(g/L)
X1
0.025
0.05
0.075
0.1
0.125
(NH
4
)
2
SO
4
(g/L)
X2
0.25
0.3
0.35
0.4
0.45
MnSO
4
(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).
