| Independent variable | Symbol | Coded levels | ||||
| −2 | −1 | 0 | 1 | 2 | ||
| TOCin (mg/L) | X1 | 50 | 450 | 850 | 1250 | 1650 |
| Flow rate (mL/min) | X2 | 15 | 45 | 75 | 105 | 135 |
| H2O2,in (mg/L) | X3 | 100 | 300 | 500 | 700 | 900 |
| pH | X4 | 3 | 5 | 7 | 9 | 11 |