| Symbol | Factors | Units | Levels | ||
| Low (−) | Mid (0) | High (+) | |||
| X1 | Dose | kg/t | 3 | 4 | 5 |
| X2 | pH | - | 2 | 3 | 4 |
| X3 | Impeller speed | rpm | 1000 | 1250 | 1500 |
| X4 | Air flow rate | L/min | 10 | 20 | 30 |