|
| Levels | |||
| Factors | Symbol | Low (−1) | Center (0) | High (+1) |
| Contact time (minutes) | A | 40 | 80 | 120 |
| Dye concentration (mg∙L−1) | B | 30 | 65 | 100 |
| pH | C | 3 | 5 | 7 |