Table 1 Experimental levels and factors of CCD.

From: Optimization pilot scale study on ammonia nitrogen removal by bio filter

Coded level

Uncoded level

C/N

pH

HL (d−1)

 − 1.68

11.59

6.66

0.16

 − 1

15.00

7.00

0.50

0

20.00

7.50

1.00

1

25.00

8.00

1.50

1.68

28.41

8.34

1.84