Table 2. Central composite design matrix and the responses of biomass and hydrocarbon production to Fe (x₁), Mn (x₂), Mo (x₃) and Ni (x₄).

Runs	Independent variables				Responses
	Coded levels				Biomass(g/L)	Hydrocarbon (%, w/w)
	x1	x2	x3	x4
1	1	1	1	1	0.246	14.82
2	−1	−1	1	1	0.292	14.31
3	1	−1	−1	1	0.251	15.45
4	−1	1	−1	1	0.296	14.56
5	1	−1	1	−1	0.124	13.99
6	−1	1	1	−1	0.120	13.42
7	1	1	−1	−1	0.136	14.83
8	−1	−1	−1	−1	0.125	13.86
9	1	−1	1	1	0.257	13.96
10	−1	1	1	1	0.320	14.12
11	1	1	−1	1	0.248	14.19
12	−1	−1	−1	1	0.306	14.00
13	1	1	1	−1	0.116	13.96
14	−1	−1	1	−1	0.121	15.26
15	1	−1	−1	−1	0.105	14.68
16	−1	1	−1	−1	0.126	13.96
17	1.72	0	0	0	0.215	20.23
18	−1.72	0	0	0	0.231	19.24
19	0	1.72	0	0	0.123	12.25
20	0	−1.72	0	0	0.121	11.59
21	0	0	1.72	0	0.118	18.57
22	0	0	−1.72	0	0.124	20.18
23	0	0	0	1.72	0.289	12.54
24	0	0	0	−1.72	0.094	11.90
25*	0	0	0	0	0.124	19.31
26*	0	0	0	0	0.120	18.46
27*	0	0	0	0	0.123	19.17
28*	0	0	0	0	0.127	20.13
29*	0	0	0	0	0.122	19.74
30*	0	0	0	0	0.126	18.45

^*Central point values contributing to the degree of freedom for pure error calculation.