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Table 2 The level of variables

From: Enhanced methane production of vinegar residue by response surface methodology (RSM)

Variables Factors Experimental design (Coded level)
(Coded Xi) −1.682 −1 0 +1 +1.682
F/I ratio X1 0.5 0.7 1 1.3 1.5
Organic loading (gVS L−1) X2 13.18 20 30 40 46.82
Initial pH X3 5.32 6 7 8 8.68