Table 4.

Analysis of variance results of regressions for total VFAs at 24 h incubation and biogas production at 12 h incubation

Response	Source	Degree of freedom	Sum of square	Mean of square	f-value	p-value
Total VFAs at 24 h	Regression	9	523.11	58.12	10.13	0.010
	Linear	3	367.83	7.79	1.36	0.356
	Square	3	38.40	10.61	1.85	0.256
	Interaction	3	116.88	38.96	6.79	0.033
	Residual error	5	28.69	5.74	-	-
	Lack-of-fit	3	24.93	8.31	4.43	0.190
	Pure error	2	3.75	1.88	-	-
	Total	14	551.80	-	-	-
Propionate at 24 h	Regression	9	35.54	3.95	13.07	0.006
	Linear	3	23.82	0.88	2.93	0.139
	Square	3	4.03	1.12	3.72	0.096
	Interaction	3	7.69	2.56	8.49	0.021
	Residual error	5	1.51	0.30	-	-
	Lack-of-fit	3	1.42	0.47	10.66	0.087
	Pure error	2	0.09	0.04	-	-
	Total	14	37.06	-	-	-
Biogas production at 12 h	Regression	9	1113.36	123.71	17.39	0.003
	Linear	3	256.48	255.02	35.85	0.001
	Square	3	829.79	280.32	39.40	0.001
	Interaction	3	27.10	9.03	1.27	0.379
	Residual error	5	35.57	7.11	-	-
	Lack-of-fit	3	22.90	7.64	1.21	0.483
	Pure error	2	12.67	6.33	-	-
	Total	14	1148.93	-	-	-

VFA, volatile fatty acid.

R² values for total VFAs, propionic acid and biogas production were 94.8%, 95.9%, and 96.9%, respectively.