Process optimization for the anaerobic digestion of poplar (Populus L.) leaves

Figures & data

Table 1. Variables and levels of factors used in the experiments

Variable	Parameters	Level
		−1	0	+1
A	Alkali concentration (%)	2	4	6
B	Bacteria concentration (‰)	0.10	0.15	0.20
C	Composting time (days)	2	5	8

Table 2. Real value of the variables and response variable

Trails	A (%)	B (‰)	C (Days)	Total Biogas Production (ml)
1	2	0.15	8	17,582
2	6	0.15	8	19,521
3	4	0.1	8	17,086
4	4	0.20	2	16,491
5	4	0.20	8	28,816
6	2	0.20	5	20,067
7	4	0.1	2	19,050
8	4	0.15	5	26,015
9	6	0.1	5	19,243
10	4	0.15	5	24,267
11	2	0.15	2	11,243
12	4	0.15	5	28,212
13	2	0.1	5	17,890
14	6	0.20	5	27,215
15	4	0.15	5	26,800
16	6	0.15	2	14,718
17	4	0.15	5	25,816

Table 3. Analysis of variance (ANOVA) of the response surface quadratic model

Source	Sum of Squares	df	Mean Square	F Value	p-value Prob > F
Model	4.36*10^8	9	4.84*10^7	34.90 ^a	< 0.0001
A-Alkali Concentration	2.42*10^7	1	2.42*10^7	17.45	0.0042
B-Bacteria Concentration	4.67*10^7	1	4.67*10^7	33.63	0.0007
C-Composting Time	5.78*10^7	1	5.78*10^7	41.66	0.0003
AB	8.40*10^6	1	8.40*10^6	6.05	0.0435
AC	5.90*10^5	1	5.90*10^5	0.43	0.5352
BC	5.10*10^7	1	5.10*10^7	36.79	0.0005
A^2	9.93*10^7	1	9.93*10^7	71.58	< 0.0001
B^2	2.88*10^5	1	2.88*10^5	0.21	0.6622
C^2	1.32*10^8	1	1.32*10^8	95.16	< 0.0001
Residual	9.71*10^6	7	1.39*10^6
Lack of Fit	1.39*10^6	3	4.62*10^5	0.22 ^b	0.8766
Pure Error	8.32*10^6	4	2.08*10^6
Core Total	4.45*10^8	16

Std. Dev. = 1177.82, Mean = 21,178.35, C.V.% = 5.56, PRESS = 3.520*10⁷,

R² = 0.9782, R²_adj = 0.9502, R²_pred = 0.9210, Adequate precision = 20.016

^asignificant with 95% confidence interval, ^b not significant

Figure 1. Response surface curves for biogas production as a function of alkali concentration and bacterial concentration. (a) Alkali concentration and composting time (b); and bacterial concentration and composting time (c)

Figure 2. Changes in the daily biogas production with fermentation time of the treated groups (t) and the untreated groups (u). (T1/U1: pH 6, TS 8%, Temperature 25°C. T2/U2, pH 6, TS 10%, Temperature 30°C. T3/U3, pH 6, TS 12%, Temperature 35°C. T4/U4, pH 7, TS 8%, Temperature 30°C. T5/U5, pH 7, TS 10%, Temperature 35°C. T6/U6, pH 7, TS 12%, Temperature 25°C. T7/U7, pH 8, TS 8%, Temperature 35°C. T8/U8, pH 6, TS 10%, Temperature 25°C. T9/U9, pH 8, TS 12%, Temperature 30°C.)

Table 4. Total biogas production of the treated and untreated groups under different fermentation conditions

Group	pH	TS (%)	Temperature (°C)	Total biogas production (ml)
				Treated	Untreated
T1/U1	6	8	25	26,183	4515
T2/U2	6	10	30	35,130	5637
T3/U3	6	12	35	26,805	4533
T4/U4	7	8	30	34,677	5411
T5/U5	7	10	35	31,418	4859
T6/U6	7	12	25	33,752	5401
T7/U7	8	8	35	23,704	3797
T8/U8	8	10	25	32,234	5005
T9/U9	8	12	30	39,625	6209

Figure 3. Changes in the biogas generation rate in the treated and untreated groups at different temperatures. A: 25°C, B: 30°C, and C:35°C. (T1/U1: pH 6, TS 8%, Temperature 25°C. T2/U2, pH 6, TS 10%, Temperature 30°C. T3/U3, pH 6, TS 12%, Temperature 35°C. T4/U4, pH 7, TS 8%, Temperature 30°C. T5/U5, pH 7, TS 10%, Temperature 35°C. T6/U6, pH 7, TS 12%, Temperature 25°C. T7/U7, pH 8, TS 8%, Temperature 35°C. T8/U8, pH 6, TS 10%, Temperature 25°C. T9/U9, pH 8, TS 12%, Temperature 30°C.)

Figure 4. Changes in the biogas generation rate in the treated and untreated groups with different TS levels. A: 8%, B: 10%, and C: 12%. (T1/U1: pH 6, TS 8%, Temperature 25°C. T2/U2, pH 6, TS 10%, Temperature 30°C. T3/U3, pH 6, TS 12%, Temperature 35°C. T4/U4, pH 7, TS 8%, Temperature 30°C. T5/U5, pH 7, TS 10%, Temperature 35°C. T6/U6, pH 7, TS 12%, Temperature 25°C. T7/U7, pH 8, TS 8%, Temperature 35°C. T8/U8, pH 6, TS 10%, Temperature 25°C. T9/U9, pH 8, TS 12%, Temperature 30°C.)

Figure 5. Changes in the biogas generation rate in the treated and untreated groups at different pH value. A: pH 6, B: pH 7, and C: pH 8. (T1/U1: pH 6, TS 8%, Temperature 25°C. T2/U2, pH 6, TS 10%, Temperature 30°C. T3/U3, pH 6, TS 12%, Temperature 35°C. T4/U4, pH 7, TS 8%, Temperature 30°C. T5/U5, pH 7, TS 10%, Temperature 35°C. T6/U6, pH 7, TS 12%, Temperature 25°C. T7/U7, pH 8, TS 8%, Temperature 35°C. T8/U8, pH 6, TS 10%, Temperature 25°C. T9/U9, pH 8, TS 12%, Temperature 30°C.)

Figure 6. Fitting of the modified Gompertz model to the experimental data of optimized fermentation