*2.3.1 The effect of total solid (TS)*

Solid-state anaerobic digestion (SS-AD) systems usually operate at TS contents of higher than 15% [4]. A research has been conducted [20] about the effect of


**Table 3.** *Research material needs.*

**Figure 5.** *Cumulative biogas yield per g TS based on C/N ratio.*

## *Biogas Production from Water Hyacinth DOI: http://dx.doi.org/10.5772/intechopen.91396*

solid-state anaerobic digestion (SS-AD) on biogas production using water hyacinth leaves. In this study to make the total solid content increased, drying method was used. The water hyacinth leaves from Rawa Pening Lake have an initial total solid content of 13.52%. The first variable was dried for 2 days, and the second variable was dried for 1 day. After that, the water hyacinth leaves that have been dried in the sun were examined for their total solid content using Eq. (1). For the first variable, the total solid content was 48.26% and for the second variable was 36.36%. After that the water hyacinth leaves that have been dried in the sun were added to the cow rumen in a ratio of 1:1. In studies using the SS-AD method, no additional water was given [20]. The combination of the variables is shown in **Table 4**.

The variable with TS of 24.34% produced biogas with a total of 34.79 ml/g TS, and the variable with TS of 17.67% obtained 52.98 m/g TS. The result is shown in **Figure 6**.

Further research had been conducted [24] to know about the optimization of total solid (TS) and carbon to nitrogen (C/N) ratio of biogas production from water hyacinth leaves by adding microbial consortium as much as 3%, 6%, and 9%. Meanwhile the total solid contents from the research were 15%, 27.5%, and 40%. And the C/N ratios were 20, 35, and 50. To get the optimum conditions, calculation had been done by the central composite design method with the following variables in **Table 5**.

Variations of variable values in each reactor were obtained using Statistica software as shown in **Table 6**.


**Table 4.**

*Initial and final total solid of water hyacinth leaves.*

**Figure 6.** *Cumulative biogas yield per gram TS.*

### *Biogas - Recent Advances and Integrated Approaches*


### **Table 5.**

*Variable values in the central composite design.*


### **Table 6.**

*Variable values in experiments using central composite design.*

In this SS-AD method, variations in the total solid concentration used were 15%, 27.5%, and 40%. The total solid for each reactor was adjusted to the total solid of the water hyacinth. Water and rumen were added to regulate the total solid in each of the reactors. **Figures 7–9** show the graphs of biogas results produced at certain reactors which were compared between reactors with the same C/N and microbial consortium ratio values against different TS values [24].

The graph in **Figure 7** shows the production of biogas produced from Reactor 1 and Reactor 5 where the reactors had the same concentration variations for the same C/N ratio and microbial consortium variables, with the lowest value of each variation of 20 for the C/N ratio and 3% for the concentration of the microbial consortium. The difference was in the total solid concentration (**Table 6**). Based on the graph in **Figure 7**, the total cumulative biogas production for Reactor 1 was 27.367 ml/g TS while for Reactor 5 was 5.1 ml/g TS. Reactor 1 with a lower TS which was 15% produced biogas with a greater total than that of the Reactor 5 with TS of 40% [24].

Reactors 4 and 8 had varying concentrations for the same C/N and microbial consortium variables (**Table 6**), namely, a C/N ratio of 50 and a microbial consortium concentration of 9%. Both of these variation values are the highest values

**Figure 7.** *Effect of TS on biogas production (Reactors 1 and 5).*

**Figure 8.** *Effect of TS on biogas production (Reactors 4 and 8).*

among the range of values for these variables. The TS concentrations in Reactors 4 and 8 were 15% (lowest value) and 40% (highest value). Reactor 4 with a lower TS value of 15% produces more biogas production than Reactor 8 with a higher TS value (40%). **Figure 8** shows that the total biogas production for Reactor 4 was 43.87 ml/g TS and for Reactor 8 was 6.15 ml/g TS [24].

The biogas production graph in **Figure 9** came from a reactor with a C/N ratio of 35 and a microbial consortium concentration of 6% (**Table 6**). This value was the middle value of the variation of concentration for each of these variables. Biogas production varies in each of the reactors. It can be seen in **Figure 9** that the total biogas production for Reactors 9, 10, and 11 was 22.65 ml/g TS, 87.85 ml/g TS, and 10.09 ml/g TS. Reactor 10 with TS concentration of 5.45%, C/N ratio of 35, and

**Figure 9.** *Effect of TS on biogas production (Reactors 9, 10, and 11).*

**Figure 10.** *Effect of C/N ratio on biogas production (Reactors 1 and 3).*

microbial consortium concentration of 6% produces the largest biogas production when among Reactors 10 and 11 [24].

## *2.3.2 The effect of C/N ratio*

The variations in the C/N ratio used in this study were 20, 35, and 50. First the C/N ratios of the water hyacinth leaves were tested. To obtain variations in the concentration of the C/N ratio as determined, urea was used to adjust the N value of the water hyacinth leaves [24].

Different C/N ratios were tested with the same total solid and microbial consortium concentration in Reactor 1 and Reactor 3 (**Table 6**). Reactor 1 with a C/N ratio

### *Biogas Production from Water Hyacinth DOI: http://dx.doi.org/10.5772/intechopen.91396*

of 20, total solid of 15%, and microbial consortium concentration of 3% produced a total biogas of 27.37 ml/g TS. Reactor 3 with a C/N ratio of 50, total solid of 15%, and microbial consortium concentration of 3% produced biogas with a total of 51 ml/g TS. Reactor 3 with a higher C/N ratio of 50 produced more biogas volume than the Reactor 1 with a C/N ratio of 20. The graph is shown in **Figure 10** [24].

The graph in **Figure 11** was a biogas graph produced from Reactors 6 and 8. The concentrations of the total solid and microbial consortium variables in Reactor 6 were the same as those in Reactor 8 which were 40% and 9%, respectively (**Table 6**). The C/N ratio of Reactor 6 was 20, while Reactor 8 is 50. For the total biogas production produced, based on the graph in **Figure 11**, it can be seen that Reactor 6 has a higher biogas than that of the Reactor 8 which was 13.14 ml/g TS for Reactor 6 and 6.15 ml/g TS for Reactor 8. Thus, reactors with lower C/N ratios produce higher biogas under conditions of total solid concentration of 40% and microbial consortium of 9% [24].

The graph in **Figure 12** was taken from the calculation of biogas production produced in Reactors 9, 12, and 13. The reactors have the same total solid concentration and

**Figure 11.** *Effect of C/N ratio on biogas production (Reactors 6 and 8).*

**Figure 12.** *Effect of C/N ratio on biogas production (Reactors 9, 12, and 13).*

microbial consortium (**Table6**) of 27.5% and 6%, respectively, with a C/N ratio different from Reactor 9 with a C/N ratio of 35, Reactor 12 with a C/N ratio of 8.54, and Reactor 13 with a C/N ratio of 61.45. The total biogas production in Reactor 9 was 22.65 ml/g TS, whereas in Reactor 12, the total biogas production was 4.76 ml/g TS. For Reactor 13, the total biogas production was 31.24 ml/g TS. The volume of biogas production in Reactor 13 was greater than the volume of biogas production in Reactors 9 and 12 [24].
