3.3. Effect of recycled digestate
Fig. 6 shows the effect of recycled digestion addition on the
biogas production using the substrate mixing ratio of 0.15 (w/w).
The use of recycled digestate improved the production of biogas by
12%. The increase of biogas production due to the recycled digestate
is attributed to the increase of active bacteria since the recycled
materials are a rich source of microorganisms. The results are in
agreement with other results of previous works (Gizem et al., 2014;
Lili et al., 2015). However, more lignin and cellulose bonds are
destroyed to smaller sucrose and glucose molecules indicating that
the existence of these bacteria could be another option to increase
the biogas generation rates. On the other hand, the accumulation of
VFA can be reduced by using the recycled material, especially when
this material has a high water content (Lin et al., 2014).