# Influences of Temperature and Substrate Particle Content on Granular Sludge Bed Anaerobic Digestion
## 3 Result and Discussion
### Simulation Results  
Comparison of the two modes of simulations revealed that classification of particles into fast and slow disintegrating fractions (Mode 2) led to a better representation of the experimental data compared to Mode 1 simulation across all four phases of the experiment. Application of the same simulation approaches to batch reactors also revealed that Mode 2 simulation led to a better fit to experimental observations [9]. Mode 1 simulation assumed that all particulates possess fast disintegration potential, whereas Mode 2 assumed that a fraction of the particulates disintegrated at a slower rate based on Equation (4). The result from the Mode 1 simulation revealed that the rate of methane production was consistently lower than the experimental data. In the batch test experiments, we observed that the Mode 1 simulation overestimated the rate of methane production, which was expected considering that more particulates have higher disintegration rate which means more production of methane. Our initial explanation for this was an accumulation of VFA, but pH data did not support it. However, data from biomass concentration (Figure 4) showed that the growth of acetate (Xac), sugar (Xsu), and fatty acid (Xfa) degrading organisms declined quite fast in the Mode 1 simulation, likely affecting the rate of methane production (Figures 5 and 6).  
Figure 4: Changes in acetate (Xac), sugar (Xsu), and fatty acid (Xfa) degrading organisms in the reactor during Mode 1 and Mode 2 simulations.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline  & \multicolumn{4}{c}{**Average Removal (\%)**} \\ \cline{2-5}
**Property** & \multicolumn{2}{c}{**Raw Feed**} & \multicolumn{2}{c}{**Centrifuged Feed**} \\ \cline{2-5}  & \(\mathbf{25}\ {}^{\circ}\mathbf{C}\) & \(\mathbf{35}\ {}^{\circ}\mathbf{C}\) & \(\mathbf{25}\ {}^{\circ}\mathbf{C}\) & \(\mathbf{35}\ {}^{\circ}\mathbf{C}\) \\ \hline TS & 31 & 42 & 17 & 29 \\ VS & 47 & 58 & 23 & 47 \\ TSS & 50 & 68 & \(-\)16 & 16 \\ VSS & 42 & 61 & \(-\)19 & 17 \\ TDS & 8 & 9 & 28 & 33 \\ VDS & 60 & 48 & 58 & 73 \\ COD\({}_{\text{total}}\) & 44 & 73 & 43 & 54 \\ COD\({}_{\text{soluble}}\) & 58 & 79 & 50 & 68 \\ VFA\({}_{\text{total}}\) & 70 & 91 & 67 & 97 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Removal efficiencies of the reactor (based on g/L measurements in Table 7).  
The Mode 2 simulation showed a more efficient particulate removal and sensitivity to changes in temperature and particulate content than the Mode 1 simulation (Figures 7 and 8). In both modes of simulations, particulates accumulated regardless of whether they were fast or slow disintegrating, and this continued until the start of Phase 3 when CF was introduced. During Phases 1 and 2, the rate of addition of particulates into the reactor was faster than the rate of disintegration and hydrolysis. After Phase 2, the particulate content started to decline. At the start of Phase 3, the particulates that were let into the reactor during Phases 1 and 2 were in the reactor for several days, meaning that even slow disintegrating portions of the particulates would have started to disintegrate. The particulate content of CF was much lower than that of RF, leading to a slower rate of addition of particulates into the reactor. A combination of these factors contributed to the decline in the particulate content in the latter two phases of the experiment. However, in the Mode 1 simulation, the decline in particulate content was short; after an initial decline early in Phase 3, particulate content seemed to stay constant. By contrast, Mode 2 showed a considerable decline in particulate content, which continued until the end of the experiment.  
Figure 5: Biogas flow rate when K\({}_{\text{dis}}\) and Xc were used (Mode 1 simulation).  
Figure 6: Biogas flow rate when K\({}_{\text{dis1}}\), K\({}_{\text{dis2}}\), Xc1, and Xc2 were used (Mode 2 simulation).