## 3 Results and Discussion  
### Effect of Thermal and Thermochemical Pretreatment on Substrate Solubilization  
Thermal and thermochemical pretreatments with different conditions were applied to excess sewage sludge and food waste slurry. The sCOD of pretreated materials, as it can reflect the effectiveness of the pretreatment method in substrate solubilization, are reported in Figure 7. As can be observed from the figure, the result of thermal pretreatment at 70 and 90 \({}^{\circ}\)C for excess sewage sludge was similar The sCOD of the substrate almost doubled, which accounted for roughly 20% of the excess sewage sludge tCOD, after 30 min of pretreatment; however, this increase did not change significantly during the 4 h of pretreatment. This suggested that increasing temperature from 70 to 90 \({}^{\circ}\)C or increasing exposure time from 30 min to 4 h does not affect thermal pretreatment. [PERSON], et al. [29] reported that thermal pretreatment at 70 and 90 \({}^{\circ}\)C led to similar increases in sCOD at short pretreatment duration (less than 12 h), however, pretreatment at 90 \({}^{\circ}\)C with duration more than 24 h showed better results. In the case of food waste slurry, both temperature and exposure time influence thermal pretreatment effectiveness. Thermal pretreatment at 90 \({}^{\circ}\)C was more effective compared to pretreatment at 70 \({}^{\circ}\)C as the increase in sCOD was between 18-33% for the 90 \({}^{\circ}\)C pretreatment and 6-21% for the 70 \({}^{\circ}\)C pretreatment at different contact times, respectively. Regarding thermochemical pretreatment of substrates, pretreatment with Ca(OH)\({}_{2}\) at 70 and 90 \({}^{\circ}\)C for both excess sewage sludge and food waste slurry resulted in similar increases in sCOD compared to thermal pretreatment at respective temperatures. However, pretreatment with NaOH showed significant increases (\(p<0.05\)) in sCOD. Similarly, [PERSON], et al. [30] and [PERSON], et al. [31] reported that thermochemical pretreatment with Ca(OH)\({}_{2}\) is less effective for solubilizing the biomass compared to NaOH. This could be due to the low solubility of Ca(OH)\({}_{2}\) in water. Pretreating excess sewage sludge with NaOH at 70 and 90 \({}^{\circ}\)C for 30 min resulted in 5.30- and 6.35-times increases in sCOD, respectively. Exposure time did not affect the pretreatment at 90 \({}^{\circ}\)C considerably but had an impact on pretreatment at 70 \({}^{\circ}\)C as the sCOD of pretreated sludge increased from 28.4 g/L after 30 min to 37.2 g/L after 4 h of pretreatment. Thermal and thermochemical pretreatment improved the disintegration of both excess sewage sludge and food waste slurry; however, this does not necessarily lead to improvement of the further biodegradation,  
Figure 6: An overview of the proposed scenario two.  
since other compounds such as heavy metals, inhibitors, and high complex compounds can be released in the water phase. Thus it is necessary to evaluate the effect of pretreatment on the anaerobic digestion. Considering the results of the pretreatment step, industrial and economic aspects of the pretreatment method, using both thermal and thermochemical (NaOH) pretreatment at 90 \({}^{\circ}\)C for 30 min, were chosen for the next phase of the experiment.