Livestock manure accounts for 80% (wet weight) of the organic waste generated in Korea. While approximately 86% of this waste is treated through composting, it is expected that the demand for compost will decrease in the future. Anaerobic digestion (AD) is a process that can treat organic waste and produce energy simultaneously and has received considerable attention as a method for treating organic waste in recent decades. Although the energy potential of livestock manure through AD is reported to be approximately 1.7 million tons of oil equivalent per year in Korea, the low anaerobic digestion efficiency of livestock manure limits the process performance. This study aimed to investigate the effect of thermal hydrolysis pretreatment (THP) on the AD of cattle manure (CM).

The THP was applied to the CM samples under various temperatures and NaOH addition conditions, and biochemical methane potential (BMP) was measured. The generation of recalcitrant and toxic substances (e.g., melanoidins and furfural) that could occur during the THP was determined, and the energy balance of the process was calculated. The results showed that increasing the NaOH concentration decreased the lignin content in the fiber and increased the solubilization of CM. The highest BMP has observed in CM treated at 160 ℃ with 2% NaOH addition, with a value of 227.0 ± 11.0 mL-CH₄/g-Volatile solid(VS), which was 25% higher than that of intact CM samples (182.2 ± 2.5 mL-CH₄/g-VS). The generation of recalcitrant substances and furfural was observed at temperatures above 180 ℃, and the production of recalcitrant substances was also promoted with increasing NaOH addition at temperatures below 180 ℃. Therefore, among the THP conditions without the generation of recalcitrant substances, the highest methane potential was observed in CM treated at 160 ℃ without NaOH addition. When applying THP to the AD of CM, it is predicted that an additional 161.4 ± 39.3 MJ/tonne-CM of energy can be produced.

A lab-scale continuously stirred tank reactor was operated under mesophilic anaerobic conditions for approximately 400 d while gradually reducing the solid retention time (SRT). The THP was performed at 160 ℃ and 6.1 atm for 30 min without the addition of NaOH. The results indicated that the THP-applied AD (THP AD) exhibited more than 1.4 times higher methane yield and VS removal efficiency than the control AD with the same SRT. Even under the SRT of 13.2 d, the THP AD showed higher performance than the control AD with a 36.0 d of SRT. However, the concentration of volatile fatty acids (VFAs) that could cause inhibition increased from 165 mg/L to 613 mg/L in THP AD as SRT reduced from 36.0 d to 13.2 d, and microbial community shifted towards an inefficient direction for the reactor performance. Thus, the stability of AD could decrease. Regardless of the application of THP, a rapid decrease in methane production was observed after 8.0 d of SRT for both THP AD and control AD. The stable operation was confirmed during the three periods of SRT at 13.2 d in this study, but stability confirmation for long-term operation is required.

ADM1 was enhanced to incorporate changes in biochemical parameters resulting from variations in SRT. Linear regression analysis was used to establish the relationship between the SRT and biochemical parameters, which were then incorporated as variables into the Dynamic ADM1. The model was calibrated using experimental data from an AD of CM and validated by simulating methane production of other reactors operated under different conditions and comparing the results. The accuracy of Dynamic ADM1 was improved by comparing it with the conventional ADM1. The same process was applied to an AD of thermally hydrolyzed CM, and the validity of the model was confirmed. According to model simulations, the application of THP resulted in a 1.5-fold increase in average methane production under SRT conditions ranging from 6.6 to 36.0 d. This was due to an increase in biodegradable substrate and maximum growth rate of microorganisms. Furthermore, THP shortened the SRT condition which demonstrated the highest concentration of microorganisms. The Dynamic ADM1 enables more precise prediction of reactor behavior in response to changes in SRT, offering benefits in determining operational conditions, enhancing design, and reducing operating costs.