## 5 Hydrothermal Pre-Treatment: Energy and Value-Added Products Recovery  
### Methane and Hydrogen  
In order to generate a sustainable energy system, it is predicted that hydrogen will soon be in global usage. Hydrogen is environment-friendly, safe to produce, quite useful for heating purpose in residences, and fuel for non-polluting transport systems and aircraft. The energy content of hydrogen (122 kJ/g) is 2.7 times higher than the gasoline. Similarly, methane gas, produced from anaerobic reactors and digesters is also used as a fuel to generate heat and light. It is a clean fuel thus used in industries, transportation, appliances and power generation [70]. [PERSON] et al. [71], studied the mesophilic anaerobic digestion of rice straw for the production of methane, which involved thermo-chemical pre-treatment. Hydrothermal treatment was given for 10 min at 200 \({}^{\circ}\)C to the ground rice straw. For the untreated sample, the methane production was 59.8 L/kg VS. Hydrothermal pre-treatment with 5% NaOH addition (for maintaining suitable pH for biogas production) enhanced the hydrolysis step of the anaerobic digestion thereby giving a methane yield of 132.7 L/kg VS, i.e., 2.22 times higherthan the untreated substrate. [PERSON] et al. [58], studied the hydrothermal pre-treatment of Giant reed biomass at temperature combinations of 150 \({}^{\circ}\)C and 180 \({}^{\circ}\)C, for a time of 10 and 20 min, in the presence and absence of catalyst. The catalyst used was 2% _w/w_ sulphuric acid, immediately before the hydrothermal pre-treatment and 24 h soaking in the catalyst. The untreated assay showed a methane yield of 273 mL/g VS, whereas the assay with no catalyst showed an increase of 10%, 7%, 23% and 4% in the 150 \({}^{\circ}\)C/10 min, 150 \({}^{\circ}\)C/20 min, 180 \({}^{\circ}\)C/10 min and 180 \({}^{\circ}\)C/20 min combination, respectively. The pre-treatments in the presence of the catalyst showed an inhibition in the methanogenic activity due to high sulphate concentration, which, led to the growth of the sulphate reducing bacteria. [PERSON] et al. [72], studied the hydrothermal pre-treatment on the methane production potential of antibiotic mycelial residue (AMR). Varying the treatment time from 0 to 60 min and temperature from 80 \({}^{\circ}\)C to 180 \({}^{\circ}\)C. The best digestion results were observed at a temperature of 120 \({}^{\circ}\)C for 60 min with a methane yield of 290 mL/g VS, which was almost 3 times the methane yield of raw AMR (100 mL/g VS). [PERSON] et al. [47] studied the hydrothermal pre-treatment followed by anaerobic digestion of five kinds of lignocellulosic biomass, namely, wheat straw, rice straw, and Douglas fir bark. Carbon dioxide was used as the catalyst in the experiment and the methane and hydrogen yield was also observed. Temperature and pressure variation of pre-treatment was from 26-175 \({}^{\circ}\)C and from 25-102 bars, respectively. 23-42% of hemicellulose destruction and 0-12% of delignification was observed. A 20-30% enhancement of hydrolysis rate was observed except for Douglas fir bark, which showed a 172% enhancement in digestion rate. For hydrothermally pre-treated biomass, a methane yield of 269 mL CH\({}_{4}\)/g VS\({}_{\text{added}}\) for wheat straw, 319 mL CH\({}_{4}\)/g VS\({}_{\text{added}}\) for rice straw, and 136 mL CH\({}_{4}\)/g VS\({}_{\text{added}}\) for Douglas fir bark was observed. Methane yield for the pre-treated substrate was higher by 5.1% in wheat straw, 2.6% in rice straw, and 39% in Douglas fir bark in comparison with non-pre-treated biomass. [PERSON] et al. [73] studied the effect of hydrothermal pre-treatment on Napier grass and the anaerobic digestion. The highest methane yield in batch studies was observed to be 248.2 NmL/VS added at a pre-treatment temperature of 175 \({}^{\circ}\)C, which was 35% greater yield in comparison with untreated biomass. [PERSON] et al. (2014) [74] studied the hydrothermal pretreatment of rice straw (20% TS) at 150 \({}^{\circ}\)C and 210 \({}^{\circ}\)C for 0 to 30 min operation time.