Thermal hydrolysis (TH) has been widely applied to sewage sludge treatment to improve downstream anaerobic digestion (AD) by enhancing organic solids hydrolysis and subsequent biogas productivity. However, TH also has several drawbacks, including the formation of recalcitrant compounds, which are anaerobically non-biodegradable and even inhibitory to the downstream AD. Melanoidins produced from the Maillard reaction have been regarded as a representative of such compounds. Theoretically, the production of melanoidins increases with increasing TH temperature. This study collaborated with TORWASH®, a patented TH technology developed by TNO, aiming to characterise the recalcitrant compounds derived under TORWASH® conditions from 180 - 210 °C. In addition, we investigated the effects of anaerobic pre-digestion on the TH performance, attempting to increase the biogas productivity of digestate. Moreover, we also sought pretreatment methods that could mitigate the production of recalcitrant compounds. By reviewing the studies of Zhang et al. (2020) and other researchers in this field, we found that acidic pretreatments could be the solution. Then, we assumed that the addition of acetic acid and CO2 gas could limit the progression of the Maillard reaction. A laboratory simulation of the TH process was performed on waste activated sludge (WAS) at 180 - 200 °C and anaerobically pre-digested WAS at 180 - 210 °C. The following TH tests with acidic pretreatments were only conducted on WAS at 190 °C. We found that the production of recalcitrant compounds (soluble humic substances) increased with increasing reaction temperature, corresponding to increased UVA254 and colour formation. Proteins were primarily hydrolysed or denatured during TH, and competition for carbohydrates between the Maillard and caramelisation reactions could occur at 190 °C and above. In addition, TH disintegrated high molecular weight DOM > 1 kDa into low molecular weight DOM < 0.1 kDa, and stronger sludge disintegration occurred at higher reaction temperatures. Moreover, BMP decreased after anaerobic pre-digestion but increased after TH treatment. The liquid fraction of pre-digested WAS could reach an equivalent degree of anaerobic biodegradability as that of WAS when applying a higher reaction temperature. All acidic pretreatments applied decreased the production of soluble humic substances by 10 %, corresponding to a 13 % decrease in UVA254. CO2 pressurisation at 10 bar could effectively reduce the formation of fulvic-acid like and humic-acid like matter, compared with the untreated sample. The effect of 20 bar CO2 was less pronounced than that of 10 bar CO2, whether or not the samples were pre-added with HAc.