Optimizing thermal hydrolysis in kraft cellulose biosludge treatment: Challenges with recalcitrant compounds and process efficiency

The use of thermal hydrolysis (TH) in the treatment of biosludge has several significant benefits, such as increased biogas generation. However, this advantage is limited by the production of recalcitrant compounds, which increase the complexity of waste management and subsequent treatment stages. It has been observed that certain undesirable processes can lead to the formation of these recalcitrant compounds during the TH of biosludge, generating compounds such as melanoidins resulting from Maillard reactions, caramelization products, soluble lignin derivatives and potential inhibitors, which can be detected mainly in the liquid phase. The detection and monitoring of the increase in these compounds by easily applicable variables and tests are crucial for process optimization. This study focused on the generation of recalcitrant compounds during the TH of sludge from the effluent treatment of the Kraft cellulose industry. Indirect quantification methods, such as the measurement of absorbance at certain wavelength characteristic of compounds such as soluble lignins (205 nm), melanoidins (420 nm) and humic-type compounds (254 nm), have been employed. Increases of up to 15 times in the absorbance at 420, 254 and 205 nm were recorded with the application of TH compared with those of the untreated samples. However, no known inhibitors, such as hydroxymethylfurfural or furfural, were found. An increase in soluble organic nitrogen and residual postdigestion chemical oxygen demand was observed, which was related to the severity of the thermal treatment, suggesting significant correlations among the analysed variables.