Alkaline Thermal Hydrolysis of Sludge : Browning Reactions and Their Influence on Anaerobic Digestion and Protein Recovery

The organic matter released during the alkaline thermal hydrolysis (ATH) of sludge bases reacts with each other to produce melanoidins, 5-hydroxymethylfurfural (HMF) and humic acids which could have an effect on subsequent anaerobic digestion and protein recovery. The effects of ATH at different temperatures and reaction times on the organic matter in sludge associated with the browning reaction and the subsequent performance of the hydrolysate for protein recovery and anaerobic digestion were systematically evaluated. The results shown that the browning of the sludge hydrolysate was related to the comprehensive effects of the Maillard reaction (MR), caramelization and humic acid desorption. The concentrations of melanoidin, HMF and humic acid in the hydrolysate were positively correlated with temperature in the range of 80 °C -150 °C. At 150 °C (pH 12 and 1 h), melanoidin and HMF were 9.65 mmol/L and 0.84 mg/L, respectively. The browning products did not change significantly in the range of 1 h to 5 h, indicating that temperature was the main influencing factor for the browning reaction. Humic acid, melanoidin, could co-precipitate with the protein, thereby reducing the purity of the protein recovery. After anaerobic digestion of the hydrolysate, the residual organic matter was mainly humic acid-like. Melanoidin has the same functional groups and fluorescent properties as humic acids, which, when combined with the results of three-dimensional fluorescence, shown that the humic acid in the supernatant after ATH of the sludge was not only converted in the adsorbed state, but also possibly generated by the reaction of the dissolved proteins with polysaccharides. This study further clarified the reaction of organics during the hydrothermal treatment of sludge and helped to improve the accuracy of subsequent studies