Study on Dissolution Characteristics of Excess Sludge by Low-Temperature Thermal Hydrolysis and Acid Production by Fermentation

To investigate the dissolution characteristics of low-temperature thermal pretreatment conditions and the process of sludge fermentation to produce acid, the influence of thermal pretreatment temperature on the dissolution of excess sludge organic composition and the mechanism of cell crushing of sludge thermal pretreatment were analyzed by an experimental method, and the performance of acid production was explored by sludge fermentation after pretreatment at different temperatures. The performance of acid production by sludge fermentation after pretreatment at different temperatures was measured. The results proved that the soluble chemical oxygen demand (SCOD) shows the largest increase in dissolution rate (11.92%) at 70 °C and in dissolution quantity (6518.33 mg/L) at 90 °C. However, at 80 °C, the solubility of total organic carbon (TOC) is the highest (3224.47 mg/L), and at 70 °C, the best dissolution conditions for soluble carbohydrate (SC) and soluble protein (SP) reached 340.07 and 80.92 mg/L, respectively. The degree of sludge breaking starts to increase at 70 °C. Correlation analysis shows that dissolved organic matter is mainly derived from the cell wall and intracellular material and SP is mainly derived from intracellular material. Excitation-emission matrix spectra and parallel factor analysis (EEM-PARAFAC) divides the sludge dissolved organic matter (DOM) into five fluorescent components, including C1 (318/366) tyrosine, C2 (418/470) UVA humic acid, C3 (282/334) tryptophan substances, C4 (322/430) UVC humic acids, and C5 (314, 382, 454/526) UVA humic substances. Fermentation acid production experiment shows that the peak concentration is highest at 80 °C, the arrival time is 2 days, and the acid production type is butyric acid fermentation. Thus, it is proved that low-temperature thermal pretreatment promotes the process of acid-producing fermentation and has no effect on the type of fermentation. The optimal condition for hydrolytic dissolution and acid production under low-temperature thermal pretreatment is 80 °C.