A new model with serial hydrolysis reactions for the anaerobic digestion of waste activated sludge under thermophilic conditions
Hydrolysis is one of the rate-limiting reactions in the anaerobic digestion (AD) of wastewater sludge; thus, reliable kinetic models for hydrolysis reactions are essential in numerical simulations of AD. In ADM1 (anaerobic digestion model no. 1), hydrolysis is defined by 3 parallel reactions, where proteins, polysaccharides, and lipids are deconstructed into their monomeric forms. Considering hydrolysis as a serial detachment of individual monomers from suspended solids (SSs), we have proposed a new model (hydrolysis digestion model, HDM) where a low volatile SS (X505) is converted into soluble organics (Sorg) in 3 sequential hydrolysis reactions via moderately volatile SS (X350) and highly volatile SS (X205). X205, X350 and X505 were quantified by burning sludge samples at 205 °C, 350 °C, and 505 °C, respectively. By comparing HDM simulations with the lab-scale anaerobic digester operation at 55 °C, the first-order kinetic constants for the serial hydrolysis reactions were estimated. The VSS fractionation provided a clear understanding of hydrolysis reactions and detailed mass flow with time in thermophilic AD of thickened waste activated sludge. We found that the yield coefficient of fermenters is higher than that previously reported at 55 °C. HDM simulation results were consistent with ADM1, allowing the reliable prediction of AD performance with readily measurable model variables (X205, X250, and X505), while proteins, polysaccharides, and lipids in ADM1 are difficult to quantify in conventional wastewater analysis. Among the 3 sequential hydrolysis reactions, the second reaction (X350 → X205 + Sorg) was found to be rate-limiting in the sensitivity analysis using HDM. The sensitivity study also found that 12 h is sufficient for the thermophilic pretreatment of thickened waste-activated sludge at 55 °C.