Doctor of Philosophy
Chemical and Biochemical Engineering
Dr. George Nakhla and Dr. Madhumita B Ray
Batch experiments were performed to investigate the effect of particulate protein particle size on the hydrolysis of casein in anaerobic degradation. While particle size did not affect the ultimate protein degradation efficiency, the hydrolysis rate coefficient increased from 0.034 to 0.298 d-1 with the change in specific surface area from 0.01 to 0.192 m2/g. The maximum methane production rate was affected by the particle size change, although the ultimate amount of methane produced was approximately the same despite the change in specific surface area. A mathematical relationship between the hydrolysis rate coefficient and specific surface area was developed and a new hydrolysis equation was proposed and verified. Ultrasound treatment of wastewater sludges prior to anaerobic digestion disrupts the flocs and causes lysis of the bacterial cells releasing both inter and intracellular materials. Primary (PS) and waste activated sludge (WAS) were treated with different ultrasonic intensities, varying sonication time and amplitude at a constant frequency. Results showed that gas production, volatile fatty acids, ratio of soluble chemical oxygen demand to total chemical oxygen demand and soluble protein increased, while particulate protein and particle size of the sludge decreased with sonication time. An empirical model was developed to determine the economic viability of ultrasound based on electrical energy input and energy obtained from enhanced methane production. Ultrasonic pretreatment is only economically viable for primary sludge at low sonication doses. The Anaerobic Digestion Model # 1 (ADM1) was applied to the batch anaerobic digestion for sonicated and non-sonicated sludge. The model successfully simulated the experimental trends. The efficiency of ultrasound as a pretreatment method for hog manure prior to anaerobic digestion was also evaluated at specific energies of 250 to 30,000 kJ/kg total solids (TS). This study confirmed that CODsolubilisation from particulates correlated well with the more labor and time intensive degree of disintegration test. The particle size distribution for hog manure was bimodal (0.6 - 2500 mu m), while ultrasound primarily impacting particles in the 0.6-60 mu m range. Hog manure is more amenable to ultrasound than waste activated sludge, as it took only 3000 kJ/kgTS to cause 15% more solubilization as compared to 25000 kJ/kg TS for waste activated sludge. Bound protein degradation during sonication was 13.5% at 5000 kJ/kg TS and remained constant thereafter for higher energy input. Biomass cell rupture occurred at specific energy of 500 kJ/kg TS. An economic evaluation indicated that only a specific energy of 500 kJ/kg TS was economical, with a net energy output valued at $ 4.1/ton of dry solids, due to a 28% increase in methane production. Degradation of odorous compounds in sludge during anaerobic digestion was systematically studied and simulated using the Anaerobic Digestion Model # 1 (ADM1). The degradation of various protein fractions (particulate, soluble and bound), VFAs, lipids and amino acids of PS and WAS were monitored during anaerobic digestion. Degradation kinetics of the odorous compounds namely, protein, amino acids, lipid and volatile fatty acids (VFAs) were determined. Relationships between protein fractions and volatile suspended solid were established. A strong relationship between bound protein, a major odors precursor, and volatile suspended solid degradation was found, while no statistically significant difference in bound protein reduction was observed between PS and WAS. ADM1 successfully simulated the lab scale continuous anaerobic digestion; model results with optimized parameters showed good agreement with the experimental data for methane production and all other sludge parameters including odor precursors such as lipids, VFAs and proteins.
Aldin, Saad, "The Effect of Particle Size on Hydrolysis and Modeling of Anaerobic Digestion" (2010). Electronic Thesis and Dissertation Repository. 60.