Cellulose mineralization in two-stage anaerobic digestion systems
The objective of this research was to evaluate the anaerobic degradation of a simulated high-strength solid waste/sludge, particulate cellulose, by two-stage anaerobic digestion systems. Cellulose hydrolysis and methanogesis were examined in both the thermo-meso and acid-methane two-stage systems under different hydraulic retention times (HRTs), feed concentrations and organic loading rates (OLRs). The emphasis of this research was to characterize the similarities and differences between these two processes;The acid-methane system consisted of two consecutive reactors, an acidogenic reactor and a methanogenic reactor with both operated at mesophilic temperatures (35°C). The thermo-meso process consisted a thermophilic reactor (55°C) followed by a mesophilic unit. All the reactors used were completely-stirred tank reactors;This research demonstrated that both thermo-meso process and acid-methane process had considerable potentials for the anaerobic degradation of cellulose with concentrations up to 60 g COD/L at system HRT of 13 to 30 days;Methane production and methane yield of the thermo-meso system were in the range of 148--1,100 mL/L-reactor·d and 0.23--0.33 L/g COD added when the OLRs were 466 to 4,000 mg/L/d. Thermophilic reactor dominated the solids destruction and methane production in the thermo-meso system. An OLR of 18.3 gCOD/L-reactor·d in conjunction with 3-day HRT was a threshold loading limit for this reactor;Under the similar OLRs, methane production rates of the acid-methane system were 71--776 mL/L·d and the methane yields were 0.16--0.27 L/g-COD. The first-stage acidogenic reactor demonstrated an average soluble organics yield of 0.352 g soluble COD/g cellulose COD added;Results from the continues runs showed that the thermophilic reactor possessed 2--4 times higher methane production rate than the methanogenic reactor. The first-order hydrolysis rate of the thermophilic reactor was 0.79 +/- 0.22 day-1 which was higher than that of the acidogenic reactor (0.26 +/- 0.2day-1);Based on the system methane generation model obtained from operating the two-stage systems, thermo-meso system demonstrated higher methane productions if treating high solid wastes with a shorter hydraulic retention time needed when compared with the acid-methane two-stage system.