Date of Award
Master of Engineering Science
Chemical and Biochemical Engineering
Dr. Madhumita B. Ray
Dr. George Nakhla
Although anaerobic digestion (AD) is a very old and widely accepted sludge stabilization process, the traditional digestion process is very slow due to the rate limiting hydrolysis step. The quality of biogas is another major concern, as the presence of volatile sulfur compounds is detrimental to thermo-catalytic conversion equipment and may generate harmful emissions. This study investigated the impact of various pretreatment techniques on volatile sulfur compounds control leading to enhanced anaerobic digestion of municipal waste activated sludge.
Initially, the effect of combination of mechanical and chemical pretreatment of municipal waste activated sludge (WAS) prior to anaerobic digestion was studied using a laboratory-scale sludge pretreatment system with an objective to decrease volatile sulfur compounds in biogas and digested sludge. Mechanical pretreatment was conducted using depressurization of WAS through a valve from a batch pretreatment reactor pressurized at 75±1 psi, while combined pretreatments were conducted using six different dosages of hydrogen peroxide (H2O2) and ferrous chloride (FeC^) along with mechanical pretreatment. About 37%- 46% removal of H2S in biogas occurred for different combined pretreatment conditions. Sludge solubilization achieved due to the mechanical pretreatment increased total cumulative methane production by 8%-10% after 30 days during the biochemical methane potential (BMP) test. The pretreatment also decreased methyl mercaptan generation potential of the digested sludge.
In the next phase, combined sono-thermal pretreatment was conducted using three
different ultrasound specific energy inputs (1000, 5000, and 10,000 kJ kg'1TSS’1) and thermal
pretreatment temperatures (50, 70 and 90°C) to enhance the anaerobic digestion of waste
activated sludge. Prior to anaerobic digestion, sono-thermal pretreatments have significantly
improved VSS destruction by 29%-38%. A maximum of 30% increase in methane production was observed for 30 minutes of thermal pretreatment at 90°C followed by ultrasound pretreatment at 10,000 kJ kg'1 TSS'1 specific energy input. Sono-thermal pretreatments have
improved the dimethyl sulfide (DMS) removal efficiency from the biogas by 42%-72%, but did not show further improvement in hydrogen sulfide (H2S) removal compared to ultrasound and thermal pretreatment alone. Economic analysis has shown that sono-thermal pretreatment combining 1000 kJ kg^TSS'1specific energy input and thermal pretreatments at different temperatures (50-90°C) can reduce the operating costs by $44-66/ton dry solid compared to conventional anaerobic digestion without any pretreatment.
In the final phase, thermo-oxidative pretreatment of municipal waste activated sludge was conducted using thermal pretreatment at 60°C in presence of 0.6 mg H2O2+I.5 mg FeC^/mg S2" as oxidants with the objective of achieving sludge disintegration for enhancing anaerobic digestion as well as to remove volatile sulfur compounds generation potential in biogas in continuous flow anaerobic digestion. For the pretreated feed digester, the hydrogen sulfide (H2S) and dimethyl sulfide (DMS) concentrations in biogas significantly decreased by average 75%, and 40%, respectively, while methanethiol (MT) removal efficiency was statistically insignificant compared to the control digester. Compared to the control, overall TSS and VSS removal efficiency were 10% and 11% higher for the pretreated feed digester operated at 10 days solid residence time (SRT), and methane production rate (L CFE/Day) increased by -20%
Dhar, Bipro Ranjan, "PRETREATMENT OF MUNICIPAL WASTE ACTIVATED SLUDGE FOR ENHANCED ANAEROBIC DIGESTION AND VOLATILE SULFUR COMPOUNDS CONTROL" (2011). Digitized Theses. 3671.