Acidogenic codigestion of fats, oils, and greases (FOG) was studied at 37"C using suspended sludge digesters operated as sequencing batch reactors (SBRs). Volatile fatty acid (VFA) production was found to increase with larger FOG loading rates, although this increase was insignificant compared the theoretical VFA production from FOG addition. Long chain fatty acids (LCFAs) were found to have accumulated in the reactor vessel in semi-solid balls that were primarily composed of saturated LCFAs. Adding high FOG loadings to an APD not acclimated to LCFAs allowed for a mass balance calculation and resulted in near complete saturation of unsaturated LCFAs and significant accumulation of LCFA material in the digester, which was found to be mostly 16:0, 18:0, and 18:1. While 18:2 and 18:3 LCFAs were nearly completely removed, 18:0 and 14:0 LCFAs were produced, most likely from the degradation of 18:2 and 18:3 LCFAs. The APD pH was found to have a significant impact on the amount of accumulated LCFA material present, with higher pH levels resulting in less accumulated material. Two-phase codigestion of FOG was also studied using an APD followed by gas-phase (GPD) digesters. The two-phase systems were compared by FOG addition to the APD versus GPD. FOG addition to the APD resulted in 88% destruction of LCFAs, whereas FOG addition to the GPD resulted in 95% destruction of LCFAs. Accumulated LCFAs in the APD receiving FOG were composed mostly of stearic acid (18:0). The low pH of the APD is likely the cause of LCFA accumulation due to saturation of unsaturated LCFAs. / Master of Science

Two different sludge pretreatments were investigated in an attempt to improve the management and performance of processes for the co-digestion of biosolids with fats, oils, and grease (FOG). The mechanisms of long chain fatty acids (LCFA) degradation in thermal hydrolysis pretreatment and the influence of pH on LCFA degradation in two-phase co-digestion systems were studied. LCFA thermal hydrolysis was investigated at different temperatures (90-250 °C) and reaction times (30 minutes and 8 hours). Approximately 1% of saturated fatty acids were degraded to shorter chain fatty acids at 140 and 160 °C (8-hr thermal hydrolysis). Only 1% or less of unsaturated fatty acids were degraded from 90 to 160 °C (8-hr thermal hydrolysis). Little degradation (< 1%) of both saturated and unsaturated LCFAs was observed at a 30-min reaction time. Both groups of LCFAs were stable up to 250 °C (30-min hydrolysis). The use of chemical-thermal treatments was also investigated. Only unsaturated LCFAs, C18:1 and C18:2, were degraded when thermally hydrolyzed with hydrogen peroxide coupled with activated carbon or copper sulfate. Semi-continuous, acid-phase digesters (APDs) under different pH conditions were studied in order to understand the effects of pH on FOG degradation. Increases in soluble chemical oxygen demand (SCOD) were observed in all APDs. However, the APDs with pH adjustment appeared to perform better than the controls in terms of solubilizing organic compounds. Approximately 38% and 29% of total COD (TCOD) was solubilized, and maximum volatile fatty acid (VFA) concentrations of 10,700 and 7,500 mg/L TCOD were achieved at pH 6 and 7, respectively; It is useful to note that the feed sludge had a VFA concentration of 2,700 mg/L COD. Higher pH (6.0-7.0) showed less accumulation of LCFA materials and more soluble LCFAs in the APDs. This indicates that the lower pH in the APDs was most likely the cause of precipitation and accumulation of LCFAs due to saturation of unsaturated LCFAs. / Master of Science

Sewage sludge is widely used as an important source for biogas production through digestion. Developing the high performance processes has a significant goal in order to promote energy efficiency and reduce the cost sewage sludge treatment. The problem of sewage sludge disposal is becoming top one which almost cost 50 % of running fee for a municipal wastewater treatment plant. This paper basically introduces three methods to improve the conventional digestion. However, they enhance the conventional digestion from different aspects. For examples, Two-phase anaerobic digestion enables to exhibit the merit of thermophilic anaerobic digestion and avoid the weak points of conventional digestion regarding odor problem. In two-phase anaerobic digestion, the acid and methane producing stages are separated. Extended solids retention time is an approach to separate the hydraulic retention time and solids retention time in an anaerobic digester by using recycle thickening. This method could benefit further de-composing the organics and increase methane formation. Dewaterability is the final step of anaerobic digestion process. Enhancing this part of process is an efficient way to increase the solid content of sludge that would reduce the transportation costs. In a nutshell, no matter on saving cost or energy perspectives, these three methods all promote biogas production efficiency up to a better performance, but various requirement of energy and cost are demanded. The paper displays and compares the advantages and disadvantages among three methods. There is no certain answer to which method is the best one; however, they can be chose to enhance digestion in different condition.