Waste sludge is frequently treated by anaerobic digestion to kill pathogens, generate methane gas and reduce odors so the sludge can be safely land applied. In an attempt to reduce sludge volumes and improve sludge dewatering properties, the use of thermal hydrolysis (TH), a sludge pretreatment method, has been adopted by numerous wastewater treatment plants, among them being the District of Columbia Water and Sewage Authority (DC WASA). The use of anaerobic digestion in collaboration with thermal hydrolysis has been shown to increase VS removal, COD removal and biogas production. The sludge generated also dewaters to a higher cake solids than from conventional anaerobic digestion. Unfortunately, DC WASA has found that the use of thermal hydrolysis had brought about two major issues. These are: (a) does thermal hydrolysis increase destruction of fats, oils and greases compared to conventional digestion? and (b) is the mixing method used at Virginia Tech (recirculating gas mixing) capable of stripping ammonia from the digester? Therefore the main purpose of this study is to evaluate these issues which occur with the use of the thermal hydrolysis process. Experiments were conducted in two phases. The first phase was to assess the performance of anaerobic digesters via their biogas production with and without long chain fatty acid addition and with or without thermal hydrolysis. This research was further carried out in two stages. First a mixture of unsaturated long chain fatty acids (hydrolyzed and unhydrolyzed) was used. The fatty acid mixture included oleic, linoleic and linolenic acids, which contain one, two and three double bonds, respectively. In the second stage, the effect of a single unsaturated fatty acid (hydrolyzed and unhydrolyzed) was analyzed. If extra gas is generated, grease addition to the digesters will be implemented. If thermal hydrolysis produces more gas, the greases will be added through the thermal hydrolysis unit rather than being added directly to the digester. The results showed that addition of long chain fatty acids greatly increased gas production and the long chain fatty acids that were thermally hydrolyzed generated more gas than the untreated long chain fatty acids, although the gain was not large. The second phase of the study was carried out by alternating the type of recirculating gas mixing (partial and continuous) in the anaerobic bioreactor. To achieve this goal, short-term anaerobic bioreactor studies were conducted by varying the frequency of the gas. The result showed that continuous gas recirculation at the bottom of the digester was responsible for stripping ammonia from the system. It appeared that up to 500 mg/L of ammonia was being stripped from the digester operating at 20 day solids retention time. This suggests that ammonia can be stripped if a reduction of ammonia in the digester was desired. / Master of Science

LD2668 .T4 1966 D37 / Master of Science

published_or_final_version / abstract / Biological Sciences / Doctoral / Doctor of Philosophy

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The long-term kinetic rates of solubilization/utilization of proteins, carbohydrates, and lipids in secondary solids from three municipal wastewater treatment plants were measured. Temperature, input sludge retention time (SRT₀), and terminal electron acceptor were used as control variables for two four-month digestion studies employing seven mixed batch reactors. Three types of adenosine triphosphate (ATP) measurements were used to differentiate between viable endogenous biomass, viable anaerobic/facultative biomass and intact non-viable solids. Solids solubilization was significantly affected by all three control variables. Temperature and terminal electron acceptor showed greater effects on solubilization than SRT₀. The effect of temperature was time-dependent and differed for the two types of secondary solids evaluated. Proteins were removed to the greatest extent, lipids to an extent comparable to that of volatile suspended solids (VSS), and carbohydrates were the most resistant component. Direct sample ATP or VSS measurements were poor indicators of the time-dependent potential of mixed microbial population for further waste stabilization. However, the analysis of the ratios of recovery ATP to sample ATP (ATP recovery ratio) provided more insights into the microbial behavior during sludge digestion. Under aerobic conditions, ATP recovery ratios generally increased with time. This result implied that during the initial phase of digestion the volatile solids consisted of a higher fraction of intact non-viable material, but after prolonged digestion the solids included a greater fraction of viable endogenous biomass. Under anaerobic conditions, ATP recovery ratios increased and then decreased with time, implying that the fermentative microbial fraction was highest at some intermediate point in the time frame employed. Four phase degradation patterns were observed in all cases, indicating an inability for any single equation to effectively model the long-term solubilization process.

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