UCLA

The sludge drying process, a crucial step in treating sludge from wastewater treatment plants, significantly reduces the sludge volume. This reduction makes it more manageable for transportation, storage, and utilization as fertilizer or soil amendment. However, despite its importance, the sludge drying process is marred by the emission of odors, which not only lead to complaints from neighboring communities but also impede the use of dried sludge as fertilizer.Given the challenges posed by the odors emitted during the sludge drying process, it becomes imperative to evaluate these odors comprehensively. The current standard method of evaluation, which focuses solely on olfactometry, falls short as it does not provide a complete understanding of the character and intensity of odors. Therefore, this dissertation delves into the use of sensory and chemical analysis to gain a more nuanced understanding of the odors emitted during various sludge drying processes and at different temperatures. The study employed the Odor Profile Method (OPM) as the sensory method, which involved a panel of at least four individuals describing the character and intensity of the odor. The chemical analysis was conducted using a Gas Chromatography-Mass Spectrometer (GC-MS), which identified the chemicals responsible for the odorants. This combined approach provided a comprehensive pattern of odor evolution during the sludge drying process, similar to the one used to evaluate drinking water odor. A pilot study at a WWTP in Limay, France, laid the foundation for developing the sludge drying odor wheel using a pilot laboratory roto evaporator system that emulated the sludge drying process at a drying facility. After 14 years, a follow-up study was done using sludge samples from Orange County Sanitation Districts (OCSD) in Southern California, USA. Following similar pilot laboratory procedures, the study confirmed they had similar odors but were not duplicable compared with the pilot study. The primary odors occurring in both studies were fecal, rotten vegetable, and rancid. OPM data was proven by finding the chemical concentration/OTC ratio. A concentration/OTC ratio of over 1 demonstrates that the panelists detected the odor during the study. The concentration/OTC ratio in the pilot study corresponded with the OPM data. Furthermore, with the occurrence of fecal odor in both the pilot study and the follow-up study, chromatographs indicated that indole was responsible for the fecal odor. To better understand the process of odor evolution during sludge drying, this dissertation explored a new approach by creating artificially undigested and digested sludge recipes to observe if the same sludge drying odors could be replicated and to understand how the sludge composition affects odor during the sludge drying process. The primary odors detected were consistent with the previous study, including the burnt odor. The simulated sludge study indicated that fecal and musty/earthy odors were also significant. A caramel odor was detected as a new odor within the burnt odor category. The sludge composition contributing to each odor was more easily studied by simulating a sludge-digesting recipe. The different recipes of simulated sludge contained specific ratios of carbohydrates, proteins, and lipids. Raw sludge was added to a recipe to stimulate anaerobic digestion. However, the odors produced from the recipe with raw sludge were much closer to the actual odors from the sludge drying process than those without adding digested sludge. The observed difference in odor suggests odor variation based on varying sludge composition and microbial activity within the sludge. Sludge drying at different temperatures can also affect the odors emitted during the process. To understand how odors change at different temperatures, thermal hydrolysis and indirect drying were carried out. The odor produced from both processes was analyzed using the Odor Profile Method (OPM) and the sludge drying odor wheel. The musty odor was the primary odor in thermal hydrolysis, while the fecal odor was the primary odor in sludge drying. Results revealed that thermal hydrolysis is less of an odor nuisance than sludge drying. The results from the studies of this dissertation revealed a new odor category (caramel odor) that needs to be further investigated to be added to the sludge drying odor wheel. Also, the production of the caramel odor needs more exploration, as caramel odor can be the ideal odor for the sludge drying process. Additionally, the thermal hydrolysis process for sludge drying suggests a more controlled process for odor nuisance in sludge drying.