Odor nuisance has been a challenge to management of wastewater treatment plants (WWTPs) and endanger the relationship between these facilities and neighbors. A systematic methodology was developed to investigate and understand odor nuisances from WWTPs, and provide practical solutions to solve these problems. The study of this methodology includes the incorporation of both chemical and sensorial methods, optimization of sampling techniques, development of analytical methods for specific odorants, and understanding the masking effect of odorous mixtures.

Both sensorial and chemical methods should be applied to investigate odor nuisance from wastewater facilities. Using chemical analysis alone is not able to clarify the problem because there is a gap between the method reporting limit (MRL) and odor threshold concentration (OTC) of odorants. Sensorial methods, such as odor profile method (OPM) and Detection/Threshold (D/T) method should be used to bridge the gap. The OPM can determine the major odors presented and their intensities, and then narrow the major potential odorants to a manageable group. D/T determines the total odor and if there is an odor nuisance problem, while the OPM can define the odors and their intensities.

The losses of volatiles and odorants in sampling bags have been reported in literature. Thus, the proper choice of sampling bags is needed for both sensorial and chemical analysis. The stability of wastewater odorants samples in sampling bags of Tedlar (polyvinyl fluoride film) and Teflon FEP (fluorinated ethylene propylene film) was evaluated and compared. Quick losses of indole and skatole were found in Tedlar bag, with less than 5% left after 15 minutes due to adsorption on the bag wall, while skatole and indole showed over 75% recovery over 6 hours in Teflon bags. Thus, Teflon bags are required for the analysis of skatole and indole, which needs to occur within 6 hours of sample collection. Teflon bags also are preferred for both chemical and sensory analysis due their lower background of chemical and odorant contamination.

Sensory- gas chromatography (Sensory GC) and GC-mass spectroscopy (GC/MS) with the OPM can identify primary odor causing chemicals that are causing the odor nuisance. GC-MS was applied to investigate the chemical sources of fecal and musty odorants identified by the OPM. Skatole and indole were found to be the primary chemicals leading to fecal odor, due to its odor concentration/ OTC (C/OTC) ratio that ranged from 2.8 to 22.5. 2-Methylisoborneol (MIB) and 2-isopropyl-3-methoxypyrazine (IPMP), that have been reported to cause musty odor in drinking water supplies, were identified as principal contributors to the musty nuisance odor at the WWTPs odor sources.

The present ability of WWTP odor control treatment of these fecal odorants by different air pollution control methods was evaluated at different locations at two WWTPs by the OPM and indole and skatole chemical analysis. Chemical scrubbing and biofiltration performed best in removing fecal odors among current control technologies.

The C/OTC ratio is a simple method to prioritize the odorants and employ both chemical and sensorial results to help eliminate them. The OTC of the nine “most detectable” odor characteristics by the OPM and D/T method and the odorants associated with these odors by the GC-MS and GC-Sensory Analyses at two WWTPs were determined based on Weber-Fechner curve. The Weber-Fechner Curve relates the Log of the Odor Intensity of each chemical causing the odor versus the Log of the Concentration of each odorant. The OTC of 2-isopropyl-3-methoxypyrazine (0.02 ng/L in air), methyl mercaptan (0.2 ng/L in air), 2-methylisoborneol (0.1 ng/L in air), and skatole (0.3 ng/L in air) were quite low. As dilution occurred, the intensities of both the fecal and the sulfur odors decreased. At the greater dilutions, musty odors appeared and the fecal and sulfur odors became undetectable. This is a masking of the odor, which is called “peeling the onion” effect. This masking effect was successfully evaluated in this study.

Thus, this thesis for the first time identified and quantified the fecal odorants (indole and skatole), the musty odorants ( IPMP and MIB) that cause odor nuisances from WWTPs, Analytical methodologies using the correct sampling bags were defined for the OPM and chemical analysis of these odorants. Also this thesis developed knowledge of the odor masking effect by odorous mixtures that are not sensorially observed at the WWTP, but could be observed off-site as an odor nuisance.