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Topic

Fate and distribution of atmospheric volatile organic compounds from wastewater treatment facilities

Department of Chemistry

Date & location

• Wednesday, April 9, 2025
• 11:00 A.M.
• Virtual Defence

Examining Committee

Supervisory Committee

• Dr. Erik Krogh, Department of Chemistry, University of Victoria (Supervisor)
• Dr. Christopher Gill, Department of Chemistry, UVic (Member)
• Dr. Laura Minet, Department of Civil Engineering, UVic (Outside Member)

External Examiner

• Dr. Naomi Zimmerman, Department of Mechanical Engineering, University of British Columbia

Chair of Oral Examination

• Dr. Caetano Dorea, Department of Civil Engineering, UVic

Abstract

Malodourous volatile organic compounds (VOCs) are found throughout the environment, from biogenic, geochemical and anthropogenic sources. A concerning anthropogenic source of malodours are wastewater treatment plants (WWTPs), where elevated levels of VOCs including several organosulfur compounds with low odour detection thresholds are produced and emitted. The ability to measure this class of malodourous compounds specifically, methanethiol (CH₃SH), dimethyl sulfide (CH₃SCH₃), and dimethyl disulfide (CH₃SSCH₃) by mobile mass spectrometry has enhanced our understanding of their fate and distribution in the environment surrounding wastewater treatment facilities. Organosulfur compounds can impact the environment including human health either through toxic effects, malodours, and/or as a source of sulfur dioxide (SO₂) thus lowering air quality.

This thesis summarizes the use of direct mass spectrometry in a purpose-built research vehicle to investigate odour control systems at three wastewater facilities on Vancouver Island both on-site in foul air collection ductwork and on-road in ambient air in the neighbouring community. The three WWTPs varied in age, size, location, and odour control technology. VOCs were measured using proton-transfer time-of-flight mass spectrometry (PTR-TOF-MS) with concentrations of methanethiol (m/z 49.01), dimethyl sulfide (m/z 63.02), and dimethyl disulfide (m/z 94.99) assessed in real-time. Other VOCs that are known to contribute to the odour profile include oxygenated hydrocarbons (e.g., acetaldehyde, acetic acid, butanal) as well as monoterpenes were also monitored. Supplemental measurements taken with sorbent tubes on-site were evaluated with lab-based thermal desorption-gas chromatography-mass spectrometry, to assess the identity of compounds such as dimethyl sulfide and ethanethiol.

The distribution of atmospheric gases emitted from these facilities varied depending on the treatment methods employed as well as on the location, topography, and meteorology. On-site measurements were aimed at assessing the efficiency of odour control technologies and the on-road measurements provided insight into the spatiotemporal distributions of malodourous VOCs. Concentrations of reduced sulfur compounds in the collected foul air ranged from 10² – 10⁴ parts per billion by volume (ppb_v). The odour control systems included physical adsorption by activated carbon and biological treatments using biofilters and bioreactor systems. A chemical scrubber and a pilot scale UV advanced oxidation treatment process were also evaluated during this study. Efficiencies of odour control techniques are described, with methanethiol removal efficiency consistently being the greatest, with typical removal of 70-90% by odour control technologies. Using biological and physical treatment dimethyl sulfide and dimethyl disulfide were found to be modestly removed, with removal efficiency of dimethyl sulfide at <40% and dimethyl disulfide removal efficiencies being low (<20%) to negligible. In some cases, we observe increases in dimethyl disulfide concentrations.

On-road measurements in the local communities around the WWTPs were also investigated for potential impacts from WWTPs. This includes mapping VOC concentrations over time and space, tracking plumes, and determining whether other sources of odours exist in the sample area. Typical ambient concentrations recorded at and within 2 km of WWTPs in the 0.2 -5 ppb_v range for reduced sulfur compounds and 2 – 10 ppm_v for methane. On-road concentrations for benzene, toluene, ethylbenzene, and xylenes (BTEX) associated with vehicle emissions were typically observed in the 0.5 – 5 ppbv range. Results from the drive portion of the campaign allowed for visualization of VOC distributions and produced neighbourhood scale information, including geospatial averages mapped at 50x50m, with detection of odourous VOCs within 1 km of WWTPs above odour thresholds under some conditions. Sources other than the WWTPs were also identified including pump stations, conveyance structures, and estuarine marine locations. This work illustrates the application of mobile real-time measurements to better understand the fate and distribution of VOCs in the community as well as characterize the effectiveness of mitigation strategies for malodourous compounds.