Degradation Kinetics of Common Odorants Emitted from WWTPs: A Methodological Approach for Estimating Half-Life Through Reactions with Hydroxyl Radicals

Abstract

In contemporary times, wastewater treatment plants (WWTPs) were recognized as substantial sources of odorous emissions, potentially impacting nearby communities’ sensory experience. This study investigates the half-lives (T½) of odorous compounds emitted from WWTPs and their degradation due to atmospheric hydroxyl radicals (•OH) in different environmental settings. The calculated half-lives of specific odorants in rural areas ranged from 31.36 min to 517.33 days, in urban areas from 42.50 min to 1550 days, and in the marine boundary layer from 42.50 min to 129,861 days. These results show that compounds with high reactivity and short T½, such as methanethiol and ethanethiol, degrade rapidly and are less likely to contribute to long-term odor nuisances. In contrast, compounds with longer half-lives, such as carbonyl sulfide and ammonia, persist longer in the atmosphere, with higher potential for sustained odor issues. The findings suggest that •OH plays a significant role in degrading odorous compounds. These insights into odorant–oxidant kinetics may aid in predicting atmospheric half-lives and their contribution to secondary aerosol formation, thus informing regulatory and mitigation strategies to improve air quality.

1. Introduction

The issue of unpleasant odors has become a pressing concern for both communities and industries, highlighting the intersection between environmental health and social wellbeing. Odor emissions from WWTPs and animal production operations represent a significant source of air quality complaints among residents living near these facilities, and this has driven increasing efforts to mitigate the impact of these atmospheric pollutants on surrounding areas [1]. The proximity to such odor-emitting sites can have profound effects on residents’ daily lives, leading to various negative outcomes, including annoyance, discomfort, and even measurable health impacts [2]. Odor nuisances can be particularly intrusive, triggering a range of physical and psychological reactions, from mild irritation to chronic stress, all of which contribute to a decreased quality of life and a diminished sense of well-being in affected communities [3].

The complexity of the odor issue is further underscored by the diverse chemical nature of emissions from WWTPs [4]. These facilities release a complex mixture of odorants into the atmosphere, originating from multiple chemical families [5]. Key odorant compounds include sulfur and nitrogen compounds, aldehydes, volatile fatty acids, alcohols, aromatic hydrocarbons, ketones, and biogenic compounds such as terpenes, indole, and skatole [6,7]. Each of these groups contributes distinctive odor characteristics, and although WWTPs emit a broad spectrum of odorants, only certain compounds are typically found in concentrations high enough to be considered malodorous [8]. For example, sulfur-containing compounds such as hydrogen sulfide and mercaptans are particularly known for their strong, unpleasant smells and low odor detection thresholds, which make them notable contributors to odor-related complaints [9]. Similarly, nitrogen-containing compounds, such as ammonia, also contribute significantly to the characteristic smells associated with wastewater facilities and have the potential to affect human health at higher concentrations.

Moreover, the impact of these odorants extends beyond just their concentration. Odor perception is influenced by a combination of factors, including the chemical nature of the compounds, their intensity, duration of exposure, and even individual sensitivity [10]. This variability makes odor control a complex task, as emissions can vary widely based on the specific processes in place, the type of waste being treated, seasonal and environmental conditions, and even the operational efficiency of odor management systems at the facility. Furthermore, exposure to certain odorants has been associated with respiratory symptoms, headaches, and stress-related health effects in individuals living near WWTPs and other odor-emitting facilities [11].

The presence of these odors is not only an issue of nuisance but also one of environmental justice, as lower-income communities often reside closer to industrial sites, bearing a disproportionate burden of exposure to these odorous emissions [12]. Addressing these issues requires a multifaceted approach that incorporates scientific understanding of odorant composition, concentration, and atmospheric behavior with robust regulatory policies and community engagement efforts. In response, there has been an increasing focus on both refining odor detection and quantification methods and developing advanced mitigation technologies to limit the dispersion and impact of these odorants on surrounding communities. Effective odor management strategies at WWTPs must therefore consider the diverse and complex chemical makeup of these emissions, as well as their varying environmental and health impacts. Through a combination of improved emissions monitoring, targeted odorant reduction strategies, and community-centered regulatory policies, it is possible to mitigate the adverse effects of WWTP odors, thus helping to preserve air quality and enhance the quality of life for nearby residents.

The atmosphere possesses oxidizing capabilities, with key atmospheric daytime oxidant •OH. Additionally, oxidation processes can be initiated by NO₃, O₃, and Cl or Br atoms, and organic peroxyl radicals (RO₂). The understanding of odorant molecules kinetics in the presence of atmospheric oxidants is crucial for elucidating their fate and impact on air quality.

•OH hold significant importance across various layers of the atmosphere [13]. Within the troposphere, it acts as the primary oxidizing agent for both natural and man-made organic compounds, contributing to ozone formation. In the stratosphere and mesosphere, •OH radicals serve as a crucial catalyst for the destruction of ozone [14]. •OH radicals are highly reactive, readily attacking a wide range of organic compounds. Their high oxidative nature stems from their oxidation potential [15]. Moreover, due to their non-selective characteristics, •OH can efficiently eliminate or break down various susceptible organic compounds (such as acids, alcohols, aldehydes, aromatics, amines, ethers, ketones, etc.) [16]. Because of their brief lifespan, •OH are generated in situ through either the oxidation of water or hydroxide ions.

The primary source of •OH radicals in the troposphere is the photolysis of O₃ to produce excited atomic oxygen (O(¹D)), which can lead to the formation of •OH radical by the reaction with water vapor:

O₃ + hv → O(¹D) + O₂

(R1)

O(¹D) + H₂O → 2•OH

(R2)

This study aims to fill an important knowledge gap regarding the reaction kinetics of odorants commonly emitted from wastewater treatment plants when exposed to atmospheric oxidants. The concentrations of these oxidants can vary significantly depending on the surrounding environment—whether marine, urban, or rural—which influences the behavior and persistence of odorant molecules in the air. Odorants with longer half-lives are more persistent in the atmosphere and, therefore, have a higher likelihood of contributing to odor nuisances over extended periods. In contrast, those that react rapidly with oxidants and have shorter atmospheric half-lives are generally considered less likely to cause prolonged odor issues. Figure 1 illustrates the transport of odorants emitted from WWTPs to different environments.

To address this, the study will focus on calculating the half-lives of various odorants under •OH oxidant concentrations, which will provide a more accurate understanding of the kinetics of these compounds in real-world environments. By examining how long specific odorants remain in the atmosphere in the presence of •OH oxidants, this research will clarify which compounds may be primary contributors to odor nuisances in different settings and under varying conditions.

Ultimately, this work aims to contribute to a deeper understanding of the environmental dynamics influencing odor persistence, particularly in areas surrounding WWTPs. The findings will be instrumental in identifying odorants with high potential for causing odor issues and may guide strategies to reduce odor impact in affected communities. This research, therefore, serves a dual purpose: enhancing scientific knowledge about odorant-oxidant interactions and providing actionable data to help mitigate odor nuisances effectively across diverse atmospheric environments.

2. Materials and Methods

Odorants commonly released from WWTPs include a range of compounds, from VOCs to sulfur-containing substances, which vary in their reaction rates with atmospheric oxidants, particularly hydroxyl (•OH) radicals. By assessing the reaction rate coefficients and mechanisms of these odorants with •OH radicals, it is possible to estimate their atmospheric half-lives. Using available data on reaction rate coefficients, along with typical oxidant and odorant concentrations from the literature, significant odorants can be identified based on their persistence in the atmosphere. This approach provides insight into which odorants are likely to linger and contribute more substantially to odor nuisances depending on their reactivity with atmospheric oxidants.

2.1. WWTP Description and Methodology for Sample Collection and Analysis

The detailed description of WWTP and the methodology of biosolids samples collection and an emission analysis is described in [9] from which the subjected odorants emission data in this study came from. Here, only a general description is presented. Biosolids samples were collected from mechanical biological WWTPs located in Sydney, New South Wales, Australia. Biosolids sampling was conducted at the dewatering stage. Each sample was evenly spread in a tray (0.6 m × 0.4 m × 0.2 m) to form a thin layer, stored in aerobic conditions at ambient temperatures ranging from 10 °C to 30 °C. Emission sampling was conducted at multiple intervals during storage, and trays were loosely sealed between events to allow moisture loss. Emission samples were collected using dynamic flux hoods designed according to U.S. EPA specifications and operated in line with AS/NZS 4323.4:2009 standards. Each hood, with a 0.03 m³ volume and 0.13 m² surface area, was purged with high-purity nitrogen gas at 5 L/min for 30 min prior to sampling to prevent interference from atmospheric oxidants. Emission samples were collected at a flow rate of 100 mL/min over 10 min using a calibrated SKC sampling pump onto Tenax TA sorbent tubes in triplicate, with each tube paired in series to monitor breakthrough. Sample analysis was performed using gas chromatography with mass selective detection (GC-MSD). Sorbent tubes were thermally desorbed, with the volatile organic compounds (VOCs) concentrated on a cold trap at −10 °C, and subsequently desorbed into a GC column with helium as the carrier gas. The GC oven was programmed to start at 50 °C, then ramp to 200 °C, and held to separate the compounds.

2.2. Odorants Selection

The identified odorants are listed in

Table 1. Selected odorants for kinetic determination.

Odorant	Reaction with •OH Radicals	k298/cm3 Molecule−1 s−1
Ammonia	•OH + NH3 → H2O + ·NH2	1.83 × 10−12
Trimethylamine	•OH + (CH3)3N → Products	5.73 × 10−11
Carbonyl sulfide	•OH + COS → Products	1.93 × 10−15
Methanethiol	•OH + CH3SH → Products	3.40 × 10−11
Carbonyl disulfide	•OH + CS2 → Products	1.91 × 10−12
Ethanethiol	•OH + C2H5SH → Products	4.61 × 10−11
Dimethyl sulfide	•OH + (CH3)2S → H2O + CH3SCH2	4.40 × 10−12
Dimethyl disulfide	•OH + (CH3S) 2 → Products	2.41 × 10−10
Hydrogen sulfide	•OH + H2S → H2O + SH	5.84 × 10−12
p-xylene	•OH + 1,4-Dimethylbenzene → Products	1.36 × 10−11
Toluene	•OH + Toluene → Products	5.70 × 10−12
1,3,5-trimethylbenzene	•OH + 1,3,5-Trimethylbenzene → Products	5.90 × 10−11
Butanol	•OH + CH3CH2CH2CH2OH → Products	8.47 × 10−12

. These odorants have undergone assessment of their reaction rate coefficients and mechanisms with the atmospheric oxidant (•OH radicals), ranging from VOCs to sulfur-containing compounds, based on data from the NIST Chemical Kinetics Database. By utilizing reaction rate coefficients (k) extracted from this database, along with information on oxidant and odorant concentrations gathered from the literature sources, it becomes possible to determine potentially significant odorants by calculating their half-lives. This process helps identify which odorants may persist in the atmosphere for longer periods based on their reactivity with atmospheric oxidants.

2.3. Hydroxyl Radicals Environment

In this research, we utilized continuous measurements of oxidants, building upon insights obtained from previous studies. To thoroughly evaluate oxidant levels, our data predominantly stemmed from monitoring efforts carried out at three distinct locations: marine environments, urban areas, and regions characterized by mountainous or rural landscapes. This methodology was designed to encompass a wide array of environmental contexts and enhance our understanding of how oxidant levels fluctuate for odorants across diverse settings.

Table 2. Levels of hydroxyl radicals across various environments.

Oxidant	Marine Site (Molecules/cm3)	Urban Site (Molecules/cm3)	Rural Site (Molecules/cm3)
•OH radical	9.60 × 105 [14]	2.65 × 106 [17]	8.00 × 106 [18]

provides detailed information regarding the concentrations of •OH radicals in different sites.

2.4. Odorants Degradation Determinations

This section utilizes the reaction data from Table 1 to develop models that examine the kinetics and reactions of •OH radicals with various odorants. Using the previously determined reaction rate coefficients at 298 K, the half-lives of these odorants are determined by applying the pseudo-first-order reaction Equation (1), where the concentration of the oxidant [•OH] is treated as constant. The pseudo-first-order reaction equation is given by the following:

$${\displaystyle \frac{\left[\mathrm{A}\right]}{\left[\mathrm{A}\mathrm{o}\right]}}={\mathrm{e}}^{-{\mathrm{k}}^{\prime }\mathrm{t}}$$

(1)

where

• [A] is the concentration of the odorant at time t
• [Ao] is the initial concentration of the odorant
• k′ is the pseudo-first-order rate constant
• t is the reaction time

3. Results and Discussion

In this section, we explore the degradation behavior of several odorants released from WWTPs upon exposure to atmospheric •OH, with a focus on how their persistence in the atmosphere varies across different environmental settings. We will discuss the findings on the calculated half-lives of these compounds in urban, marine, and rural areas, providing insights into their potential to contribute to sustained odor pollution.

3.1. Half-Life Calculations and Persistence of Odorants

As previously described, the half-life of a compound is the time it takes for half of its initial concentration to be degraded by reaction with •OH. The •OH is a highly reactive atmospheric species, playing a critical role in the degradation of volatile organic compounds (VOCs) and other odorants. The reaction rate coefficients (k values) for each odorant were obtained from the NIST Chemical Kinetics Database, and their half-lives (T½) were calculated based on the •OH concentrations in urban, marine, and rural environments.

Table 3. The half-lives of odorants in different environments.

Odorants	T½ Rural Site (min)	T½ Marine Site (min)	T½ Urban Site (min)	T½ Class
Ammonia	781.67	4.57 *	1.75 *	Long
Trimethylamine	25.30	210.00	75.79	Short
Carbonyl sulfide	517.36 *	129,861.00 *	1550.00 *	Long
Methanethiol	42.50	353.00	128.11	Short
Carbonyl disulfide	753.33	4.50 *	986.50	Intermediate
Ethanethiol	31.36	261.50	94.80	Short
Dimethyl sulfide	326.70	425.00	989.70	Intermediate
Dimethyl disulfide	18.10	50.00	65.75	Short
Hydrogen sulfide	248.40	1.50 *	750.00	Short
p-xylene	320.00	886.70	1.00 *	Intermediate
Toluene	253.33	1.50 *	766.67	Intermediate
1,3,5-trimethylbenzene	25.83	203.33	73.33	Short
Butanol	170.00	1.00 *	513.33	Long

* Values expressed in days for better readability.

summarizes the half-lives of odorants in the different environments studied. The calculated half-lives provide valuable insight into which compounds are more likely to persist in the atmosphere and contribute to sustained odor issues.

3.1.1. Short-Lived Odorants

Several odorants exhibited short half-lives, indicating that they degrade almost instantaneously upon release. These include trimethylamine, 1,3,5-trimethylbenzene, methanethiol, ethanethiol, dimethyl disulfide, and hydrogen sulfide. For example, trimethylamine had a half-life of 42.50 min in rural areas, 353.00 min in marine environments, and 128.11 min in urban sites, making it highly reactive and unlikely to contribute to long-term odor issues. Similarly, ethanethiol showed a half-life ranging from 31.36 to 261.50 min, underscoring its rapid degradation.

These compounds, while known for their strong and unpleasant odors, do not pose significant long-term air quality risks due to their relatively short atmospheric lifetimes. Their degradation is rapid, preventing them from causing sustained odor nuisances over large distances or periods. Figure 2 illustrates the degradation of methanethiol as an example, since all odorants classified as short-lived have relatively similar half-lives.

3.1.2. Intermediate-Persistence Odorants

A group of odorants displayed intermediate atmospheric persistence. These include carbonyl disulfide, dimethyl sulfide, and toluene, which exhibited half-lives ranging from 253.33 min to 4.50 days depending on the environmental setting. For example, p-xylene, a common aromatic compound found in WWTPs emissions, had half-lives of 886.70 min in marine environments and one day in rural areas. The relatively longer half-lives of these compounds suggest they are more likely to contribute to temporary odor nuisances, particularly in areas with continuous or frequent emissions.

Despite their intermediate lifetimes, these odorants degrade at a moderate rate, which may limit their contribution to long-term odor pollution. However, in urban or industrial areas where emissions are frequent and concentrated, these compounds could still contribute to localized and temporary odor issues. Figure 3 illustrates the degradation of toluene as an example, given that the four odorants classified within the intermediate-persistence category exhibit relatively similar half-lives.

3.1.3. Long-Persistence Odorants

Among the odorants examined, ammonia, carbonyl sulfide, and butanol displayed significantly longer half-lives. Ammonia exhibited half-lives of 781.67 min in rural areas, 4.57 days in marine environments, and 1.75 days in urban settings. Carbonyl sulfide showed an even more extended persistence, with half-lives ranging from 517.36 days in rural settings to 1550 days in urban sites. Similarly, butanol lasted 170.00 min in rural environments and one day in marine environments.

These extended half-lives suggest that these compounds may persist in the atmosphere for much longer periods, even in the presence of an atmospheric oxidant. However, it is likely that dispersion plays a more crucial role than oxidation in transporting these compounds over long distances, potentially contributing to persistent odor pollution far from their emission sources, although this remains uncertain.

The longer half-lives of these odorants underscore their potential to cause long-term odor issues, particularly in areas where emissions are not rapidly diluted by atmospheric processes. Therefore, carbonyl sulfide, ammonia, and butanol may be key contributors to persistent odor nuisances. Figure 4 illustrates the degradation of carbonyl sulfide as a representative example, since it has the highest half-life among the odorants in the long-persistence category.

3.2. Environmental Influence on Odorant Degradation

The results of this study highlight the important role environmental conditions play in the degradation of odorants. As observed, the •OH concentrations varied across the three studied environments—urban, marine, and rural—leading to differences in odorant half-lives. Urban areas tend to have higher concentrations of •OH radicals due to pollution-driven photochemical reactions, which can accelerate the degradation of odorants. In contrast, rural and marine environments, where •OH levels are relatively lower, tend to have slightly longer half-lives for many of the compounds analyzed.

For example, trimethylamine and ethanethiol exhibit relatively short half-lives in all environments, ranging from 25.30 min to 353.00 min. These compounds degrade so rapidly that environmental differences have little effect on their persistence. However, other compounds, such as carbonyl sulfide, butanol, and p-xylene, show more significant variations in half-lives across environments.

In marine environments, which generally have higher humidity and more active chemical processes as noted in [19,20], the degradation of odorants like carbonyl sulfide occurs more slowly, with a half-life of 129,861 days, compared to 517.36 days in rural environments. This difference can be attributed to the complex interplay of factors in marine atmospheres. While the higher humidity can often facilitate the breakdown of volatile compounds through reactions like hydrolysis, other factors such as salt aerosols, chemical composition, and marine-specific reactions may contribute to a slower overall degradation of carbonyl sulfide in marine environments, despite the increased moisture.

4. Conclusions

In conclusion, this study provides a comprehensive evaluation of the atmospheric degradation behavior of odorants emitted from WWTPs, focusing on their reaction with •OH across different environmental conditions. The calculated half-lives of these odorants in urban, marine, and rural settings reveal significant variations in their persistence, allowing us to classify them into short-lived, intermediate-persistence, and long-persistence compounds. Understanding these degradation patterns is crucial for assessing their potential contribution to sustained odor pollution and for developing effective odor management strategies.

Short-lived odorants, including trimethylamine, methanethiol, ethanethiol, dimethyl disulfide, and hydrogen sulfide, exhibited rapid degradation, with half-lives ranging from 31.36 to 353.00 min. These highly reactive compounds are unlikely to cause persistent odor issues as they degrade quickly in the atmosphere. Intermediate-persistence odorants, such as carbonyl disulfide, dimethyl sulfide, and toluene, demonstrated moderate atmospheric lifetimes, with half-lives spanning from 253.33 min to 4.50 days. While these compounds do not persist indefinitely, they can contribute to temporary odor nuisances, particularly in urban and industrial areas where emissions are continuous or frequently occurring.

In contrast, long-persistence odorants, including ammonia, carbonyl sulfide, and butanol, exhibited significantly longer half-lives, in some cases extending from days to even years. For example, carbonyl sulfide had a half-life of 517.36 days in rural areas and up to 129,861 days in marine environments. The extended persistence of these compounds suggests their potential to travel long distances and contribute to prolonged odor pollution far from their emission sources. Unlike short-lived odorants, which degrade slightly quickly upon release, long-lived odorants require targeted mitigation strategies due to their potential for long-term environmental impact. The results further highlight the role of environmental conditions in determining odorant degradation rates. The degradation of some compounds, such as carbonyl sulfide, is particularly slow in marine environments.

Overall, this study enhances our understanding of the atmospheric fate of odorants from WWTP emissions. The findings underscore the need for effective odor management strategies, particularly for persistent odorants that remain in the atmosphere for extended periods. While short-lived compounds do not pose significant long-term risks, intermediate- and long-persistent odorants may require additional treatment measures to minimize their impact. Future research should explore the influence of other atmospheric oxidants and environmental conditions on odorant degradation to further refine predictive models and mitigation approaches.