Fate and emission of methyl mercaptan in a full-scale MBBR process by TOXCHEM simulation

The biological decomposition of organic materials in sewage treatment plants produces noxious odors due to anaerobic breakdown (Asadi & McPhedran 2021). The odors of a wastewater treatment plant (WWTP) may be the result of sewerage odorants emitted into the air during the plant's operations (Gao et al. 2021). Biodegradability in wastewater can also lead to the formation of odorants (Wiśniewska et al. 2021). Chemical substances such as hydrogen sulfide, methane, and ammonia, as well as organic chemicals such as mercaptans, amine, indole, and skatole are produced during anaerobic processes (Andraskar et al. 2021). Sulfurous chemicals, such as hydrogen sulfide and mercaptans, which include methyl, ethyl, propyl, and butyl mercaptans all have a generally disagreeable odor and are common in WWTPs (Grzelka et al. 2019).

Volatile sulfur compounds (VSCs) are potentially the most significant of the odorous compounds emitted by WWTPs as they can create health-related issues such as discomfort and a non-carcinogenic danger. Hydrogen sulfide (H₂S), methyl mercaptan (MT), dimethyl sulfide (DMS), carbon disulfide (CS₂), and dimethyl disulfide (DMDS) are the most common VSCs emitted from WWTPs. Detecting VSCs emitted from WWTP units in the ambient air has been the topic of many earlier investigations examining VSC discharge. For example, a review article considered 24 studies that had examined VSC concentrations in WWTPs’ ambient air and found a wide range for H₂S (0.1–20,480 μg/m³), MT (0.4–2.4 μg/m³), DMS (0.4–5,450 μg/m³), CS₂ (3.06–9.82 μg/m³), and DMDS (0.62–1,600 μg/m³). Direct monitoring at the water–air interface is typically used to determine the concentrations of VSCs emitted from WWTPs. VSC emission fluxes from a main and secondary clarifier, for example, were assessed over an 8-day period using water–air interface sampling and included H₂S (0.004–1.404), MT (0.004–0.684), DMS (0.086–0.612), CS₂ (0.005–0.012), and DMDS (0.002–2.988) (Li et al. 2021).

The organic molecule methanethiol (also known as methyl mercaptan or thiolmethane) has the formula CH₃SH or CH₄S (Kleinhenz 2003). Methyl mercaptan is a colorless gas with a distinct dry smell (Zhou et al. 2018), and is a naturally occurring substance found in the blood and brains of humans and animals, as well as plant tissues. Animals remove it through their feces. Some foods, such as nuts and cheese, naturally contain CH₄S. It is also one of the key chemicals responsible for bad breath and the smell of flatulence.

Methanethiol, abbreviated as MeSH, is a thiol that is extremely flammable. Food and feed manufacturers use methanethiol to make methionine. Methanethiol is also employed as a free-radical polymerase in the plastics sector, and is also used in the pesticide industry (Vision & Mission 2013). H₂S odor detection thresholds have fluctuated from 0.491 to 0.97 μg/m³ over the previous 20 years. Different smell thresholds exist for mercaptans, chemical substances found in wastewater treatment operations (ranging from 0.01 μg/m³ for butyl mercaptan to 2.16 μg/m³ for CH₃SH) (Grzelka et al. 2019). When methyl mercaptan combines with water, steam, or acids, it creates flammable vapors that are dangerous on inhalation. CH₄S has the potential to cause respiratory paralysis, which can easily result in death. It is irritating to the eyes and respiratory tract, with pneumoedema, hepatic, and renal impairment all being symptoms of exposure.

An OSHA PEL (permissible exposure limit) of 10 ppm (20 mg/m³) is the standard and guideline for methyl mercaptan, with a NIOSH REL (recommended exposure limit) of 0.5 ppm and NIOSH IDLH (immediately dangerous to life or health) of 150 ppm The AIHA ERPG-2 (where many people might be exposed for up to 1 h without experiencing or developing permanent or other serious health problems or symptoms that could hinder their ability to take protective action) is 25 ppm (Nile 2018; Michaels & Wagner 2020). Methyl mercaptan is a relatively volatile organic compound (Weihua et al. 2021). The volatile organic compounds produced in WWTPs are affected by a number of factors, the most significant of which are stripping, decomposition, and sorption by mixed liquid suspended solids (MLSS) or biofilm (Zwain et al. 2020a, 2021). The organic and mineral materials represented by the MLSS with the reactor can adsorb and absorb organic materials and heavy elements. Bhatti et al. (2020) studied the treatment of arsenic from water by absorbing it with polyacrylonitrile fiber (organic) and iron ore. There are several models used to simulate the emission of volatile organic compounds in WWTPs (Buaisha et al. 2020) and TOXCHEM is considered one of the most important tools for determining the fate and emission of volatile organic compounds in sewage treatment plants (Zwain et al. 2020b, 2021). The majority of previous studies have focused on eliminating the methyl compound on emission from the treatment plant facilities and thus treating it in the gaseous state, but there are no studies to date about the fate of this compound within the processes of the treatment plant, i.e., while it is still within the wastewater.

TOXCHEM was developed in the early 1990s to replace the EPA's Water8 (Water9) software, which had certain limitations such as lack of enhanced mass transfer methods, sorption of pollutants to solids, and a peer-reviewed compound database of physical, chemical, and biological processes. TOXCHEM is a popular tool for calculating volatile organic carbon (VOC) emissions from sewage intake, storage (preliminary, primary, and secondary), and sludge treatment. Site-specific drainage parameters, pollutant qualities, and process design and operation statistics are used to estimate VOC contamination concentrations. TOXCHEM research centers on mass transfer equations and balances, such as stripping and volatilization reduction methods, and biodegradation and sorption processes. Not all of these processes would be active for all compounds. It would be necessary to use sorption (and/or precipitation, which might be confused with sorption) to remove heavy metals. Removal of VOCs is mostly accomplished utilizing biodegradation and volatilization, with sorption playing only a small part. Some hydrophobic organic molecules can be effectively removed by all three techniques. Photolysis and hydrolysis are difficult to distinguish from biodegradation. Under steady-state or dynamic settings, it may be used to forecast the fate of any chemical species synthesized in WWTPs (Masoomi et al. 2021).

The lack of a slope in the sewage network for the residential complex in Al-Hur favors anaerobic conditions and, combined with the high temperatures found in Iraq, this promotes the emission of sulfur compounds. In this study, the fate and emission of methyl mercaptan and the factors affecting its emission will be determined via the TOXCHEM 4.1 model in Al-Hur's moving bed bioreactor (MBBR) treatment plant. The novelty of this study lies in the determination of the plant's strengths and limitations, as well as the identification of areas of concern for methyl mercaptan emission. This risk can be addressed without incurring additional expenditures, but the biological processes can only be regulated by recognizing the system's important processes, as outlined in the preceding section. This is the first time that methyl mercaptan, a sulfur compound with a disagreeable odor, has been evaluated and investigated in the MBBR system and by the TOXCHEM model. The investigation provides precise information on the location of the irritating gas emissions, allowing appropriate precautions and safety procedures for workers to be put in place.

The residential complex treatment plant (RCTP-MBBR) is located in the city of Al-Hur, about 10 km away from the center of the Karbala governorate and about 110 km from the Iraqi capital, Baghdad, located at coordinates of 32.645454 °N and 43.973323 °E (Figure 1). The plant technique is an MBBR process, as shown in Figure 2. Designed on the basis of a flow rate of 800 m³/d and a service of 4,000 capita, the station contains the following four stages: the first level is preliminary treatment, which includes sand removal basins and a balance basin; the second level includes aeration basins; the third is the tertiary treatment, which includes disinfection (by calcium hypochlorite) and filters; and the fourth level includes sludge treatment by filter press and operations according to the data shown in Table 1. The study methodology was conducted according to the procedure shown in Figure 3.

To assess the performance of the RCTP, laboratory samples were collected and analyzed for a period of 1 year from 1/7/2020 to 1/7/2021, as shown in Table 2. All the tests shown in Schedule 1 were carried out in accordance with standard methods for the examination of water and wastewater (Mohsen et al. 2020). These results were compared with the Iraqi Law on Wastewater Determinants treated with the exception of methyl mercaptan, which will be compared to the invasive condition with the determinants in the Acute Exposure Guideline Levels for Selected Airborne Chemicals: Volume 15, National Research Council, shown in Table 3.

Sensitivity analysis is one of several important methods used to understand the impact of various operational parameters on the fate and emission of CH₄S. Using the key factors that influence the treatment process used in MBBR systems, sensitivity analysis was used to understand the fate and emission of CH₄S in this study, which includes the thickness of the biofilm layer, surface area for media, media fill fraction, and aeration flowrate, different thicknesses of the biofilm layer (0.5–7 mm), surface area for media (250–1,250 m²/m³), media fill fraction (30–70%), and aeration flowrate (225–900 m³/h) were considered.

The performance of RCTP-MBBR is shown in Table 2 and Figure 4, which operates very well compared to the Iraqi standard, as it achieved a removal efficiency of 93, 97, 60, 86, 97, 97, and 96% for COD and BOD5, PO₄, oil and grease, NH₄, total suspended solids (TSS), and 97% for methyl mercaptan, respectively, while the plant generated nitrate and sulfate at 95 and 12.5%, respectively. The plant is divided into two parts, the first for the treatment of substrate by heterotrophic bacteria, and the second for the treatment of nutrients by autotrophic bacteria. It was observed that the plant treats both the substrate and nutrients well. Phosphate removal was observed to be 60% due to the growth of phosphorous-accumulating bacteria in the equalization basin due to anaerobic processes, and therefore these bacteria move to the aeration basin for biological complementary processes, to be removed with the generated sludge (Faris et al. 2022). The generation of a high concentrations of nitrates was observed due to the absence of the anoxic basin in the plant as well as due to the nitrification process that oxidizes ammonia, proteins, and amino acids in the wastewater, all of which contribute to the increase in nitrate concentrations, reaching a generation rate of 95% (Daims et al. 2015). High sulfate concentrations were also observed in the treated wastewater due to the oxidation of sulfur compounds that takes place in the aeration basin, such as for hydrogen sulfide gas and proteins that contain a higher percentage of sulfur in addition to methyl mercaptan, where the generation rate reached more than 12% (Syed et al. 2006). High concentrations of methyl compounds were observed in the influent wastewater, which led to the emission of unpleasant odors that negatively affected the workers in this plant and the residents of the residential complex. Processing stages and temperatures greatly affected the emission of these odors (Andraskar et al. 2021). It was observed that the concentration of methyl mercaptan compounds in the atmosphere exceeded the threshold limit compared to the parameters in Table 3. Methyl mercaptan concentrations in the plant's atmosphere reached about 14 ppm_v. These concentrations are undesirable as they lead to health problems amongst the workers in the station and people in the surrounding areas. Therefore, the necessary measures must be taken to determine the causes of emission, the factors affecting it, and how to reduce these odors.

The TOXCHEM model is one of the best models to determine the fate and emission of volatile organic compounds (Behnami et al. 2019). To gain results as close to the real-world observations as possible, a calibration and validation process must be carried out. Figure 5 compares actual concentration and simulated concentration prior to validation (default), and simulated concentration after validation (predicted). The model was run on the basis of default data, so the exit results for the methyl mercaptan pollutant were far from the real-world observations, where the concentration of methyl mercaptan in the treated wastewater was 0.0024 mg/L, and the results validation for and R were 1.55 and 0.0003, respectively. To make the predicted results closer to the actual, the following parameters were changed: the volatile suspended solids (VSS)/TSS ratio from 75 to 80%, the oxygen transfer efficiency from 10 to 18%, the thickness of the biofilm layer from 1 to 5 mm, the media volume inside the reactor from 35 to 45%, whilst the concentration of sludge at the bottom of the sedimentation tank was taken to be in the range from 6,000 to 9,000 mg/L and the MLSS concentration from 500 to 800 mg/L (Cao et al. 2021). The results were 0.00077 mg/L of methyl mercaptan in the treated wastewater, and the validation result for RMSE was 1.2, and for R was 0.9.

An average of 24 g/d of methyl mercaptan enters the plant. In Figure 6, it can be observed that this compound was affected by several factors, including temperature, stripping, surface area of the reactor, and depth of liquid in the reactor (Padalkar & Kumar 2018). The temperature affected the emission and fate of this compound during winter and summer. In this study, the average temperature during winter was 12 °C, and during summer was 32 °C. It was observed that in the winter and at an average temperature of 12 °C, the rate of emission of this compound was about 10%, but in the summer, it reached 13% of the total amount entering the plant, so the increase in temperature is considered to represent a negative factor with regard to the emission of these compounds into the atmosphere of the plant. It was also observed that for both winter and summer, the emission concentration exceeded the threshold limit at 1.38 and 1.8 ppm, respectively.

Figure 6 shows the emission of methyl mercaptan compound in the treatment stages during the winter and summer seasons. The highest emissions during both winter and summer were observed in the aeration basin due to the stripping that takes place for this compound and the transition of the mass from the liquid state to the gaseous state (Lv et al. 2021). The aeration basin follows the equalization basin, and the reason for the emission of these quantities of methyl compounds is that this basin was affected by the following three important factors, namely the process of stripping by mixing, the large associated surface area, which was 110 m², and the height of the water, which was not large at 1.5 m (Wang et al. 2021). All of these factors contribute to the emission of this compound from the equalization basin, while in the remaining basins it was only present in very low concentrations, as shown in Figure 7.

As for the fate of this compound, its decomposition during winter and summer was the highest in terms of stripping and absorption. Methyl mercaptan decomposition occurs via two processes, the first being anaerobic in the equalization basin, with the conversion of a part of this compound into methane and hydrogen sulfide gas, while the second, which is predominant, took place in the aeration basin through the reaction of methyl mercaptan with dissolved oxygen, resulting in carbon dioxide, water, and sulfates (Lu et al. 2018). Emission follows the decomposition process as shown in Figure 8. The aeration process contributed mainly to the elimination of this compound through its oxidation via dissolved oxygen, but the process of stirring and stripping contributed greatly to the emission of this compound into the atmosphere (Cheng et al. 2021).

The MBBR treatment process is affected by several factors, where the most important operational factors that affect the sensitivity of this system are temperature, pH, wind speed, the amount of media and its surface area, the thickness of the biofilm layer on the media, and the concentration of the pollutant, among others (Saidulu et al. 2021).

Figure 9(a) shows the effect of the thickness of the biofilm layer on the decomposition and emission of methyl mercaptan. It was observed that when the thickness of the biofilm layer increased from 0.5 to 7 mm, the biodegradation process increased from 74 to 86%. This is logical as the increase in the biofilm layer increases the bacterial population that degrades this compound and eliminates it instead of releasing it into space or expelling it into sludge. It was also observed that when the biofilm layer was increased, the emission rate decreased from 20 to 11%, i.e., there was an inverse relationship between the increase in the biofilm layer thickness and the emission of this compound. By increasing the biofilm layer, the concentrations of the methyl mercaptan compound in the effluent wastewater decreased from 5 to 2%. It was also observed that this compound was affected in the process of absorption by the sludge. When the biolayer thickness was increased, the amount of the compound did not decrease in the resulting sludge due to the ability of microorganisms to effect its decomposition (Bhattacharya & Mazumder 2021).

Figure 9(b) shows the extent of the impact of the surface area on the decomposition and emission of methyl mercaptan. As observed when using media with a surface area of 250 m²/m³; the process of biodegradation decreased to 73%, resulting in an increase in the emission of the compound into the atmosphere of the plant as well as high concentrations of methyl mercaptan in the effluent wastewater. But when using media with a surface area of 1,250 m²/m³, the decomposition process increased to 93%, and odor emissions reached 6.5%. The surface area provided a suitable and sufficient environment for the growth of bacteria that decompose organic compounds, thus good efficiency was achieved in the system. The use of media with a surface area of 1,250 m²/m³ also positively affected the effluent wastewater quality, reducing its concentration from 5% to less than 1%, while reducing the proportion of this compound in the resulting sludge to less than 0.2% (Bachari 2021; Sun et al. 2021).

Figure 9(c) shows that when the proportion of media inside the reactor was 22.5%, the emissions due to methyl mercaptan were 20% higher, while the degradation rate, which was 72%, reached more than 75% after the increase in media inside the reactor. The decomposition rate of the methyl mercaptan compound increased to more than 90%, while the emission reduction was observed to reach less than 8% (Khan et al. 2021).

Figure 9(d) illustrates that increasing the air flow rate from 225 to 900 m³/h led to an increase in the emission of odors from the plant space from 8 to 18%, and which also caused the rate of biological decomposition to decrease from 89 to 79% with a slight change in the sludge and wastewater lines (Zwain et al. 2020a, 2021; Khalid 2021).

The novelty of this study was the determination of the strengths and weaknesses of the plant's operations and the identification of regions of concern regarding the emission of this compound. It is possible to address this issue without incurring additional costs, but the biological processes can only be controlled by identifying the relevant processes of the system as described in the above discussion. This is the first time that one of the sulfur compounds with an unpleasant smell has been comprehensively assessed via the TOXCHEM model in an MBBR. This provides accurate data on the location of the noxious gas emissions, facilitating remedial action by the specialists at this plant.

The emission of methyl mercaptan from WWTPs presents a hazard risk and odor nuisance to operatives and the local populace. In this study, the biological elimination of methyl mercaptan in a MBBR was modeled using TOXCHEM V4.1. The results show that biological degradation, stripping, compound sorption, and discharge with effluents are the main processes that influence the fate of methyl mercaptan in the MBBR system. It was further shown that operating the MBBR system with (i) a thick biofilm layer, (ii) large specific surface area of media, (iii) greater media fill fraction, and (iv) increased aeration all contribute significantly to the decomposition of methyl mercaptan and decreased emissions into the atmosphere. This demonstrates that a TOXCHEM V4.1 model can be used to estimate the dispersion and fate of methyl mercaptan within a WWTP and hence inform policy regarding this toxin. To summarize, the main findings are as follows:

• biological degradation, stripping, compound sorption, and discharge with effluents are the main processes that influence the fate of methyl mercaptan in the MBBR system.

• operating the MBBR system with (i) a thick biofilm layer, (ii) large specific surface area of media, (iii) greater media fill fraction, and (iv) increased aeration all contribute significantly to the decomposition of methyl mercaptan and decreased emissions into the atmosphere.

• The TOXCHEM V4.1 model can be used to estimate the dispersion and fate of methyl mercaptan within a WWTP.

The authors thank the Kerbala Sewerage Directorate for providing the required materials and instruments for this work.

All relevant data are included in the paper or its Supplementary Information.