Modelling of hydrogen sulfide fate and emissions in extended aeration sewage treatment plant using TOXCHEM simulations

Odors due to the emission of hydrogen sulfide (H2S) have been a concern in the sewage treatment plants over the last decades. H2S fate and emissions from extended aeration activated sludge (EAAS) system in Muharram Aisha-sewage treatment plant (MA-STP) were studied using TOXCHEM model. Sensitivity analysis at different aeration flowrate, H2S loading rate, wastewater pH, wastewater temperature and wind speed were studied. The predicted data were validated against actual results, where all the data were validated within the limits, and the statistical evaluation of normalized mean square error (NMSE), geometric variance (VG), and correlation coefficient (R) were close to the ideal fit. The results showed that the major processes occurring in the system were degradation and emission. During summer (27 °C) and winter (12 °C), about 25 and 23%, 1 and 2%, 2 and 2%, and 72 and 73% were fated as emitted to air, discharged with effluent, sorbed to sludge, and biodegraded, respectively. At summer and winter, the total emitted concentrations of H2S were 6.403 and 5.614 ppm, respectively. The sensitivity results indicated that aeration flowrate, H2S loading rate and wastewater pH highly influenced the emission and degradation of H2S processes compared to wastewater temperature and wind speed. To conclude, TOXCHEM model successfully predicted the H2S fate and emissions in EAAS system.

www.nature.com/scientificreports/ Despite that, there is a high lack of clear legal acts and guidelines regulating H 2 S emission and dispersion. Hence, a proper control of it is important to reduce nuisances experienced by the exposed populations. The direct way of controlling human exposure to odor is by avoiding the discharge of odor from the origin. Baawain et al. 4 reported that specific odor exposure can primarily be quantified by the integral results of sources of emission, dispersion route, and characteristics of receptor. Different methods (i.e. models, surveys and chamber monitoring) have been used to study odor nuisance to estimate the degree of odors emission from STP 1,2,5,6 . The management of H 2 S can be assisted by mathematical models to understand its fate and emission. The mechanisms of pollutants removal in these models are the degradation and volatilization from different processes. Such mechanisms depends on biological reactions and mass transfer in the liquid and gas phases 7 . Accordingly, TOXCHEM model is an efficient tool for the prediction of volatile organic compounds (VOCs) fate and hazardous air pollutants (HAPs) emission within/from wastewater treatments plants (WWTPs). As an alternative to Water9 software, TOXCHEM was first developed in early 1990s by US Environmental Protection Agency to overcome limitations of Water9. It is based on mass balance of several compounds in WWTPs for each operation unit, taking into account many physical, chemical, and biological processes such as sorption, stripping, volatilization and biodegradation. It is mainly used for VOC air emissions estimates from wastewater collection systems, WWTPs, and disposal facilities. This also include the reduction of air emission by planning, designing and optimization of process projects. In addition, it can also be implemented to predict the loads/ concentrations of contaminants in the water effluent, and residual solids streams 8 .
Karbala state is geologically characterized by gypsum soil and high levels of groundwater, especially in the district where the plant is located. Hence, the groundwater contains very high concentrations of sulfide (SO 4 2− ) compounds that infiltrate into the sewer system, leading to increased SO 4 2− concentrations in sewage, in addition to several other wastewater sources containing SO 4 2− . In that sewer systems, sewage are going through oxygen depletion, variable flow rate and velocity, and long retention time that consequently lead to the decomposition of SO 4 2− to H 2 S gas dissolved in wastewater. As a results, Muharram Aisha sewage treatment plant (MA-STP) is receiving high concentrations of H 2 S that result in the emission of sever odors in the area, which casing problems to the workers and people in the surrounding area. Studies on the modelling of odors exposure associated with H 2 S from STPs' are very limited. Therefore, the study aims to model the H 2 S fate at different treatment units of MA-STP and emission from these units to the atmosphere, during summer and winter, using TOXCHEM V4.1 simulations. In addition, sensitivity analysis is conducted to understand the effect of variation in aeration flowrate, H 2 S loading rate, wastewater pH level, wastewater temperature and wind speed on the fate and emission of H 2 S.

Materials and methods
Site location and description. Muhhram Aisha sewage treatment plant (MA-STP) is located in Al-Hindiya District, at about 20 km from the center of the Karbala, and nearly 110 km to the south of Baghdad, the capital of Iraq.
The geographical coordinates of MA-STP are 32° 31′ 41.4516′′ N and 44° 13′ 12.2664′′ E (Latitude: 32.528181 and Longitude: 44.220074). The treatment system used is extended aeration activated sludge (EAAS), as shown in Fig. 1. It is designed to serve 50,000 people with an estimated discharge flow rate of 8000 m 3 /day, and the operational conditions are listed in Table 1. The system consisted of aerated grit chamber with oil-water separator (API) unit, diffused aerated activated sludge unit, secondary clarifier, chlorine disinfection unit, and drying beds unit for sludge management.  , H 2 S, Oil & grease and PO 4 3− as specified by standard procedures for analysis of water and waste water 9 , and only average values were reported. The influent and effluent characteristics is tabulated in Table 2, and the effluents were compared with the Iraqi effluent standard 10 . Throughout the year, the wind speed was ranged from 5 to 25 km/h, and the direction was mostly north-west (305°). The conversation of soluble H 2 S in wastewater to gas emitted from STP is calculated based on the following equation derived from the ideal gas law at standard conditions 11 : where V is volume occupied by the gas (L) = 22.414 L at standard temperature.

Model development.
To simulate the H 2 S fate and emission from various treatment processes in MA-STP during summer and winter, TOXCHEM V4.1 simulation was used. In TOXCHEM V4.1, H 2 S has similar features like volatile organic compounds (VOC), which can be removed by liquid-gas mass transfer and biodegradation processes. In EAAS system, liquid-gas mass transfer occurs by two mechanisms: first is by volatilization to the atmosphere that is due to striping by diffused bubble aeration and volatilization from open surfaces; second is by sorption process of H 2 S to the sludge. Fate and emission processes of H 2 S in the MA-STP can be summarized in the following four methods: 1. Biological sorption of H 2 S from liquid phase to the sludge formed in the system.   www.nature.com/scientificreports/ In the MA-STP system, air diffusers have been used to provide aeration at the grit chamber and activated sludge tank. For diffused bubble aeration, the rate of stripping is represented by concentration of pollutants in the wastewater and is written as: where r d is diffused aeration stripping rate (mg/h), k d is diffused aeration stripping constant, C is volatile compound concentration in the water (mg/m 3 ), f non is pH dependent fraction of non-dissociated compound, and V is aeration basin volume (m 3 ).
It is assumed that the motion of air on the top of basin (i.e. open system) is adequate to volatilize H 2 S, thus the volatilization rate is given by: where r v is rate of volatilization (mg/h), and k v is volatilization rate constant (1/h).
Due to that MA-STP system is based on suspended growth mechanisms, suspended growth biodegradation was used in the model. Subsequently, H 2 S biodegradation is expressed by Monod reaction as shown in the following equation: where r b is the biodegradation rate (mg/h), k b is the coefficient of first order biodegradation rate (L/mg VSS/h), X is the biomass concentration (mg/L), and K s is the half saturation constant (mg/L).
H 2 S transfers from the liquid phase to the suspended solids and to the residual dead biomass by mean of sorption. Sorption of H 2 S onto the sludge is described by a linear isotherm in low pollutant concentrations and it is computed by the following equation: where q is the pollutant concentration in solid phase (µg/g), and K p is the coefficient of sorption partition (L/g).

Model validation.
To evaluate the characteristic of data predicted by TOXCHEM V4.1, all measured and predicted data were compared using the statistical parameters recommended by Chang and Hanna 12 , which include fractional bias (FB), geometric mean bias (MG), normalized mean square error (NMSE), geometric variance (VG), correlation coefficient (R), and fraction of predictions within a factor of two observations (FAC2). Results of measured H 2 S values discharged with effluent and emitted to atmosphere in 12 months are compared with the predicted H 2 S values by TOXCHEM V4.1. The statistical parameters used are presented in Eqs. (6)-(11): where C o is the measured H 2 S value, C p is the predicted H 2 S value, C o is the average over measured data, C p is the average over predicted data, and σ is the standard deviation over the dataset. The acceptable limits for these statistical parameters are shown in Table 3.
Sensitivity analysis. Among many crucial processes to understand the effect of various operational parameters on the fate and emission of H 2 S is sensitivity analysis. In this investigation, sensitivity analysis was applied to comprehend the fate and emission of H 2 S by using the major influencing parameters on the treatment process of extended aeration systems, which include aeration flowrate, H 2 S loading rate (MLSS concentration in the diffused aerated activated sludge reactor), wastewater pH level, wastewater temperature and wind speed. Differ-

Results and discussion
The performance evaluation of MA-STP. . High influent SO 4 2− concentration and oxidation of H 2 S result in excess presence of SO 4 2− concentration in the system 13 , therefor it is higher in effluent than the influent. However, neutral pH and high degradation of organics indicated a stable biological process. NH 4 + and NO 2 was not detected in the effluent due to complete nitrification process achieved by the EAAS system 8 .
In contrast, about 35 mg/L of NO 3 − was observed in the effluent because the treatment system does not include denitrification process that need to be considered to improve the system performance. Furthermore, high oil & grease removal attributed to the application of oil-water separator (API) in the aerated grit chamber. Besides, dissolved H 2 S was detected at trace level in the effluent, because most of it was degraded in the treatment process and the rest was emitted to the atmosphere.
Results validation. TOXCHEM V4.1 model is used to simulate the H 2 S fate throughout the MA-STP and emission out of it. The influent characteristics and EAAS system operational conditions were the inputs, and H 2 S fate (% and mg/L) and emission values (ppm) were the output of the model. From these applied characteristic and operational variables, model simulations were generated and compared with H 2 S analysis in the sampling points of emitted H 2 S at the top of each treatment unit and dissolved H 2 S with effluent. Table 3 presents statistical data validation of predicted and measured H 2 S emitted to atmosphere and discharged with effluent. In comparison, all data were validated within the limits, and dataset NMSE, VG, and R were close to the ideal fit.
FB and MG measure mean bias and indicate systematic errors which lead to underestimate or overestimate the measured data. FB of 0.25 (more than zero) and MG of 1.28 (more than one) evidence that TOXCHEM under predicted H 2 S concentration discharged with effluent, while FB of − 0.29 (less than zero) and MG of 0.75 (less than one) indicate that the model over predicted H 2 S emission to atmosphere. However, both of FB and MG showed that the error in all data are within acceptable limits and less than 30%. NMSE and VG showed that data scattering around the true value and they both reflected systematic random errors from unpredictable fluctuations. The results of both of NMSE and VG are very close to ideal fit, indicating that there is no random error for the predicted data over measured.
The coefficient of correlation (R) reflects the linear relationship between modeled and observed data. Both R values, 0.9 for H 2 S concentration discharged with effluent and 0.89 for H 2 S emission to atmosphere, indicated a strong correlation between predicted and measured data. The highest R values is required but not sufficient, therefore FAC2 is important factor for evaluation and validation as it's the most robust measure that is not affected by either low or high outliers. The results of FAC2 revealed that 78% (FAC2 = 0.78) of H 2 S concentration discharged with effluent and 75% (FAC2 = 1.34) of H 2 S emission to atmosphere were within a factor of two of the measured data.
In addition, Fig. 2 shows a scattering comparison of measured and predicted H 2 S that are emitted to atmosphere and discharged with effluent. Distribution of data and coefficient of determination (R 2 ) are adopted to check the goodness of model fit. The results showed that the predicted and measured data were well scattered around the linear line, where measured emission was slightly less than predicted and measured discharged concentration was slightly higher than the predicted, and R 2 values showed that data are in a good fit. The TOX-CHEM model could sufficiently describe the experimental data of H 2 S fate and emission. In comparison with other studies on the modeling of H 2 S using AERMOD 7 , CALPUFF 14 , and GOSTELOW 15 , statistical analysis of TOXCHEM V4.1 model are very satisfactory to study H 2 S due to valid prediction with less limitations and errors. H 2 S fate and emission. STP are a major source of gaseous emissions that contain odorants and greenhouse gases. Figure 3 shows the H 2 S fate (%) throughout the MA-STP. The EAAS system receives about 280 kg/day of   www.nature.com/scientificreports/ of activated sludge process is to eliminate organic pollutants, the EAAS system has successfully achieved desulfurization of about 74% of H 2 S (degradation and sorption).
In an aerobic conditions, natural microorganisms called sulfide oxidizing bacteria (SOB) play a major role in the desulfurization of H 2 S. H 2 S is oxidized by chemolithoautotrophic bacteria from the genus Thiobacillus group that has high affinity to sulfide compounds (H 2 S, HS − and S 2− ) 16 . In aqueous solution, H 2 S presents in forms that are highly depending on pH level. As the sewage pH is about 7 at STP, H 2 S is primarily dissociates to form bisulfide (HS − ) (Eq. (12)). Sulfide (S 2− ) is another form of H 2 S (Eq. (13)) that is generally neglected because of its insignificant presence except at very high pH, and H 2 S form may be predominant below pH 5 13 .
In STP where aeration is provided, HS − is biologically oxidized to firstly elemental sulfur (S 0 ) and subsequently to sulfate (SO 4 2− ), as shown in the following reactions 17 : Complete oxidation of HS − to SO 4 2− requires the consumption of two oxygen molecules, but this reaction is reversible if limited amount of oxygen is supplied and elemental S 0 might accumulate 13 . However, elemental S 0 is end-product of oxidation process that is necessary for the growth of microorganisms and directly consumed for the synthesis of cellular protein needed for new cells production 18 . Excess amounts of elemental S 0 and SO 4 2− are sorbed to the biomass and/or released with the effluent.
Volatilization describes the process whereby an odorant (H 2 S) is transferred from an area source such as the surface of diffused aerated activated sludge reactor to the atmosphere 15 . Figure 3 displays that about 70 kg/ day (23%) of total H 2 S was volatized from the MA-STP to atmosphere, and Fig. 4 shows the emission distribution of H 2 S from each unit (% of the total emission). The results revealed that summer has emitted higher H 2 S compared to winter, in which most of it was from diffused aerated activated sludge reactor (> 50%), followed by aerated grit chamber (API) (25-50%) and sludge drying beds (25-50%). The mechanism of H 2 S emission is volatilization by air stripping and open surfaces. The H 2 S emission is a physicochemical process that contains (12) H 2 S (aqueous) → HS − + H + pKa = 7.05, 5 < pH ≤ 10 (13) HS − → S 2− + H + pKa = 12.9, pH ≥ 11  A STP brings huge quantities of sewage into contact with air that boost the stripping of odorants, which can be significant odor source. Due to aeration process, biological oxidation tends to decrease liquid-phase odorant concentrations. However, recycling of activated sludge is a notable way for odor control due to the recycling of biomass containing sulfur compounds from secondary clarifiers to the aerobic activated sludge reactors. This fosters the consumption of odor compounds before they volatilize from the liquid phase to the atmosphere 16 . Furthermore, the available surface area for gas transfer is believed to affect the emission of H 2 S from open surfaces to the atmosphere. This was also proved by Parsons et al. 20 whom found that the greater open surface area of the source the greater H 2 S concentration emitted to atmosphere. Figure 5 shows the concentration of H 2 S (ppm) emitted to atmosphere from each unit. At summer and winter, the total emission were 6.403 and 5.614 ppm, from diffused aerated activated sludge reactor were 4.492 and 4.035 ppm, from aerated grit chamber (API) were 0.768 and 0.507 ppm, from sludge drying beds were 0.718 and 0.475 ppm, from secondary clarifier 0.379 and 0.541 ppm, and from chlorine disinfection were 0.046 and 0.056 ppm, respectively. The results indicated that H 2 S emission from all units was within the human odor threshold (0.0005-1.5 ppm) 3 , except for diffused aerated activated sludge reactor that was much higher. Long human's exposure (8 h) to concentrations higher than 5 ppm (total emission in this study) may cause headache, nausea and tearing of eyes. Therefore, MA-STP workers are exposed to health rick due to their exposure to high concentrations of H 2 S that required odor control system (especially at diffused aerated activated sludge reactor), the modification of operational process, or/and shorter working schedule. Sensitivity analysis. H 2 S fate and emission within/from EAAS system are affected by operational parameters such as aeration flowrate, H 2 S loading rate, wastewater pH level, wastewater temperature and wind speed. Figure 6a demonstrates the effect of aeration flowrate (2500-15,000 m 3 /h) on the fate of H 2 S. H 2 S sorption to sludge and discharge with effluent was not affected by change in aeration flowrate, compared to biodegradation and volatilization processes. Interestingly, increase in aeration flowrate from 2500 to 15,000 m 3 /h has increased the emission of H 2 S to atmosphere from 18 to 45%, and decreased biodegradation process from 80 to 52%. The authors cannot negate that there are several evidences on aeration causes odorants stripping by air bubbles, for instance, Baawain et al. 2 reported that emissions of H 2 S was intensified by air bubbles during the aeration process. In another study by Tzvi and Paz 13 , they stated that 15-30% of H 2 S was evaporated within the air bubbles introduced to the system and released to the atmosphere, which was much higher than operating the system in absence of bubbles streams. In aeration reactor, H 2 S emission to the atmosphere by stripping and volatilization from open surfaces may occur first, then followed by oxidation of H 2 S by aerobic microorganisms. Hence, monitoring H 2 S emission from aeration stream is not only necessary to evaluate H 2 S fate but also for safety aspects. Therefore, operating the EAAS system at lowest aeration flowrate will reduce the emission of odorants and increase biodegradation treatment. Figure 6b describes the effect of H 2 S loading rate (MLSS concentration in the diffused aerated activated sludge reactor) on the fate of H 2 S. It is notable that decrease in the H 2 S loading rate (increase in MLSS concentration) from 35 to 5 mg H 2 S/g MLSS/day has enhanced biodegradation process from 45 to 82%, improved sorption process from 1 to 4%, decreased emission to atmosphere from 50 to 13%, and reduced discharge with effluent from 4 to 1%. There is an inverse correlation between H 2 S loading rate and removal efficiency 21 , in which an increase in H 2 S loading rate will first decrease the biomass activity resulting in lower biodegradation process leading to decreased H 2 S removal efficiency, and second increase aqueous H 2 S concentration available for H 2 S emission to atmosphere and/or discharged with effluent. www.nature.com/scientificreports/ Figure 6c displays the effect of wastewater pH level on the fate of H 2 S, in which it effects first the dissociation of H 2 S in aqueous solution, and second the mechanism of H 2 S removal. The results vouch that increase in pH between 5 and 10 decreased the emission of H 2 S to atmosphere from 35 to 5%, making it more dissolved in sewage and increased biodegradation process from 62 to 90%, whereas sorption to sludge and discharge with effluent were not much effected. As pH increases, the fraction of available H 2 S decreases due to its dissociation into HS − (Eqs. (12), (13)). Higher pH solution led to less H 2 S available for transferring from STP treatment units into the atmosphere, whereas H 2 S stripping is favored under acidic conditions 17 . Chaiprapat et al. 22 observed that as pH of the wastewater decreased, the efficiency of H 2 S removal of the system slightly decreased due to lowered solubility of H 2 S, which lead to higher ionic strength of wastewater. Low solubility makes H 2 S and O 2 in gas form become deficient for SOB to execute biochemical reactions in the liquid form.
Moreover, the oxidation of sulfide compounds produces H + (Eq. (17)), leading to drop in the pH in the aerobic unit of STP. Low pH level may inhibit biodegradation process because under acidic environment, H 2 S is unionized and has neutral molecule that is very toxic to microorganism in the system as it can permeate through the cell membrane better than HSand S 2-1 . However, continuous wastewater feeding in STP provides recirculation and alkalinity buffering to maintain pH and hinder acidity. This indicate that using aeration for biological oxidation wouldn't results in external release of H 2 S, and decrease the risk of H 2 S stripping. www.nature.com/scientificreports/ Temperature is another key factor influencing the physicochemical properties of gases, influencing the Henry gas law and kinetics of biological processes. Figure 6d presents the effect of wastewater temperature ranges from 5 to 35 °C on the fate of H 2 S. It was observed that temperature mainly effected the mass transfer of H 2 S, either dissolved in wastewater or volatized to atmosphere. Increase in temperature from 5 to 35 has increased the emission of H 2 S from 22 to 27% and decreased its content in wastewater from 3 to 0%, while effects on degradation and sorption processes were limited. The findings evince that aqueous H 2 S condensed at a lower temperature and emitted to atmosphere at high temperature. Similarly, Baawain et al. 2 confirmed that high temperature has increased H 2 S emissions from ponds sewage treatment system. Other studies also reported that aqueous solution temperature highly effected the mass transfer rate of H 2 S, in which the overall mass transfer from liquid phase to gas phase increases with temperature 6 . Figure 6e display the effect of wind speed (friction velocity) on the emission of H 2 S from STP to the atmosphere. It was seen that increase in wind speed from 5 to 35 km/h has slightly increased the volatilization of H 2 S from 25 to 27% and decreased degradation process from 72 to 70%, whereas sorption and dissolution of gas in wastewater processes were not affected. However, this can show that wind speed has limited effect on the gas emission but of course will highly influence the dispersion of odors away from its generation source. Similarly, slight higher emission rate of H 2 S was observed with higher wind speed 2 . Wind speed is usually correlated with mass transfer and emission, where it is evident on wind speeds over 4 m/s, and nearly undetectable below this speed 5 . However, the wind speed reported to be associated with H 2 S concentration more than the emission rate, in which higher wind speed dilutes the concentration of H 2 S and disperses it for long distance 6 . In this regards, Santos et al. 15 reported that wind speed did not have a significant effect on overall mass transfer of H 2 S, suggesting that volatilization will depend more on turbulence of liquid phase than wind speed.