Case study of hydrogen sulfide release in the sulfate-rich sewage drop structure

One of the most important pieces of infrastructure for contemporary urban society is the sanitary sewer system (Faris et al. 2022). Most sewer design standards primarily optimize the sewer system's liquid phase conveying capacity, without taking sewer air flow into account (Edwini-Bonsu & Steffler 2004). H₂S, which predominately results from anaerobic microbial sulfate respiration in slimes and sediments, is the principal component of the pungent gases found in sewers (Jung et al. 2017). It is commonly recognized that H₂S can have negative impacts such as sewer corrosion, odor annoyance, and even risk of death for those tasked with maintaining sewer networks. H₂S and other pungent substances leak into the air through cracks in manhole pick holes, pumping stations, etc., irritating adjacent inhabitants with their pungent stench (Carrera et al. 2016). Moreover, sulfide-induced concrete corrosion can result in concrete mass loss, sewer pipe fracture, and finally structural collapse (Vollertsen et al. 2008). To maintain sewer systems and enhance neighborhood livability, it is crucial to understand H₂S generation and transit in sewer systems.

Sewer construction features such as junctions, manholes, bends, weirs, and dips disrupt the hydraulic conditions that exist under normal flow conditions and increase turbulence. This is particularly relevant at sewage falls and drops, where phenomena like splashing droplets and entrainment of air in the water phase occur, amplifying the mass transfer between water and air (Qian et al. 2017). Hence, this type of structure encourages H₂S stripping (Guo et al. 2018).

Previous research has shown that the rate at which air is ventilated from sewers to the atmosphere above ground affects H₂S-related odor issues in sewer systems (Edwini-Bonsu & Steffler 2006). Models have been created for calculating the air flow in the sewer system and studies on the interaction of water and air in sewer pipes have been carried out (Qian et al. 2017). Also, drop structures, which are frequently utilized in sewer systems to transport waste from higher to lower elevations, have been shown to be another significant component in ventilation based on laboratory findings and field measurements (Guo et al. 2018). However, there have been no studies of hydrogen sulfide gas emission in the drop structure by simulating the parameters using the TOXCHEM model. Furthermore, the impact of stream width on hydrogen sulfide gas emissions in the drop structure has not been covered in any of the studies.

Jung et al. (2017) studied the gas transport of H₂S at a waterfall in a discharge manhole. Hydrogen sulfide emission was shown to be strongly influenced by waterfall height, hydrogen sulfide liquid concentration, and fluid velocity. Guo et al. (2018) studied the outcomes of a field monitoring study on the H₂S levels in an upstream section of a 3-km-long sanitary sewage system in Edmonton, Alberta, Canada, that features drop structures. Six drop structures were among the 19 manholes that were monitored. The flow rate of sewage and the patterns of diurnal H₂S concentration were shown to be positively correlated. Yang et al. (2020) investigated the emission of H₂S and its movement in a sewage trunk with drops. It was discovered that the sewer system's drops had a significant impact on the release of H₂S and that even very modest sulfide levels might locally produce substantial H₂S gas concentrations.

This work for the first time studied the emission of H₂S gas in sewage networks by simulating the TOXCHEM program in the Karbala Governorate.

The Al-Hur region in the Karbala governorate is about 100 km south of Baghdad, the capital of Iraq, and within the coordinates 32.645454°N and 43.973323°E were chosen for the study. Al-Hur has a land area of approximately 2,014.8 hectares and is served by a 300 km long network of sewage pipes with diverse diameters ranging from 250 to 1,000 mm. In this study, locations where hydrogen sulfide gas emissions cause unpleasant odors to be released were chosen to assess the impact of drop structures on H₂S release into the air phase in the pipe. The network is deemed ‘recent construction’ as it was created between 2006 and 2010, and all pipes are manufactured from unplasticized polyvinyl chloride (UPVC). A drop structure with a diameter of 315 mm and a height of 0.9 m was chosen for the focus of the study.

In order to examine and study hydrogen sulfide emissions, the drop structure's necessary test parameters were gathered. In accordance with the determinants in the standard procedure for the testing of water and wastewater, the examinations were conducted on average throughout four seasons (Rice et al. 2012).

The case study includes 24 experiments, conducted over the course of a full year (2021). Each season saw the completion of six experiments, or two each month on average. Moreover, two samples are taken for each experiment; one before and one after the drop structure.

The values for COD, BOD, TSS, pH, temperature, oil and grease, SO₄, and H₂S_(aq) were measured from a sample taken before the drop structure and entered into the TOXCHEM model. The TOXCHEM model was then used to predict values for H₂S_(aq) after the drop structure. The expected value of hydrogen sulfide concentration for each season is represented by the average of these data. Then, the predicted values of H₂S_(aq) from the TOXCHEM model were compared with the actual measured values of hydrogen sulfide concentration for each season as shown in Table 1.

In this investigation, hydrogen sulfide gas emission in a drop structure in the Al-Hur district of the Karbala governorate was examined and studied using the TOXCHEM model. The fate and emission of hazardous organics are represented in numerous unit processes in the TOXCHEM models. In mass balance equations, biodegradation, air stripping (volatilization), and sorption are the three fundamental removal methods that are taken into account. The mass balances on contamination at each unit process serve as the mathematical foundation for the hazardous pollutant process modeling.

The headspace in the drop structure is assumed to be well-mixed and finitely ventilated when modeling the closed drop structure unit process object. To estimate the liquid and gas phase concentrations, the gas-phase steady-state mass balance is used in conjunction with the relationship between influent and effluent contaminant concentration in the liquid and gas phases that was created in the volatilization at drops section (HESS 2012).

Calibration is important since the model has default values that do not accurately reflect the conditions and requirements of the sewage system in Al-Hur city. Therefore, the system needs to be calibrated and adjusted in accordance with the requirements and specifications of the Al-Hur drop structure. Data from the spring and summer seasons, which were acquired for the study drop's testing, were used in this study to calibrate the model. Two further seasons were used to test the model, but their results were not factored into the calibration (Hassan et al. 2021).

The statistical criteria reasonable limits are 1 ≥ R > 0.8, 1 ≥ R² ≥ 0, and 0 ≤ RMSE < 1.5.

Sensitivity analysis is a crucial tool for assessing how different process variables affect the fate and emission of H₂S. Sensitivity analysis was performed to understand the fate and emission of H₂S using the main influencing parameters of the drop structure, such as tailwater, drop height, stream width, sewer ventilation rate, and value pH level.

Sewage samples from the drop structure were taken for testing in order to assess the quality of the local sewage. In Table 1, the general sewage quality is displayed and the range for the measured average temperature was between 17 and 27 °C. The network's organic matter decomposition was hastened by the high temperatures in Al-Hur city's sewage (Yang et al. 2019). The wastewater is turbid due to the presence of suspended particles, which can occasionally exceed 167 mg/L. Organic or inorganic compounds are among the chemical characteristics of wastewater. According to COD and BOD measurements, the level of organic pollution in the wastewater for Al-Hur city is between 250 and 320 mg/L and 150 and 180 mg/L, respectively. Due to the high amounts of sulfate in the wastewater from Al-Hur city (500 to 800 mg/L), the presence of organic materials, and the anaerobic conditions in the network, sulfate is converted to hydrogen sulfide gas. Al-Hur city's wastewater has extremely high levels of hydrogen sulfide gas, which can reach 33 mg/L in the winter and 42 mg/L in the summer. This causes network-wide technical and environmental issues. As the pH of the effluent was near 7, not all of the hydrogen sulfide concentrations in the network were released. The concentration of oil and grease ranges between 17 and 27 mg/L and in locations where the flow is stable, this fat forms a very thin layer on the surface of the sewage water, which retards the release of hydrogen sulfide gas (Zwain et al. 2020).

The main point of entry into the model for analysis, design, or realistic research is the calibration procedure. Without calibrating the model, the study becomes implausible and meaningless. A drop structure with an average daily flow of 1,250 m³/day was calibrated for this study for the year 2021. Table 2 shows the required data measured in the drop structure in spring (March, April, and May) and summer (June, July, and August). These values were used to calibrate the TOXCHEM model to predict more realistic results.

After entering all the values obtained from the laboratory tests in the TOXCHEM model, and after running the model, the results for the spring and summer seasons were determined and are shown in Table 3.

After modifying the influencing parameters of wastewater and drop structure for spring and summer, the outcomes were identical to the R², R, and RMSE as shown in Table 4. The table shows all the values of the statistical limits (R², R, and RMSE) for the spring and summer seasons, and all values were within the permissible ranges for R², R, and RMSE.

The validation of the model comes after the calibration has been established and is successful. Model validation is defined as an agreement within the allowed range between the model's predictions and a different set of data that was not used in the model's construction (Mu'azu et al. 2020). After data from the spring and summer seasons were utilized in the calibration, the corresponding values from the autumn and winter seasons were used to validate and compare the anticipated results with the actual outcomes.

Table 5 shows the required data measured in the drop structure in autumn (September, October, and November) and winter (December, January, and February) to validate the TOXCHEM model.

The results for the autumn and winter seasons are provided in Table 6. All the values from the laboratory tests were entered into the TOXCHEM system and the model was run.

The results were identical to the R², R, and RMSE as presented in Table 7 even after wastewater and drop structure impacting parameters were changed for autumn and winter. All of the statistical limits (R², R, and RMSE) for the autumn and winter seasons are shown in Table 3, and each number is within the ranges that are acceptable for R², R, and RMSE.

Emission is the term used to describe the mass transfer of H₂S from the liquid to the gas phase. Hydrogen sulfide gas was produced in the study drop structure in appreciable quantities due to the high sulfate content and the anaerobic surroundings. The release of hydrogen sulfide gas has previously been linked to significant issues including release of unpleasant odors and erosion of manholes and other infrastructure. More seriously, it has been implicated as the cause of fatalities involving network maintenance workers (Raveena et al. 2020).

In each model, there are certain parameters related to the characteristics and quantities of wastewater that can be defined as insensitive, sensitive, and extremely sensitive in terms of the outcomes for both calibration and validation processes (Cao et al. 2021). In this study, TSS concentrations, oil and grease ratios, and VSS/TSS ratios were discounted due to their negligible effects but the flow rate and temperature were all recognized as sensitive variables that when altered, affected the results. However, it was discovered that pH was the most sensitive and significant determinant in the outcomes.

For factors related to the characteristics of the drop structure, the tailwater depth, stream width, and sewer ventilation rate were all regarded as sensitive in this investigation and influenced the outcomes when changed. Drop height, however, was found to be the most sensitive and influential structural factor in the results.

The sensitivity of pH value was investigated in this study in relation to the quantity and features of this wastewater, on the one hand, and to the sensitivity of the characteristics of the drop structure, on the other hand.

A calibrated and validated TOXCHEM analysis was used to ascertain the sensitivity of the parameters influencing hydrogen sulfide gas emission in drop structures. The study was based on statistical analyses of data collected over a year, as well as grab samples collected over a period of 24 days from the inlet and outlet of the drop structure. The most significant factors affecting hydrogen sulfide emission were found to be drop height, pH level, tailwater depth, stream width, and sewer ventilation rate. The drop height and pH were major determinants of H₂S emission in drop structures, according to the TOXCHEM software, and drop height was found to be the most important characteristic. As a result of this study, the following conclusions were reached:

• 1.

  The greatest emission of hydrogen sulfide occurs in summer due to the high temperatures, lower pH, and decreased flow. This is followed by spring and then autumn.

• 2.

  Increasing the drop height, tailwater depth, stream width, and sewer ventilation rate of the drop structure will raise the emission of hydrogen sulfide gas.

• 3.

  Less hydrogen sulfide gas is released in the drop structure at higher pH levels.

• 4.

  Emission can be reduced by increasing the pH through the addition of an alkaline substance. The emission becomes close to nil with pH ≥ 10.

The results of this study will be helpful in creating odor control strategies for sewers and limiting the detrimental effects on human health and sewer infrastructure from H₂S release. However, additional research is required to fully explain some of the observed events, such as the H₂S burst at multiple locations. To further investigate the H₂S creation process and its rates of emission, liquid field samples should also be gathered and examined, as well as sewage biofilm data.

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

The authors declare there is no conflict.