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Article

Exploring Oxygen and Harmful Gas Distribution in Wastewater Treatment Tanks of Industrial Enterprises

by
Chunli Yang
* and
Yan Liu
*
Institute of Urban Safety and Environmental Science, Beijing Academy of Science and Technology, Beijing 100054, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2026, 16(2), 1034; https://doi.org/10.3390/app16021034
Submission received: 5 November 2025 / Revised: 9 January 2026 / Accepted: 14 January 2026 / Published: 20 January 2026
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Abstract

Many confined-space accidents have happened in wastewater treatment tanks, mainly caused by hazard gases. To identify the factors affecting the distribution of toxic and harmful gases in wastewater treatment tanks, in this study, we collected data on confined-space accidents occurring in wastewater treatment tanks in China and analyzed accident types, the substances that caused the accidents and the purpose of entry. We carried out field tests to detect the concentrations of oxygen, hydrogen sulfide, combustible gas and carbon monoxide in 222 wastewater treatment tanks from 28 industrial enterprises and investigated the influence of wastewater treatment tank type, cover type and industry type on gas distribution. Through continuous monitoring, the concentrations of hydrogen sulfide and carbon monoxide in the regulating tanks of two industrial enterprises were monitored for a few days. The mechanism of harmful gas generation and control approaches were explored and analyzed. The results showed that more than 90% of confined-space accidents in wastewater treatment tanks were poisoning accidents, and the levels of harmful gas in wastewater collection tanks, regulating tanks, hydrolysis acidification tanks, sedimentation tanks and sludge tanks were high, qualifying them as high-risk wastewater treatment tanks prone to accidents. Without disturbance, there is basically no harmful gas in wastewater treatment tanks with completely uncovered tops. In addition, the concentration of toxic and hazardous gases in wastewater treatment tanks is not always stable, instead fluctuating greatly with time. The main purposes of this study are to identify the factors affecting the concentration of toxic and harmful gases in wastewater treatment tanks and to assess the risks of using wastewater treatment tanks.

1. Introduction

Industrial enterprises may produce wastewater, which contains a significant amount of organic matter, toxic and hazardous substances, etc. Wastewater must be treated before it is discharged [1,2]. Generally, approaches such as the activated sludge method and the biofilm method can be adopted for wastewater treatment. A wastewater treatment system may comprise different types of tanks with different functions, primarily including collection tanks, regulating tanks, sedimentation tanks, anaerobic tanks, anoxic tanks, aerobic tanks, MBR tanks, dissolved air flotation tanks, sludge tanks, clean water tanks, etc. [1,3]. Figure 1 shows an enterprise’s wastewater treatment system, where wastewater flows in the direction indicated by the arrow. The role of the collection tank is to collect wastewater, and the role of a regulating tank is to adjust the pH value and outlet flow rates. Dissolved air flotation tanks are mainly used to remove suspended solids and oil. The role of a sludge tank is to collect and treat settled sludge. The role of a middle tank is to settle and regulate water volume, regulate water quality and store water. The main function of an aerobic tank is to remove organic matter from wastewater, carry out nitrification reactions, and absorb phosphorus. The role of a biological contact oxidation tank is to remove organic matter from water and maximize water quality. The main function of sedimentation tanks is to remove suspended particles through gravity settling. A sand filter is mainly used to filter suspended solids, colloids, bacteria and other impurities in water. A clean water tank is used to store treated wastewater [1,3].
Some wastewater tanks are uncovered, and others are covered at the top as a closed vessel, meaning that they are normally closed but can be opened when needed. According to the Occupational Safety and Health Administration (OSHA) [4], confined spaces can be classified into two categories: (1) open-topped enclosures with depths that restrict the natural movement of air and (2) enclosures with limited openings for entry and exit. Based on the definition of confined space in the Safety Regulations for Working Confined Spaces in the Industrial and Trade Enterprises, these wastewater treatment tanks are typical confined spaces. The wastewater treatment process is a complex system, in which different microorganisms interact to generate methane, hydrogen sulfide, carbon dioxide, ammonia and other toxic and hazardous gases, while at the same time consuming oxygen [5], posing great risks for workers who enter them. According to the data of the Ministry of Emergency Management of the People’s Republic of China [6], 98 confined-space accidents occurred in the industrial and trade industry during 2013~2023, of which 41 (41.8%) occurred in wastewater treatment systems. In the United States, sewage system accidents account for 31% of confined space poisoning accidents [7].
Some researchers have conducted studies on gas distribution in confined spaces. J. Zhao studied gas evacuation or oxygen recovery times for fan-ventilated confined-space manure pits [8,9] and found that the time required to reduce the H2S concentration from a documented high level (10,000 ppm) to the OSHA PEL level (10 ppm) was equal to or greater than those for other gases. G. S. Michaelsen studied oxygen level in street manholes and analyzed the cause of oxygen decrease [10,11], finding that the oxygen levels in some wells decreased very quickly, decreasing from 16.1% to 0 within 14 days, and the composition of the soil around the well was the reason for the decrease in oxygen content. However, little research on gas distribution in wastewater treatment tanks exists conducted at present [12]. In this study, we mainly focus on investigating the emission of malodorous gases and greenhouse gases [13,14,15,16] from an environmental perspective and the impact of gases released into the atmosphere on human health [17,18]. The accounting methods, emission characteristics and influencing factors for gases are studied [19,20].
Since a wastewater treatment tank is a high-risk place for confined-space accidents at present, we must urgently conduct more in-depth and systematic studies on the distribution of harmful gases and oxygen in them. Therefore, in this study, we focused on wastewater treatment tanks at industrial enterprises. Examples of confined-space accidents that occurred in wastewater treatment tanks were collected. The accident type, specific accident tank and purpose of entry were analyzed. O2, H2S, CO and combustible gases from different tanks at different enterprises were tested, and two tanks were selected for long-term monitoring. The main goals of this study are to obtain the factors affecting the concentration of toxic and harmful gases in wastewater treatment tanks and to assess the risks posed by wastewater treatment tanks.

2. Methodology

2.1. Accident Analysis

In this study, we collected investigation reports of confined-space accidents in China before 2024 from the websites of emergency management bureaus in all provinces and cities [21,22,23,24] and then read the reports and selected the accidents that occurred in wastewater treatment tanks.
The accident occurrence time, location, type of tank and purpose of entry were obtained from the reports.

2.2. Field Test

In the field test, the oxygen, hydrogen sulfide, combustible gas and carbon monoxide levels in the tank were tested via two portable gas detectors: the first was MSA Altair 4XR combined with the MSA Altair Pump Probe (MSA (China) Safety Equipment Co., Ltd., Suzhou, Jangsu, China), and the second was MultiRAE LITE PGM6208 (MultiRAE Scientific Instrument (Shanghai) Co., Ltd., Shanghai, China). Both instruments have four sensors: a hydrogen sensor, sulfide oxygen sensor, combustible gas sensor and carbon monoxide sensor. The hydrogen sensor, oxygen sensor and carbon monoxide sensor are electrochemical sensors. The range and resolution of the two instruments are detailed in Table 1. “ppm” represents the volume fraction of target gas per million parts of gas mixture. “%LEL” represents the percentage of the lower explosive limit of combustible gases.
These two types of gas detectors do not measure specific types of combustible gas. The sensor indirectly measures the concentration of combustible gas in the environment by detecting heat change [25,26]. Therefore, gases that can burn and release heat can be detected, and the measured combustible gas is the total concentration of combustible gases present in confined space. The main type of combustible gas generated by wastewater is methane [27,28], and the sensor is calibrated to detect it. Therefore, methane was the primary combustible gas tested in this experiment.
In total, 28 companies with wastewater treatment systems were selected to conduct the field test. The business activities of these enterprises included food manufacturing (17); equipment manufacturing (4); automobile manufacturing (2); medicine manufacturing (2); wine, beverage and refined tea manufacturing (2); and furniture manufacturing (1).
The portable detectors, which were calibrated according to national standards, were taken to every tank tested. The test point was set as close as possible to the wastewater surface. Each point was tested three times, and the average value of the three tested values was recorded. All wastewater treatment tanks of the 28 enterprises were tested. A total of 222 wastewater treatment tanks were ultimately tested.

2.3. Field Continuous Monitoring

The hydrogen sulfide detector (GBPRO-H2S MFA110078, Industrial Scientific Sensing Instruments (Shanghai) Co., Ltd., Shanghai, China) and carbon monoxide detector (GBPRO-CO MFA110078, Industrial Scientific Sensing Instruments (Shanghai) Co., Ltd., Shanghai, China) were used for continuous monitoring. The detectors recorded the gas concentration data at regular intervals and downloaded the recorded data through the data link. Using a replaceable CR2 lithium battery, they can run for 2600 h when fully charged. However, if used in a high-temperature and high-humidity environment such as a wastewater treatment tank, their battery power may be consumed more quickly due to the exception alarm; thus, the running time may be greatly shortened. In addition, the instrument can measure and record the temperature at the same time. The basic parameters of the detectors are shown in Table 2.
The wastewater treatment tanks with high concentrations of hydrogen sulfide and carbon monoxide were potential test sites. After communicating with these enterprises and determining their willingness to participate in testing, a food manufacturing enterprise was ultimately selected.
Continuous monitoring was carried out twice in December. The installation position and duration are shown in Table 3.

3. Results

3.1. Analysis of Confined-Space Accidents in Wastewater Treatment Tanks

Thirty-eight confined-space accidents were collected, distributed in 19 provinces and cities in China. The years in which the accidents occurred are shown in Figure 2.
Among the thirty-eight accidents, there were thirty-six poisoning accidents, one hypoxia accident and one explosion accident, showing that poisoning accidents accounted for the vast majority of accidents.
The gases causing the accidents include hydrogen sulfide, carbon monoxide, ammonia, benzene series, volatile organic compounds, hydrogen phosphine, ammonia and methane, as shown in Table 4. Methane is a substance that causes explosions [4,9], carbon dioxide is a substance that causes anoxic asphyxia [4,9], and hydrogen sulfide is one of the main substances that causes poisoning. Although we recorded 15 accidents in which the type of poisoning gases was unknown, hydrogen sulfide was speculated as being the main gas causing such incidents [4,9].
The type of tanks in which the 38 accidents occurred were counted. The results are shown in Figure 3. Figure 3 shows that wastewater collection tanks, regulating tanks, sedimentation tanks and sludge tanks are subject to frequent accidents, so it can be inferred that these places accumulate large amounts of harmful gases and, thus, pose a relatively high risk. These are the most commonly used tanks in wastewater treatment systems [1], which are rich in organic matter, and they are covered at the top. Hence, there is a high possibility of accumulating toxic and hazardous gases. The function of a sedimentation tank is to separate suspended solids, so sludge in wastewater will accumulate at the bottom of sedimentation tanks, which may produce harmful gas.
Confined spaces are not designed for regular workplaces, where workers may enter them when necessary. The purposes of entry were analyzed and the results are shown in Figure 4. We found that the purposes of entry included the overhaul and maintenance of equipment and facilities (e.g., water pump, overflow pipe, photo-oxidation machine), sludge removal, inspection, anti-corrosion tasks, etc. Maintenance and sludge removal are the most frequent reasons for entry. The equipment used for overhaul and maintenance is mainly water pumps, followed by aeration facilities, overflow pipes and photo-oxidation machines. As these facilities are installed in wastewater collection tanks, regulating tanks, sedimentation tanks, etc., once they stop working, workers enter tanks to perform maintenance. Moreover, sludge is easily deposited in these tanks. Thus, if an automatic sludge discharge meter is not included in the design process, the tank will likely require entry for sludge removal. During dredging operations, accidents frequently occur due to insufficient understanding of the distribution of harmful gases and failure to implement relevant safety measures.

3.2. Distribution of Oxygen and Harmful Gases in Wastewater Treatment Tanks

3.2.1. Overall Situation

The gas concentrations of the 222 wastewater treatment tanks are shown in Figure 5. The maximum value that can be registered via the instrument was used to determine if the tested value exceeded the range of the detectors.
The average concentrations of oxygen, hydrogen sulfide, carbon monoxide and combustible gas were 20.6% VOL, 10.4 ppm, 14.8 ppm and 4.3% LEL, respectively. According to the criteria for gas exposure limits in China [29,30], the exposure limit for oxygen is 19.5%, and the exposure limits for hydrogen sulfide, carbon monoxide and combustible gas are 7 ppm, 25 ppm and 10% LEL, respectively. The tanks in which hydrogen sulfide, carbon monoxide and combustible gas exceed their exposure limits accounted for 4.95%, 17.57%, 5.86% and 10.36%, respectively.
The box plot, shown in Figure 6, further reveals the heterogeneous risk pattern of gas distribution. The distribution of oxygen concentration detection data is relatively concentrated with no significant anomalies. The oxygen concentration in the vast majority of sewage tanks is maintained at around 20%, which is within a safe range. The slight extension of the lower part of the kernel density curve indicates that although a few sewage tanks have a mild hypoxic condition, a majority of the tanks have not entered a severe hypoxic state, and the overall risk of hypoxia is relatively low. The concentration of hydrogen sulfide varies greatly with significant fluctuations, and the number of samples exceeding the standard is the highest among the four gases, fully reflecting its position as a frequently co-occurring toxic gas in the sewage treatment process. The kernel density curve shows an upper-skewed distribution, indicating that concentrations in some tanks are extremely high with many extreme outliers. In certain areas, the hydrogen sulfide concentration exceeds 100 ppm, significantly exceeding the exposure limit, highlighting the high safety hazard associated with exposure in specific areas. Although the overall distribution of the carbon monoxide concentration is slightly lower than that of hydrogen sulfide, it also shows considerable fluctuations. It is mainly concentrated within a certain range. However, there are also some extreme outliers, indicating that although it is not a major harmful gas, it still poses potential risks in certain specific tank types. Finally, multiple samples of flammable gas concentrations significantly exceed the standard, constituting a risk source that cannot be ignored and requiring attention to be paid to abnormal data and potential safety hazards.

3.2.2. Distribution of Wastewater Treatment Tanks with Harmful Gases at High Concentrations

The 222 sewage tanks tested in this study represent 60 types of wastewater treatment tanks. In this study, we summarize the types with more than two examples, totaling 15 types. The results are shown in Table 5. These 15 types of tanks are widely distributed in industrial enterprises. Table 5 shows that the tanks with high average concentrations of hydrogen sulfide are hydrolysis acidification pools, middle tanks and regulating tanks, with the average value being higher than 30 ppm. The tanks with high average concentrations of carbon monoxide are regulating tanks and hydrolysis acidification tanks: the average value of carbon monoxide in regulating tanks is far higher than those for other tanks, and the tanks with the highest average concentrations of combustible gas are hydrolysis acidification tanks and regulating tanks.
According to the regulations for confined spaces [29,30], if the gas concentration in a wastewater treatment tank exceeds the exposure limits, workers will not be permitted to enter the confined space until it has been ventilated to a satisfactory gas concentration. Tanks in which the gas concentration exceeds the standard are called gas unqualified tanks in this study. The proportions of oxygen, hydrogen sulfide, carbon monoxide and combustible gases in unqualified tanks for the 15 types of tanks are shown in Figure 7. The hydrolysis acidification tank has the highest proportion of unqualified oxygen, accounting for 33.3% of total gas unqualified pools. Hydrolysis acidification tanks have the highest proportion of unqualified hydrogen sulfide tanks (83.8%). Middle tanks have the highest proportion of unqualified carbon monoxide tanks (25%). The hydrolysis acidification tanks have the highest proportions of unqualified combustible gas tanks (66.6%).
To analyze the differences in harmful gas concentration distribution in diversified wastewater treatment tanks, statistics were produced regarding the wastewater treatment tanks with hydrogen sulfide and carbon monoxide gas concentrations higher than 50 ppm and combustible gas concentrations higher than 30% LEL. The statistical results showed that there were 12 wastewater treatment tanks of 6 types with hydrogen sulfide concentrations higher than 50 ppm, as well as 8 wastewater treatment tanks of 4 types with carbon monoxide concentrations higher than 50 ppm and combustible gas concentrations higher than 30% LEL. Figure 8 shows the specific types and numbers of wastewater treatment tanks. Based on the figure, the tanks with high concentrations of hydrogen sulfide and combustible gas were primarily regulating tanks and the hydrolysis acidification tanks, and the tanks with high concentrations of carbon monoxide gas were primarily regulating tanks. This is consistent with the statistical analysis of accidents: The regulating tanks have high concentrations of harmful gases, with high working risk, as they are prone to causing accidents. The concentration of hydrogen sulfide in the hydrolysis acidification tanks is high. The reason for this is that the main function of a hydrolysis acidification tank is to transform macromolecular substances that are difficult to biodegrade into small molecular substances that are easy to biodegrade; this degradation process is accompanied by the generation of significant levels of toxic and hazardous gases. Although the concentration of toxic and hazardous gases in hydrolysis acidification tanks is high, the accident sites do not include the hydrolysis acidification tanks. There is no equipment or facilities installed in the hydrolysis acidification tanks; thus, there is no need for entry. The main reason for the high levels of toxic and harmful gases being present in these tanks is that they are front-end tanks and are rich in organic matter, which produces toxic and harmful gases.

3.2.3. Influence of Wastewater Treatment Tank Coverings on Gas Distribution

Wastewater treatment tanks can be roughly divided into two categories: those that are completely uncovered on the top in routine use, and those that are covered or mostly covered on the top (with only a small opening) in routine use.
Among the 222 wastewater treatment tanks tested, there were 75 completely uncovered wastewater treatment tanks in daily operation; the oxygen content of the 74 wastewater treatment tanks was above 20.6%, and the contents of hydrogen sulfide, carbon monoxide and combustible gas were zero. If an uncovered wastewater treatment tank was not disturbed, the risk was low.

3.2.4. Influence of Industry on Gas Distribution

A total of 141 wastewater treatment tanks were tested at 17 food manufacturing enterprises, and 81 were tested at 11 other enterprises, which included 4 used in equipment manufacturing; 2 used in automobile manufacturing; 2 used in medicine manufacturing; 2 used in wine, beverage and refined tea manufacturing; and 1 used in furniture manufacturing.
To analyze the correlation between the industry and the hazardous gas concentration, statistics were produced on the average gas concentrations of different industries. The results are shown in Table 6. The concentrations of hydrogen sulfide, carbon monoxide and combustible gas in the food manufacturing industry are much higher than those in other industries. There is little difference in oxygen concentration between industries, and the risk of oxygen deficiency is low. The reason for these findings is that wastewater from the food manufacturing industry is rich in organic matter, which may produce methane and hydrogen sulfide. Higher contents of organic matter will accelerate the consumption of oxygen and the formation of an anaerobic state, as well as provide a carbon source for SRB and MR, which is beneficial to the production of hydrogen sulfide and methane gas. Hence, the concentrations of hydrogen sulfide and combustible gas in the food manufacturing industry are high. At present, the biological nature of CO production is poorly understood [31,32]. The research of Rich, GM showed that CO production is observed in the presence of glucose and formate [33]. Therefore, we speculate that due to the high concentrations of sugar and starch in the wastewater of food manufacturing enterprises, the concentrations of glucose and formate inside the tanks are relatively high, resulting in significant carbon monoxide production.

3.2.5. Correlation of Different Gases

To analyze whether there is an accompanying correlation between hydrogen sulfide, carbon monoxide, combustible gas and oxygen, the Pearson correlation coefficients between the four gases were calculated via SPSS 16.0 software. The results are shown in Table 7.
Table 7 indicates that the correlation coefficient between oxygen and combustible gas is −0.74, which presents the strongest, but negative, correlation. It indicates that tanks with high combustible gas concentrations are more likely to have low oxygen concentrations. There is a positive correlation between carbon monoxide and combustible gas, meaning that a wastewater treatment tank with a high combustible gas concentration is more likely to have a high carbon monoxide concentration.

3.3. Changes in Toxic Gases over Time in Wastewater Treatment Tanks

3.3.1. Hydrogen Sulfide

Figure 9 and Figure 10 show changes in hydrogen sulfide concentration and temperature identified via two hydrogen sulfide gas detectors in the first test. Figure 10 and Figure 11 show the changes in hydrogen sulfide concentration and temperature obtained via three hydrogen sulfide gas detectors in the second test.
The concentration of hydrogen sulfide in the regulating tank was not constant but fluctuated with time, and it even increased rapidly at one time. The maximum value of the hydrogen sulfide concentration was about 70 times higher than the minimum value. We speculate that stirring performed by facilities in the regulating tank or the inflow/outflow of water will cause disturbance, causing significant hydrogen sulfide gas release from the wastewater. As shown in Figure 10 and Figure 12, the temperature was between 20 and 30 °C. At the time of testing, Beijing was in winter, with an outdoor temperature of 3 °C, and the temperature in the tank was higher than the outdoor temperature.

3.3.2. Carbon Monoxide

Figure 13 shows the change in the carbon monoxide concentration. The figure shows that the concentration of carbon monoxide in the regulating tank is beyond the range of the instrument (1500 ppm), and it was high and changed with time. Because the measuring point is close to the outlet, the change in carbon monoxide concentration at the measuring point may be influenced by the external air. It can be inferred that the carbon monoxide concentration in the regulating tank may more than 1500 ppm at all times.

4. Discussion

4.1. Risk Level of Different Tanks

Considering historical accidents in wastewater treatment tanks and concentrations of toxic and harmful gases, the risks posed by different tanks are assessed and graded. The indexes and grading criteria are shown in Table 8. Historical accidents are based on the data in Figure 2, and the concentration of toxic and harmful gases is based on data in Table 4. The highest and average concentration scoring criteria for the same toxic and harmful gas are the same. The maximum score is 60 points. When the scores are between 0 and 15, 15 and 30, 30 and 45, and 45 and 60, the risk levels are extreme high, high, medium and low, respectively. The classification results are shown in Table 9. The tank with the highest risk level is the regulating tank, followed by the sedimentation tank, the collection tank and the hydrolysis acidification tank.

4.2. The Main Sources and Forms of Harmful Gases in Wastewater Treatment Tanks

Toxic and hazardous gases in wastewater treatment tanks are the main cause of confined-space accidents. Hydrogen sulfide has always been the primary cause of accidents in wastewater treatment tanks [4], and it is an accompanying byproduct in the wastewater treatment process. Sulfate-reducing bacteria (SRB) in wastewater use sulfate as an electron acceptor of respiratory metabolism and organic matter as an electron donor to produce H2S and other sulfides through dissimilatory reduction [34,35,36].
Due to the special physical and chemical properties of hydrogen sulfide, its dispersion after production is mainly affected by pH and disturbance. Sulfide produced by sulfate reduction exists in wastewater treatment tanks in four forms: hydrogen sulfide gas (H2S(g)), hydrated hydrogen sulfide (H2S(aq)), hydrogen sulfide ion (HS−) and sulfur ion (S2−); these forms are subject to dynamic change, and only hydrated hydrogen sulfide can be directly transformed into hydrogen sulfide gas [37,38]. Hence, when the pH value is greater than 12, sulfide mainly exists in the form of S2−, and when the pH value is less than 5, it almost always exists in the form of hydrated hydrogen sulfide. The correlation between gaseous hydrogen sulfide and hydrated hydrogen sulfide concentration is compliant with Henry’s law. The correlation between sulfide types and pH is shown in Figure 14.
The production of hydrogen sulfide is influenced by SRB, MA, dissolved oxygen (DO), pH value, temperature, hydraulic retention time, oxidation-reduction potential (ORP), sulfate concentration, etc. [39,40,41]. In wastewater treatment tanks, these factors are mainly influenced by the raw and auxiliary materials produced by enterprises.
Raw and auxiliary materials of many enterprises, such as food enterprises, are rich in organic matter and sulfur-containing substances, which are more likely to produce toxic gases such as hydrogen sulfide.
If strong acids such as sulfuric acid are added to adjust the pH or the wastewater itself is rich in strong acid substances, significant levels of hydrogen sulfide gas may be produced from wastewater, which is the main cause of many accidents in wastewater treatment tanks. For instance, this gas caused the “5.29” sludge tank accident at Nanyang Lvyuan Ecological Protection Co., Ltd. in Nanyang, Henan Province in 2024 [42], the “1.10” wastewater collection tank accident at Shaoxing Fuqiang Hongtai Co., Ltd. in Shaoxing, Zhejiang Province in 2023 [43], and the “9.14” wastewater treatment tank accident at Zhangye Yaobang Chemical Technology Co., Ltd. in Zhangye, Gansu Province in 2020 [44]. These accidents were caused by adding strong acids such as sulfuric acid and hydrochloric acid into wastewater treatment tanks, resulting in a large concentration of hydrogen sulfide gas rapidly overflowing in a short period, leading to poisoning accidents and a large number of casualties.

4.3. Prevention and Control of Gas Hazards in Wastewater Treatment Tanks

Regulations and standards for confined-space working safety have been published in countries such as the United States [45], Canada [46], the United Kingdom [47], Australia [48], and China [49]. These regulations require workers to be trained and qualified to work in these environments. Workers must understand the hazards present in tanks and be trained to safely use operating equipment, emergency rescue equipment and related facilities. Before entry, oxygen, toxic and harmful gases, and flammable and explosive gases must be detected. If the gas levels do not meet the necessary safety requirement, ventilation should be performed to eliminate toxic and harmful gases. At the same time, workers must wear appropriate personal protective equipment when entering confined spaces.
When workers enter a wastewater treatment tank, even if hydrogen sulfide gas has not been detected before entry, the solid–liquid mixed state of the wastewater treatment tank will gradually enter a suspended state due to the disturbance, which may lead to the transformation of liquid hydrogen sulfide in the sludge mixture into gaseous hydrogen sulfide, resulting in the release of a large amount of hydrogen sulfide. Many accidents occur because such agitation accelerates the overflow of hydrogen sulfide, leading to personnel poisoning; for example, in the “2.15” accident at Shuangzhou Paper Co., Ltd. in Dongguan, Guangdong, in 2019 [50], hydrogen sulfide was released into the air in the wastewater regulating tank due to violent disturbances caused by high-pressure water used for fire-fighting during the dredging operation, resulting in seven deaths. Therefore, when entering the wastewater treatment tank for dredging operations, operators must wear appropriate respiratory protective equipment. A type of positive-pressure tight-fitting full-face piece respirator is the first choice, such as an electric long-tube respirator, a high-pressure long-tube respirator or a positive pressure air respirator. However, filter respiratory protective equipment, such as gas masks, should not be worn for the following reasons: Firstly, there are many different types of toxic gases in wastewater treatment tanks, and different toxic gases may be present at the same time. In addition to the common toxic gases such as hydrogen sulfide, carbon monoxide and methane, there may be other toxic gases, such as hydrogen cyanide [51] and phosphine [52,53,54]. According to national standards [5], the occupational exposure limit for hydrogen cyanide is 1 mg/m3, and that for phosphine is 0.3 mg/m3, meaning that even a small amount will cause poisoning. Most of these gases are not tested at present. Filter-type respiratory protective equipment cannot protect the operator from a variety of toxic gases. Secondly, when the concentration of toxic and hazardous gases is too high, they may instantly penetrate the filter element and the respiratory protective equipment may lose its protective effect. Thirdly, filter-type respiratory protective equipment cannot be used under anoxic conditions.
The best way to reduce the risk posed by wastewater treatment tanks is to reduce risk at the source. Firstly, reducing the production of toxic and harmful gases could reduce risk. Wastewater can be filtered to remove some large substances to reduce the accumulation of organic matter before it flows into the treatment system. At the same time, by changing the structure of the tanks, such as changing the bottom of the tank to have a conical shape, the sludge can be easily discharged at the bottom. Meanwhile, to avoid reducing treatment effectiveness, adjusting the pH and adding inhibitors can inhibit the generation of harmful gases. Secondly, optimizing sewage treatment design to reduce the need for entry could reduce risk. For example, organizations should not install equipment in the tank. If doing so is unavoidable, it is better to design the pumps and other equipment to act a lifting device that can be lifted outside the tank for maintenance.

4.4. Limitations

When analyzing confined-space accidents in wastewater treatment tanks, we only obtained accident investigation reports from official websites, so the data do not include all confined-space accidents, and officials did not publish all relevant data for confined-space accidents that occurred during this period. The data may only reflect the situation for these 38 accidents.
When carrying out field tests, we only tested levels of hydrogen sulfide, carbon monoxide, combustible gases and oxygen. Other gases such as phosphine, hydrogen cyanide, ammonia, benzene derivatives and volatile organic compounds that may exist in wastewater treatment tanks were not tested. Although these gases are not the main hazardous gases causing accidents in wastewater treatment tanks, they pose a potential risk to the health of workers. Moreover, the scale of the testing industry is limited, with testing mainly performed in the food manufacturing industry and not covering all industries. The testing results provide a basis for performing risk assessments of wastewater treatment tanks in other industries, but they cannot fully represent the situations in such industries. In the future, it will be necessary to test wastewater tanks used in industries such as papermaking, printing and dyeing, leather processing and beverage manufacturing that may produce wastewater containing large amounts of organic matter. The ranges of the existing instrument also constrain our study, because concentrations of H2S and CO in some tanks exceeded the ranges of our detectors, so the real concentrations are unknown.
Due to the difficulty in contacting companies that can conduct field continuous monitoring, it was only carried out at one company. Therefore, the next step in our research needs to supplement and improve on the continuous monitoring data.
Despite these limitations, our study still provides valuable references for understanding and promoting the development of the safety of confined spaces and offers new directions for future research.

5. Conclusions

The conclusions of this study are as follows:
(1)
In confined-space accidents that occur in wastewater treatment tanks, about 95% are poisoning accidents, mainly caused by hydrogen sulfide. The main purpose of entry is equipment maintenance and cleaning, accounting for 86% of all entries.
(2)
We found that the type and cover status of tanks had a greater impact on the distribution of toxic, harmful, flammable and explosive gases from wastewater treatment tanks. The type of sewage treatment tank with the highest risk level is the regulating tank, followed by the sedimentation tank, collection tank and hydrolysis acidification tank. There are no toxic or harmful substances in uncovered tanks.
(3)
The concentration of hydrogen sulfide in wastewater treatment tanks undergoes dynamic change over time, and is not always stable. The maximum value of hydrogen sulfide concentration is about 70 times the minimum value. This may be caused by internal disturbances such as water discharge and stirring in the tanks.

Author Contributions

C.Y. conceived of this study, conducted the experiments, and analyzed the results. Y.L. contributed to the data collection, analyzed the data, and assisted in interpreting the findings. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the National Key Research and Development Program of China (Grant No. 2024YFC3015001) and the Key Science and Technology Project of the Ministry of Emergency Management of the People’s Republic of China (2024EMST141406).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. A schematic diagram of an enterprise’s wastewater treatment system.
Figure 1. A schematic diagram of an enterprise’s wastewater treatment system.
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Figure 2. The years in which the thirty-eight accidents occurred.
Figure 2. The years in which the thirty-eight accidents occurred.
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Figure 3. The types of wastewater treatment tanks in which the 38 accidents occurred.
Figure 3. The types of wastewater treatment tanks in which the 38 accidents occurred.
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Figure 4. Purpose of entry for the 38 confined-space accidents.
Figure 4. Purpose of entry for the 38 confined-space accidents.
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Figure 5. Concentrations of oxygen and harmful gases in the 222 wastewater treatment tanks.
Figure 5. Concentrations of oxygen and harmful gases in the 222 wastewater treatment tanks.
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Figure 6. A box plot depicting concentrations of oxygen and harmful gases in the 222 wastewater treatment tanks.
Figure 6. A box plot depicting concentrations of oxygen and harmful gases in the 222 wastewater treatment tanks.
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Figure 7. The proportion of gas unqualified tanks.
Figure 7. The proportion of gas unqualified tanks.
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Figure 8. Wastewater treatment tanks with highest concentrations of toxic and hazardous gases. (a) Concentration of CO Higher than 50 ppm. (b) Concentration of H2S Higher than 50 ppm. (c) Concentration of Combustion Gas Higher than 30%LEL.
Figure 8. Wastewater treatment tanks with highest concentrations of toxic and hazardous gases. (a) Concentration of CO Higher than 50 ppm. (b) Concentration of H2S Higher than 50 ppm. (c) Concentration of Combustion Gas Higher than 30%LEL.
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Figure 9. The variation law of the hydrogen sulfide concentration with time (first test).
Figure 9. The variation law of the hydrogen sulfide concentration with time (first test).
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Figure 10. The variation law of temperature with time (first test).
Figure 10. The variation law of temperature with time (first test).
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Figure 11. The variation law of the hydrogen sulfide concentration with time (second test).
Figure 11. The variation law of the hydrogen sulfide concentration with time (second test).
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Figure 12. The variation law of temperature with time (second test).
Figure 12. The variation law of temperature with time (second test).
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Figure 13. The variation law of carbon monoxide gas.
Figure 13. The variation law of carbon monoxide gas.
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Figure 14. The correlation between sulfide species and pH value.
Figure 14. The correlation between sulfide species and pH value.
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Table 1. The range and resolution of the instruments.
Table 1. The range and resolution of the instruments.
Detectors BrandResolution and Range of Four Gases
Hydrogen Sulfide/ppmCarbon Monoxide/ppmOxygen/%VOLCombustible Gas/%LEL
ResolutionRangeResolutionRangeResolutionRangeResolutionRange
MSA10–20010–19990.10–3010–100
MultiRAE10–10010–5000.10–3010–100
Table 2. The parameters of the detectors.
Table 2. The parameters of the detectors.
DetectorsResolution/ppmRange/ppmOperating Temperature/°COperating Humidity/%
Industrial Scientific
GBpro hydrogen sulfide		0.10–500−40–600–99
Industrial Scientific
GBpro carbon monoxide		10–1500
Table 3. The installation position and duration of the detector.
Table 3. The installation position and duration of the detector.
TimesDetectorInstallation Position (Distance from the Entrance/m)Duration
First timeTwo hydrogen sulfide detectors1Four days
Second timethree hydrogen sulfide detectors0.5/1/1.5Two days
One carbon monoxide detectors0.5
Table 4. The harmful gases causing the 38 wastewater treatment tank accidents.
Table 4. The harmful gases causing the 38 wastewater treatment tank accidents.
No.Accident TypeNumber of AccidentsToxic and Hazardous Gas Causing Accidents
1Explosion1Methane
2Anoxic asphyxia1Carbon dioxide
3Poisoning21Hydrogen sulfide
15Not described
Table 5. The levels of oxygen and toxic and harmful gases in different wastewater treatment tanks.
Table 5. The levels of oxygen and toxic and harmful gases in different wastewater treatment tanks.
Type of TanksNumber of TanksO2/%H2S/ppmCO/ppmCombustible Gas/%LEL
Average ValueMinimum ValueAverage ValueMaximum ValueAverage ValueMaximum ValueAverage ValueMaximum Value
Sedimentation tank2920.617.51253.1901.320
Regulating tank2420.113.630200102199911.5100
Collection tank2320.718.24.4272.2423.321
Aeration tank1620.820.26.5675.1411.17
Sludge tank1320.719.92.4112.9382.821
Dissolved air flotation tank820.920.55.1150000
Aerobic tank820.619.8000000
Anaerobic tank820.518.218.12001.5125.824
Oil separator tank620.820.30.681.8003.219
Hydrolysis acidification tank62017.8821708.51639.3100
Contact oxidation tank520.219000000
Clear water tank520.920.9000000
Hypoxia tank520.619.8000000
Reaction tank420.820.4000000
Middle tank420.319.732.51286237.726
Table 6. The average concentrations of oxygen and hazardous gases in wastewater treatment tanks used in different industries.
Table 6. The average concentrations of oxygen and hazardous gases in wastewater treatment tanks used in different industries.
Industry TypeNumber of TanksAverage Gas Concentration
Hydrogen Sulfide/ppmCarbon Monoxide/ppmCombustible Gas/%LELOxygen/%VOL
Food manufacturing industry14112.9235.820.5
Other industries815.670.381.620.7
Table 7. The correlation coefficients between the four gas concentrations.
Table 7. The correlation coefficients between the four gas concentrations.
FactorsCorrelation Coefficient
OxygenHydrogen SulfideCarbon MonoxideCombustible Gas
Oxygen1−0.211 **−0.658 **−0.740 **
Hydrogen sulfide−0.211 **10.0720.355 **
Carbon monoxide−0.658 **0.07210.488
Combustible gas−0.740 **0.355 **0.488 **1
**: It means significant at the level of 0.01 (two-sided test).
Table 8. Wastewater risk assessment indexes and grading criteria.
Table 8. Wastewater risk assessment indexes and grading criteria.
IndexesGrading Criteria
Historical accidents1. No historical accident found: 10 score
2. Accidents have occurred in history: 0 score
1. Maximum concentration of hydrogen sulfide
2. Average concentration of hydrogen sulfide		1. More than 50 ppm: 10 score
2. Between 7 and 50 ppm: 5 score
3. Less than 7 ppm: 0 score
1. Maximum concentration of carbon monoxide
2. Average concentration of carbon monoxide		1. More than 50 ppm: 10 score
2. Between 25 and 50 ppm: 5 score
3. Less than 25 ppm: 0 score
1. Maximum concentration of combustible gases
2. Average concentration of combustible gases		1. More than 30% LEL: 10 score
2. Between 10 and 30% LEL: 5 score
3. Less than 10% LEL: 0 score
Table 9. Risk levels of different wastewater treatment tanks.
Table 9. Risk levels of different wastewater treatment tanks.
Type of TankScoreRisk Level
Regulating tank55extreme high
Sedimentation tank30high
Collection tank30high
Hydrolysis acidification tank30high
Sludge tank25medium
Middle tank20medium
Oil separator tank15medium
Aerobic tank10low
Anaerobic tank10low
Aeration tank5low
Dissolved air flotation tank5low
Contact oxidation tank0low
Clear water tank0low
Hypoxia tank0low
Reaction tank0low
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Yang, C.; Liu, Y. Exploring Oxygen and Harmful Gas Distribution in Wastewater Treatment Tanks of Industrial Enterprises. Appl. Sci. 2026, 16, 1034. https://doi.org/10.3390/app16021034

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Yang C, Liu Y. Exploring Oxygen and Harmful Gas Distribution in Wastewater Treatment Tanks of Industrial Enterprises. Applied Sciences. 2026; 16(2):1034. https://doi.org/10.3390/app16021034

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Yang, Chunli, and Yan Liu. 2026. "Exploring Oxygen and Harmful Gas Distribution in Wastewater Treatment Tanks of Industrial Enterprises" Applied Sciences 16, no. 2: 1034. https://doi.org/10.3390/app16021034

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Yang, C., & Liu, Y. (2026). Exploring Oxygen and Harmful Gas Distribution in Wastewater Treatment Tanks of Industrial Enterprises. Applied Sciences, 16(2), 1034. https://doi.org/10.3390/app16021034

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