Review and discussion of domestic and foreign research status of harmful gas control in urban drainage pipes Xu Xiaobing Wang Yiwang Sheping 2, Zhu Dujie 1 (1. School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, Shaanxi; 2. Xi'an Municipal Design and Research Institute Co., Ltd., Xi'an, Shaanxi 710068), analyzed the generation mechanism and hazards of harmful gases in drainage pipes, and introduced the current research status of harmful gas control in urban drainage pipes at home and abroad, with a view to controlling the main harmful gases in drainage pipes, hydrogen sulfide and methane Provide important.

The main function of urban drainage pipes is to collect and transport sewage. Previous studies have focused on hydraulics (such as the rational design and optimization of pipe slope, cross section, and flow velocity) to ensure the effective transportation of solid materials in sewage. In recent years, with the increase in the scale of sewage treatment plants, long-distance transportation has caused the residence time of sewage in drainage pipes to be extended, making harmful gases generated by biological and chemical actions in drainage pipes a problem that cannot be ignored. In this regard, the current domestic and foreign research progress on the control of harmful gases in drainage pipes is introduced, with a view to providing for the control of harmful gases in drainage pipes.

Due to the relatively closed space of the drainage pipe and poor air circulation, it is easy to form an anaerobic environment. In this environment, domestic sewage entering the pipeline degrades complex organic matter and produces harmful gases under the action of a series of flora. The main mechanism is: complex organic matter is converted into small molecule organic matter (mainly VFA) under the action of hydrolytic bacteria, and into hydrogen and ethyl gold under the action of hydrogen-producing acetogens: Ministry of Education "Yangtze River Scholars and Innovation Team" The "development plan" innovation team funded the project (IRT0853) acid, and further produced methane and carbon dioxide under the action of the methanogen (MA). On the other hand, in the presence of sulfate, sulfate reducing bacteria (SRB) in pipeline biofilms or sediments can reduce sulfate to hydrogen sulfide gas. Therefore, drainage pipes with a high proportion of domestic sewage components will produce harmful gases such as H2S, CH4, and C2.

In addition, when industrial wastewater containing organic compounds such as proteins, carbohydrates, oils, fats, organic acids, alcohols, aldehydes, hydrazines, etc. enters the drainage pipeline, these substances will also be decomposed and transformed under the action of microorganisms or with some strong oxidants A chemical reaction occurs, which generates harmful gases such as HCN, O2, and NO2. 10. The harmful gases generated in the drainage pipeline will cause gas accumulation if not properly managed, and leakage will cause personnel poisoning when it reaches a certain concentration, and will explode in the event of an open flame, causing Casualty accident. For example, in the past five years, there have been 22 explosions caused by harmful gases in drainage pipes and manure treatment facilities in the main urban area of ​​Chongqing alone. Methane and hydrogen sulfide are the main harmful gases that cause major harm to the drainage pipe system. In addition, the hydrogen sulfide gas generated in the drainage pipe rises to the upper space of the sewage, which will corrode the pipe and release an unpleasant smell, and will also endanger the health of the personnel of the drainage pipe. Hvitved-acob-sen and other studies have shown that when the total concentration of sulfide in the sewage is 2.0 mg / L, the drainage pipeline will be severely corroded. The EPA has investigated the drainage pipelines of 34 cities and found that the concrete pipeline The corrosion rate is 2.5 ~ 10mm / aH, which mainly promotes the first push of hydrogen sulfide gas. In China, according to incomplete statistics, there were more than 40 casualties in hydrogen sulfide poisoning accidents that occurred in Beijing, Shanghai, and Tianjin in 2004-2005.

2 Research status of controlling harmful gas in drainage pipes 2.1 Domestic research on harmful gas in drainage pipes is still in the exploratory stage, and it is currently mainly focused on monitoring and early warning. The cities that carry out toxic and harmful gas monitoring are currently mainly concentrated in large cities such as Beijing, Guangzhou, and Shanghai. The monitoring methods are manual sampling analysis or portable instrument analysis. In view of the many toxic and harmful gas explosions in Chongqing's drainage pipelines, Chongqing University has devoted itself to the research on the monitoring and early warning system of combustible and harmful gases in urban drainage pipelines in recent years, such as Bao Liang and other infrared sensor technologies and GPRS wireless data transmission The technically designed remote monitoring and early warning system can realize the real-time monitoring and early warning of the concentration of combustible gas in the drainage pipeline. 6. However, there is still little domestic research on the control of harmful gas in the drainage pipeline.

2.2 Due to the installation of specialized ventilation equipment abroad, there are few explosions caused by excessive harmful gas in drainage pipes in developed countries. However, in terms of the control of harmful gases (especially hydrogen sulfide and methane) in drainage pipes, a lot of research is still conducted abroad. The control methods are mainly divided into three categories: â‘ Increase the redox potential of the pipeline water environment, control the generation of sulfide and methane; â‘¡Increase the pH value of the pipeline water environment, inhibit the activity of MA and SRB; â‘¢Add metal salt to control sulfide And methane.

2.2.1 Increasing the oxidation-reduction potential The main ways to increase the oxidation-reduction potential of the pipeline water environment include injecting oxygen, adding nitrate or nitrite.

Injecting oxygen into the drainage pipe can oxidize the sulfide generated in the sewage, thereby reducing the release of molecular hydrogen sulfide in the pipe. Gutierrez simulated the pressure pipeline reactor to study the effect of oxygen injection on the biofilm activity of the pipeline, and found that injecting high concentration oxygen at the inlet of the reactor can reduce the total amount of sulfide emissions at the outlet by 65%. However, regardless of the oxygen in the liquid phase How high is the concentration, and sulfide is still formed in the deep layer of the biofilm, indicating that oxygen cannot inhibit the generation of sulfide. When the oxygen is exhausted, the accumulation of sulfide will immediately recover. Therefore, oxygen has no toxic effect on SRB in the biofilm.

Since oxygen is relatively inexpensive, for pressure pipes with large diffusion power, injecting oxygen has certain appeal. However, because the aerobic environment upstream of the pipeline increases the concentration of sulfate in the downstream pipeline, which promotes the activity of SRB in the biofilm of the downstream pipeline, therefore, the method of oxygen injection is used to control the production of hydrogen sulfide in the entire pipeline. limited. However, due to the sensitivity of MA to oxygen, the injection of oxygen can have a significant inhibitory effect on the generation of methane gas.

The addition of nitrate can increase the oxidation-reduction potential of sewage, and can effectively suppress the production of sulfide and methane. The mechanism is that the addition of nitrate can stimulate the autotrophic denitrification process, increase the activity of denitrifying desulfurization bacteria (NR-SOB), and thus reduce the accumulation of sulfides.

At the same time, denitrification increases the pH of the liquid phase, thereby reducing the amount of hydrogen sulfide released into the gas phase. The control of nitrate on methane may be due to the formation of denitrification intermediate products (NO and squad 0) and have a toxic effect on MA 08, or it may be caused by nitrate changing the redox potential and reducing the activity of MA. Bentzen et al. Added 10mg / L of NO3- -N to a section of pressure pipeline with a length of 2.4km to reduce the concentration of sulfide from 4.2mg / L to a pressure pipeline with a length of 0km ~ 4d and found that the addition of nitrate Reducing the concentration of sulfide from 10 ~ 20mg / L to 2 ~ -N can fully suppress the generation of sulfide. 111. Mohanakrishnan and other nitrates are continuously added at the beginning of the pressure pipeline for 5 months to vulcanize the outlet The content of the substance is reduced by 66% M. Similar to oxygen injection, nitrate has no inhibitory or toxic effect on SRB. The long-term addition of nitrate actually stimulates the activity of downstream SRB. Only continuous addition can ensure the control of sulfide. Similarly, when the nitrate is exhausted, the activity of MA will be restored. M. Nitrite added to the drainage pipeline can inhibit the activity of SRB for a long time, greatly reducing the sulfate reduction and methanogenic capacity of the biofilm. Mohanakrishnan et al. Added -N to the simulated sewage pipeline for 25 days continuously, and found that the sulfate reduction capacity and methane production capacity of the biofilm were significantly reduced. The system had no sulfide and methane accumulation. After stopping the dosing, the activity of SRB and MA was 2.5 months M.iang et al.'S research showed that the concentration of nitrite that inhibits methane production may be lower due to the sensitivity of MA. In short, the injection of oxygen, the addition of nitrate or nitrite The methods can control the generation of harmful gases in the drainage pipeline to varying degrees, and will not cause serious environmental pollution, but the above control methods increase the consumption of volatile fatty acids while increasing the redox potential of the pipe network system, thus Downstream biological denitrification and phosphorus removal have adverse effects. In general, oxygen injection is more suitable for pressure pipelines, while nitrate and nitrite have higher solubility in water and can be used in gravity pipelines. Because the consumption rate of nitrite is lower than the consumption rate of nitrate, it may require less dosage and can be intermittently added, which can save processing costs. 1314. Therefore, the sulfide in the drainage pipe is controlled by adding nitrite The production of hydrogen and methane has great potential.

2.2.2 Increasing the pH value Hydrogen sulfide in the sewage pipeline exists in the form of S or HS-, depending on the pH value M of the sewage, increasing the pH value can greatly reduce the concentration of dissolved molecular hydrogen sulfide. Studies have shown that when the pH value is 8.0, only about 8% of the sulfide exists in the form of H2S; when the pH value is 9.0, H2S drops to less than 1%. In addition, increasing the pH value can increase the concentration of free ammonia in sewage , Disturbing the proton power and the homeostasis of MA, thereby inhibiting the growth of MA. 16. Gutierrez et al. Added sodium hydroxide to the simulated pipeline reactor to study the effect of pH on the activity of SRB and MA in biofilms. Compared with the reactor (pH value of 7.6), the activity of SRB in the biofilm was reduced by 30% and 50% at pH values ​​of 8.6 and 9.0, respectively, and there was no accumulation of methane. At the same time, they added magnesium hydroxide to the pressure pipeline, and found that high pH (9.0 ~ 9.3) can significantly reduce the concentration of soluble sulfide, and almost no methane produced E7. However, continuous addition of alkali substances to maintain the pipeline The high pH value will increase the cost of harmful gas control and will also interfere with downstream sewage treatment. Therefore, this method needs to be further improved before being applied.

2.2.3 Adding metal salts Adding metal salts (iron, zinc, lead and copper salts, etc.) to the sewage can cause sulfides to precipitate in the form of insoluble metal sulfides8, of which iron salts are widely used in the control of drainage pipes Sulfide generation. Fe2 + reacts with sulfides to form ferrous sulfide precipitates, thereby reducing the generation of hydrogen sulfide. Fe3 + can also effectively reduce the generation of hydrogen sulfide in the pipeline. The mechanism is that Fe3 + first converts the sulfide to elemental sulfur through chemical oxidation, and then reduces it to Fe2 +, and then continues to precipitate sulfide. Zhang et al. Added Fe3 + to the simulated pressure pipeline and found that Fe3 + not only can precipitate sulfide, but also 1mg / Inhibit the activity of SRB and MA in the pipeline biofilm, reduce the sulfate reduction rate and methane production rate by 60% and 80%, respectively. 120. Firer and other studies found that in order to reduce the concentration of sulfide in sewage to 0.1 mg / Below L, when using Fe2 +, the ratio of the minimum amount of Fe and S is about 1.3: 1; when using Fe3 +, the minimum amount of Fe and S is actually applied, and the hydrolysis that occurs after the addition of iron salts needs to be considered Reaction, because the pH value of urban domestic sewage is 6.8 ~ 8.5, Fe3 + can be quickly hydrolyzed to Fe (OH) 3, so iron salt needs to be added in excess. In addition, although the solubility of ferrous sulfide is very small, it is also difficult to reduce the concentration of soluble sulfide to below 0.2 mg / L.

3 Conclusion Controlling the generation of harmful gases (mainly hydrogen sulfide and methane) in drainage pipes is not only important for preventing the poisoning and explosion incidents caused by it, but also has certain practicality for effectively reducing the maintenance and repair costs of drainage pipes. value. Although a large number of foreign countries have conducted research on the control methods of harmful gases in drainage pipelines, many methods need to be further improved and optimized in practical applications due to the gap between experimental simulation pipelines and real drainage pipelines, which is also the future work Need further research.

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