By consulting experts from Beijing Drainage Group and visiting the Beijing Drainage Museum, we learned about the potential safety hazards caused by SRB, the hydrogen sulfide gas SRB produces in sewers, and the current measures for treatment.
Odor, corrosion, and worker safety down the well are all public safety hazards posed by hydrogen sulfide in sewers. The causes and current solutions are displayed in the table below.
| Concern | Reasons | Measures |
|---|---|---|
| Odor | Mainly from septic tanks, dirty mixed flow, and dischared sewage with incomplete deodorization. |
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| Corrosion |
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| Harm workers' health | Hydrogen sulfide is neurotoxic to humans | When dealing with pipeline problems, follow the principle of giving priority to above-ground operations over underground operations. For situations where underground work is necessary, workers must wear work clothes (especially oxygen masks) and various protective gear under safety regulations. |
Table 1. Concerns and Current Measures
Although corroded parts can be repaired, unfortunately, there are no effective treatments for the microorganisms that cause corrosion. Experts from Beijing Drainage Group said that the current cleaning method can only ensure the removal of sludge, but cannot remove the biofilm formed on the pipe wall below the sludge. The treatment of biofilm in sewers is currently a popular research topic, and it is also a future trend in drain maintenance.
To make our topic better address the local problems, we visited the Agricultural and Water Affairs Bureau, Chenghua District, Chengdu, and Chengdu Xingrong Municipal Facilities Management Co., Ltd. to learn about the general design of the drainage network and operation and maintenance status.
The total area of Chengdu's "5+2" urban area is about 6170,000m2, and the municipal drainage pipe network is about 7400km long, of which the rainwater pipe is about 4400km long, the sewage pipe network is about 3000km long, accounting for 40% of the pipe network.
Urban sewage is divided into domestic sewage and industrial sewage. Domestic sewage will be discharged into septic tanks for simple fermentation before being merged into municipal pipes. This part of the sewage pipeline is mainly operated and maintained by property management companies. Industrial sewage must be pre-treated and can only be discharged into municipal pipelines after meeting discharge standards. Municipal drainage pipes adopt strict rainwater and sewage diversion, and the end of the network is the wastewater treatment plant. The purified wastewater will be discharged into natural water systems.
Fig.1 Schematic diagram of urban drainage system
According to Technical specification for inspection and evaluation of urban sewer, sewer problems are divided into two categories: functional damage (e.g. blockage) and structural damage (e.g. pipe rupture, deformation or collapse), and categorized into level 1-4 according to the severity of the disease. Level 3 and above are severe damages that require prompt treatment.
After inspections of 7,400 kilometers of municipal pipelines, 100,000 severe damages of level 3 and above have been discovered, and more than 1,300 kilometers have now been rectified. In addition to the municipal pipe network, according to government statistics, the length of the pipe network in the community exceeds 10,000km, of which 70% have damages.
Two main reasons are contributing to such a high damage rate. First, the pipe material is not as corrosion-resistant as municipal pipelines. Second, different property management companies are responsible for different neighborhoods and attach different importance to pipeline maintenance. The drainage pipelines in many old neighborhoods lack regular maintenance. It indicates that our engineered bacteria may apply to control SRB growth in household pipelines and primary pipelines.
Because the drainage system in Chengdu differs from that in Beijing, we consulted the pipeline maintenance department about Chengdu’s SRB and hydrogen sulfide control measures. The results show that the measures are similar in the two places. At the same time, Mr. Ouyang from Chengdu Xingrong Municipal Facilities Management Co., Ltd. said that Chengdu pipe network maintenance has not yet paid attention to the issue of microbial corrosion. But with the further construction of intelligent pipeline networks, the government will install more monitoring equipment in the pipes. And may gradually pay attention to monitoring and treating biofilm in the sewers.