with the continuous development of ecological civilization and urbanization, urban underground sewage treatment plant has become an important direction of comprehensive utilization and sustainable development of underground space. therefore, it is of great practical significance to analyze the development status and sludge treatment of municipal underground sewage treatment plants.
main problems in construction of underground sewage treatment plant
although the underground sewage treatment plant has been well developed, there are still the following main problems in the construction process: first, the planning and construction requirements are high, the main structure of the underground sewage treatment plant is built underground, once the construction is completed, the difficulty of reconstruction and expansion will be greatly increased. therefore, in the stage of engineering feasibility study and the early stage of design, the overall planning and construction requirements of underground sewage treatment plants are relatively higher; secondly, the design and construction is difficult and the construction cost is high. the construction of underground sewage treatment plant is composed of many sub-projects, which involve ground landscape, excavation and support of deep and large foundation pits, ventilation and lighting, etc. these sub-facilities often increase in scale or improve construction standards according to actual conditions. the corresponding design and construction are more difficult in quantity and content, and the cost is also increased accordingly. thirdly, the risk factors are high. underground sewage treatment plants may have toxic gases, sewage leakage and other situations, once they occur, the consequences will be very serious; finally, the ground landscape design is single. at present, china's sewage treatment plant landscape construction is relatively single, and the design concept is relatively backward. large-scale construction of gardens, wetlands, leisure and entertainment places and parks is relatively small, and the overall function is not perfect.
ii. present situation of urban underground sewage treatment plant construction at home and abroad
large-scale underground drainage and sewage treatment systems in many foreign cities, such as finland, sweden, norway, the united states and the united kingdom, have made good progress. the development and construction of urban underground sewage treatment plant has gone through a long time. sweden has a large underground sewage treatment plant in the city. the sewage treatment plants of the whole sewage system are all built underground. they are the world's leading in treatment efficiency and scale. stockholm alone has a large drainage tunnel of 200 km. for example, the design of kappala sewage treatment plant usually uses many parallel chambers to purify water. generally, the length of the chamber is 300 m and the cross section is 100 m2. each chamber of the treatment plant can be satisfied. most of the underground treatment plants in japan also adopt the medium tunnel type, such as yeshan town sewage treatment plant in kanagawa county. it occupies a small area of land and saves a lot of land resources. in addition, although the cost of tunnel construction is relatively high, there is no need for basic works and backfilling, and the overall cost is not much different. on the other hand, the airtight space of the tunnel has a better sealing effect on the toxic gases discharged from sewage, which may cause less environmental pollution. domestic underground sewage treatment mainly refers to the concept and technology of foreign underground sewage treatment. in the process of domestic application, many key problems have been found and solved, and a set of theory and technology has been formed, and good results have been achieved. for example, the successful application of hong kong stanley underground sewage treatment plant, shenzhen buji sewage treatment plant, guangzhou jingxi sewage treatment plant and kunming no. 10 sewage treatment plant has proved the important development status and huge market potential of underground sewage treatment plant in sewage treatment.
3. analysis of sludge treatment in underground sewage treatment plant
underground sewage treatment plants often leave a large amount of sludge after sewage treatment discharge, and there are a large number of pollutants in these sludge, including harmful heavy metals, pathogenic substances and non-decomposable chemical substances and so on. the sludge produced by most sewage treatment plants in our country is directly landfilled or dumped, which not only pollutes the ecological environment, but also threatens the safety of human life. therefore, how to treat sludge reasonably has become an urgent problem to be solved.
(1) typical sludge treatment technologies
at present, the commonly used sludge treatment technologies at home and abroad mainly include the following six categories: anaerobic digestion, i.e. oxidation treatment by a variety of microorganisms, can transform high-energy organic matter into low-molecular substances, and reduce the toxicity of sludge. aerobic fermentation of sludge is mainly to decompose organic matter into humus through various bacteria under aerobic conditions. sludge lime is stable, that is, adding a certain amount of lime into the sludge, the lime reacts with the sludge to form solid compounds, thus passivating heavy metals. in addition, a large amount of heat will be generated in the process of chemical reaction to achieve bactericidal effect. thermal drying technology is a process of dewatering sludge through heat transfer with heat medium. after thermal drying, the microorganisms in the sludge are basically inactivated, and the dry sludge can be used as fertilizer and building materials. in addition, sludge incineration and bioleaching are also common sludge treatment technologies. sludge incineration can make the sludge harmless and reduce as much as possible, but the process requirements such as pollutant gas emissions are higher. bioleaching mainly relies on special bacteria such as thiobacillus to oxidize sludge deeply, remove heavy metals, and then reduce the toxicity of sludge. the technology is simple and low cost, and has been applied in major sewage treatment plants [2].
(2) practical application of engineering
the sewage treatment scale of one underground sewage treatment plant in china is 110,000 m3/d. the main residual sludge components after sewage treatment are excess sludge, primary sedimentation sludge, cement regionalized sludge and leakage sludge. according to the preliminary statistics, the sludge amount is 2346 m3/d, the sludge content is large and the production speed is fast, so it is very important to select a reasonable sludge treatment technology. as mentioned above, there are many commonly used sewage treatment technologies at present, but the selection of suitable methods for this treatment plant needs to be combined with the actual situation of the project, such as project cost, operation, and so on, while making necessary improvements and derivations. according to the actual situation of this project, three technologies of conditioning and pressing sludge drying, sludge calcium drying and bioleaching drying were preliminarily determined and compared.
(3) contrastive analysis
from the aspects of water content, solid content of sludge before and after sludge treatment, environmental impact before and after treatment, and economic cost of engineering construction and operation, the three processes of conditioning and pressing sludge drying, sludge calcium drying and bioleaching drying were compared and analyzed. from the point of view of technical performance, conditioning and pressing sludge drying technology is superior to sludge calcium drying technology and bioleaching drying technology. from the point of view of economic cost, the operation cost of sludge plus calcium drying technology is the highest, but the engineering cost is the lowest, while that of bioleaching drying technology is the lowest and the operation cost is the highest. on the other hand, considering the better combination of bioleaching and drying with the original design process, the treatment technology and method effect are in line with the requirements of the current environment, laws and regulations in china. therefore, a comprehensive analysis of the treatment plant using bioleaching and drying sludge treatment technology [4].
conclusion
in short, although the construction and development of urban underground sewage treatment plants in china is becoming more and more mature, there are still some problems. it involves planning and construction, investment and operation, landscape design and so on. in future construction, macro-long-term and reasonable planning is fundamental, and keeping vigilance is the most important thing. in addition, continuous innovation, strengthening and rational utilization of land resources and optimizing the design of land landscape need further research and wide application. in addition, through comparative analysis of different sludge treatment technologies, it is considered that bioleaching and drying sludge treatment technology is more suitable for sludge treatment in underground sewage treatment plants. the research results can provide some reference for similar projects. finally, the benefits brought by urban underground sewage treatment plants to the construction of ecological civilization and the development of human environmental friendliness are enormous. large-scale construction and application will be the main development direction of the city from now on to the future. at the same time, with the new rise of the city's comprehensive pipe corridor and sponge city in recent years, reasonable combination of underground sewage treatment plant and comprehensive pipe corridor is also a new technological revolution.
source: polaris environmental protection network
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