With the rapid development of national economy and the comprehensive development of municipal infrastructure construction in China, especially the increasing number of environmental protection projects such as sewage treatment plants, a considerable number of sewage treatment plants are built around residential areas, such as anaerobic tanks, sludge concentration tanks and bioflocculation tanks. The environment, style and odor of sewage pools directly affect people's lives and health. In order to care for the people, build a harmonious society and create a beautiful living environment, many cities have asked to cover the sewage treatment ponds of sewage treatment plants for waste gas collection and treatment.

一 Brief Introduction of  Membrane Structure System
Membrane structure is also called fabric structure. It uses soft fabric with excellent performance as material, supports membrane surface by internal air pressure, or uses flexible steel cables and rigid support structure to produce a certain pre-force on membrane surface, thus forming a new space structure system with certain stiffness and can cover large space. Membrane structure can be generally divided into three types: air-supported membrane structure (i.e. inflatable membrane structure), tensioned membrane structure and skeleton-supported membrane structure. They have different structural characteristics, architectural manifestations and are suitable for different applications. The steel-supported reverse suspension fluorocarbon fiber membrane structure is a new type of structure specially designed for the cover of sewage pool. Corrosion-resistant fluorocarbon fiber membrane is selected as the covering material, and the corrosive environment of sewage pool is adapted by reverse suspension. In Haining Dingqiao Sewage Treatment Plant, Yancang Sewage Treatment Plant, Lucun Sewage Treatment Plant, Jiaojiang Sewage Treatment Plant, Jiangyin Nanzha Sewage Treatment Plant and other sewage treatment plants have taken the lead in using steel support and reverse suspension of fluorocarbon fiber membrane to cover the sewage tank, and achieved good results.

二 Characteristics of Fluorocarbon Fiber Membrane Structure for Steel Supported Reverse Suspension
1. Durability

Steel-supported reverse-suspension fluorocarbon fiber membrane structure was successfully applied to SEINNE AMONT sewage tank in Paris in 1999. The plant has 10 circular tanks (55m in diameter) as shown in the figure below. The first settling tank (45m in diameter) of Lucun Sewage Treatment Plant, a similar project built in China, is a live photo of Lucun Sewage Treatment Plant.

In this project, the imported Milo membrane products from Germany were selected. The samples of the membrane materials were collected before use and installation, and the relevant observations were made. These tests confirmed that the fluorocarbon fiber membrane was completely suitable for wastewater treatment environment. The manufacturers also analyzed the effects of chemicals on the damage and aging of membranes. The tests included: (1) soaking in different aqueous solutions containing NH3 and H2S04 for an extra time. (2) The change of tensile strength was detected before and after immersion to evaluate the change of mechanical properties of fabrics. (3) Anti-hydrolysis: strictly in accordance with NFG37122. After the above anti-corrosion test verification, can provide a detailed anti-corrosion durability report, with 15 years of quality assurance. Since its construction in 1999, it has achieved good results. The ingenuity of the steel support reverse suspension fluorocarbon fiber membrane structure lies in the "reverse suspension".

The fluorocarbon fiber membrane with strong corrosion resistance is used to cover the exhaust gas, and the steel structure suspends the fluorocarbon fiber membrane on the outside. In this way, not only the corrosion resistance of fluorocarbon fibre film is fully exerted, but also the corrosion problem caused by the contact between steel structure and corrosive gas is fundamentally solved. Therefore, steel components can be considered according to the grade of common building structure, have 50 years of service life, give full play to the structural performance of steel support, and achieve the perfect combination of structural skeleton and covering material performance.
2. Safety

Steel-supported reverse-suspension CFRP membrane structure has stronger resistance to load. The common structural loads are self-weight, wind load, snow load and earthquake action.

 (1) The self-weight of fluorocarbon fiber membranes is generally only about 1 kg/m2, which belongs to light-weight and high-strength materials. It is especially suitable for large-span ponds such as sedimentation tanks, which minimizes the influence of the self-weight load of covering materials. In addition, a large number of purlins are needed to support the sunshine plate and FRP, and the membrane is a material reflecting the structural behavior through pre-tension, so it can achieve a large span without any supporting rods in the middle, so the weight of steel members is greatly reduced.
(2) The shape of the fluorocarbon fiber membrane is mostly conical, and because of the requirement of gas collection, the wind will slide across the membrane surface of the curve. Referring to the "Code for Load of Building Structures", the figure coefficient of the fluorocarbon fiber membrane is 0.5-0.8, and the data of wind tunnel tests are smaller, which weakens the influence of wind load on the structure. The sewage pool of Jiaojiang Sewage Treatment Plant in Taizhou area adopts steel-supported anti-suspension fluorocarbon fiber membrane structure. It has experienced the strongest typhoon "Kanu" (maximum wind force level 17). The structure is intact and intact, which fully reflects its wind resistance advantages.
Below is a live photo of Jiaojiang Sewage Treatment Plant.

（3）Snow-loaded fluorocarbon fiber membrane structure belongs to tension system.It is necessary to form a relatively large negative Gauss hyperboloid to effectively apply prestressing force.Therefore,the film surface must have a large height difference,that is,a relatively large slope can be formed. In addition, the surface of the membrane is very smooty,which makes it difficult for snow to accumulate on the membrane surface.The snow cover coefficient can be selected according to the height difference annording to the "Code for Load of Building Structures",and skiing test can be done on its own.

(4) Because of the small self-weight of the membrane and the long natural vibration period of the structure, according to the formula of seismic inertia force l(t)=-m[g(t)+t], it can be seen that when the mass m is small and the natural vibration period T is large,the seismic action is very small,so the influence  of seismic action can be greatly reduced.

3.Installation is fast, maintenance is convenient

(1) Installation is fast. Processing of steel supports and membranes is carried out in factories, and processing quality is guaranteed reliably. The site installation time is short, which reduces the occupation of the site. Especially the old pool reconstruction project can adopt integral lifting structure without affecting the operation of equipment inside the pool.

(2) Maintenance is convenient. Due to technological requirements, regular maintenance and inspection of equipment are required. It can be solved by reserving doors and passages on the side membrane. See the drawing of the sewage pool in Paris, it is very convenient to open doors and windows, which can meet any technological requirements.
4.Aesthetics

(1) Flexible and diverse forms. The structure of steel-supported reverse-suspension fluorocarbon fibre membrane can be designed according to the different site conditions and coordinated with the surrounding environment, fully showing the characteristics of flexible structure.

(2) The transmittance of optical aesthetic fluorocarbon film material is about 6%. The structure has transparency and brightness under sunshine, which can meet the needs of maintenance of internal equipment. When illuminated at night, the light can pass through the film material to achieve special architectural effect.

(3) The surface treatment of self-cleaning fluorocarbon fiber membrane has good self-cleaning performance, which limits the accumulation of dust and facilitates maintenance. Its anti-pollution ability is far superior to that of traditional covering materials and can keep the building clean.

(4) Beautiful in appearance, the steel structure skeleton is rough and spectacular, full of masculine force beauty, while the fluorocarbon fiber membrane is elegant in appearance, light and elegant, full of feminine curvilinear beauty, which can be described as both rigid and flexible, complementing each other. In the plain factory area will certainly form a beautiful scenery, become the factory's landmark building, make it full of the flavor of industrial modernization.

5. Comprehensive economic comparison

Economic analysis takes 15 years as a calculation period to compare three kinds of sewage tank capping systems (see capping scheme description). Considering the economy of long-term investment (15 years), the structure form of common carbon steel (reverse suspension)+fluorocarbon fiber membrane is far more economical than the other two structures, and the second replacement only needs to replace the part of fluorocarbon fiber membrane. The other two structures need to be replaced as a whole. As a long-term behavior, the anti-suspension fluorocarbon fiber membrane structure of steel structure should be preferred in a far-reaching sense.
三 Concluding remarks
In summary, the field of gas treatment is still a relatively new topic in China, and the development of Europe has matured. The steel-supported reverse suspension fluorocarbon fiber membrane structure has been widely used in sewage tank capping project. In our country, the traditional way of sewage tank capping has restricted the development of gas collection and treatment because of the defect of anti-corrosion performance. The membrane structure capping with corrosion resistance, light weight and high strength, various shapes and good light transmission performance has achieved ideal results through engineering practice, and it is worth expanding and popularizing.