Using PVDF or PTFE membranes has the characteristics of weatherability, good antimicrobial property, good elasticity and self-cleaning, which can effectively get rid of the disadvantages of traditional structures, not only ensure the corrosion resistance, air tightness and structural safety, but also make the steel structure skeleton, including joints, completely isolated from the corrosive environment, so as to make the building safe and applicable. And durability is guaranteed. Nowadays, many domestic sewage treatment plants are constructed in this way. Membrane materials are generally about 1000g/m2, and the tensile strength can reach 4500-5500KN/5CM, and the structure does not need secondary structure support, so the overall weight of the structure is effectively controlled, thus reducing the damage of the structure to the concrete pool. Because of the need of prestressing in the structure tension system, we design a larger height difference of the membrane surface, which forms a larger negative Gauss surface, which also solves the problem of water accumulation and snow accumulation. Membrane structure node processing considerations: Joint connection requires sufficient strength, stiffness and stability, must ensure that the node can offset the reliable load transfer in various working conditions, not prior to the main material and construction damage. The connection of the joints should also maintain flexibility and freedom, and the stress released by the deformation and vibration of the membrane structure under wind load must be guaranteed. Joint design should also avoid stress concentration at the joint of membrane system and steel structure, and avoid sharp prominent points at the joint.
Membrane structure is mainly covered by flexible coated fabric, which supports membrane stiffness by air pressure difference between indoor and outdoor or produces certain prestressing force by connecting steel cables with rigid support structure, thus making the whole building or structure form a relatively rigid space structure. Some scholars also call membrane structure soft sculpture.
With the industrialization process, a variety of pollution sources are emerging. When the wastewater discharged from the source of pollution can not meet the discharge standard because of the high concentration or total amount of pollutants, which will make the water environmental quality or function target not meet the requirements, it must be strengthened manually, so that the wastewater treatment plant emerges as the times require. Although China's sewage treatment industry started relatively late, in recent years, with the rapid economic development and the improvement of people's living standards, it has promoted the environmental engineering industry represented by sewage treatment.
Sewage treatment process is divided into three stages: first stage treatment through mechanical treatment such as grille, sedimentation and air flotation, removal of stones, sand and grease contained in sewage; second stage treatment: biological treatment, pollutants in sewage are degraded and converted into sludge under the action of microorganisms; three-stage treatment: advanced treatment of sewage, including removal of nutrients and disinfection of sewage by chlorination, ultraviolet or ozone technology.
Based on the treatment process, it is necessary to construct a structure which can complete the mixing, flocculation, sedimentation and other processes, such as grille, sand sink, initial condensation tank, aeration tank, etc.
Although the location of sewage treatment plants keeps a certain distance from urban industrial areas, living areas and rural residential areas, the distance should not be too far, so as to avoid increasing the degree of pipelines and thus increasing the operating costs. This has caused some damage to the environment due to the escape of various toxic and harmful or flammable gases, including hydrogen sulfide, ammonia, methyl mercaptan, methyl sulfide, methane, chlorine and so on. So the closed collection of toxic and harmful gases in the pool can obviously restrain the damage of the gas to the environment from the source, and the capping of the pool is obviously one of the more effective methods for the closed collection of gases (Fig. 1).

In the early stage, common carbon steel or austenitic stainless steel were used to make support skeleton, which was covered with sunshine plate, PC endurance plate, glass fiber reinforced plastic, glass and so on. Carbon steel is very easy to be corroded and destroyed because of its long-term difficult corrosion environment, so the life of buildings and structures is very short. Although austenitic stainless steel can effectively solve the problem of corrosion resistance, its cost is relatively high, and because the secondary structure outside the main structure increases the self-weight, the amount of steel used is greatly increased, which will cause excessive load on the pool and easy to cause concrete cracking.
Successful application of membrane in wastewater treatment plant opens a new chapter of pond cover. The main feature of this structure is that the main structure is a common carbon steel external reverse suspension membrane structure. Nowadays, many domestic sewage treatment plants are constructed in this way.