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Construction scheme of floating mask for soil membrane of sewage adjustment pool in garbage landfill

Number of hits:1062018-12-07 00:00:00

Construction scheme of floating mask for soil membrane of sewage adjustment pool in garbage landfill
Some landfill sites in China have adopted the HDPE membrane floating mask for sewage treatment tank.
The geotechnical membrane floating cover of the cesspool is a type of geotechnical film that is sealed and connected to the wall of the pool. The membrane floats on the surface of the water to form a closed space surface cover. Relative to the surface rigid cover, it is light in weight, low in cost, simple in structure, and easy in large areas. Construction, Therefore, more and more applications have been made to the surface floating cover of geotechnical membrane. The floating cover of landfill sewage regulating pool mainly includes the following parts: covering system, Anchor system, rainwater harvesting system and air guide system
Floating film is a key part of the entire flotation system. The adjustment pool area is generally relatively large. The covering film has long been in a state of two-way tensile stress. The exposed film is directly exposed to ultraviolet radiation from the sun, and it is also subject to the test of winter cold and low temperatures. The impact of environmental temperature changes is greater. Therefore, the anti-tension and anti-aging properties of the coating film are relatively high, and these two requirements can not be achieved by ordinary HDPE geotechnical membrane materials.
Therefore, it is recommended that the tensile strength of the coating material should be tensile fracture strength, standard oxidation induction time and environmental stress cracking.
In addition to the high requirements for the tensile performance, aging resistance, and corrosion resistance of the membrane using the environment, it is also required that the density of the membrane be preferably smaller than that of water to float on the surface of the water. At present, most HDPE membranes are used as covering membranes at home and abroad. The thickness of the membrane is generally 2.0 ~ 2.5 mm.
1. Anchorage system
Mainly structural measures for anchoring the covered film, and sometimes also anchoring the balanced weight distribution system. For concrete side walls, prebury bolts on the top of the wall or walls, fixed by splint, and pull out the film for prevention, A thicker flange can be made around the membrane. For soil slopes, Anchorage trenches can be fixed at the top of the slope. The depth of the ditch is generally 600 ~ 800mm and the width is generally 800 ~ 1000mm. The concrete should be calculated and determined. Trench backfill material can be compacted clay or low-standard concrete.
2. Rainwater harvesting system
In foreign countries, the membrane pressure tube and the submembrane matching floater and the geotechnical membrane between the two form a fixed location drainage tank. In China, only the pressure pipe is used to form a groove in the membrane pressure pool. Rainwater collection and drainage are generally carried out with light, hand-held pumps. Domestic floating covers allow full film storage. Some projects maintain a depth of 20 to 40 cm on the film, Some people think that proper water storage on the membrane can play the role of stabilizing the flotation membrane. I think that there should be 10 to 20 cm deep water storage on the membrane, which is conducive to avoiding direct sunlight. The film covers the film and delays the aging of the film.
3. Gas guidance system
The landfill leachate in the regulating tank will produce a large amount of gas. If it is not ruled out, it will accumulate under the sealing membrane and swell the membrane to form a large area of drum or a local drum. Safety problems may occur under strong winds. Therefore, These gases must be discharged in a timely manner. First, HDPE collection tubes are installed around the top of the membrane pool to collect the gas in the pool. Then passive exhaust measures or active exhaust measures are used to export the collected gas from the membrane exhaust pipe, directly burning or other treatment.
In the large regulating tank, further measures need to be taken on the trachea setting of the flotation system. That is, the "flotation" of foam is used to form a grid of "exhaust rafts" under the membrane so that the flotation film and the polluted water surface are always held. Complete gas channel, Otherwise, if the methane accumulates between the membrane and the sewage for a long time, it will expand like a balloon. The HDPE open tube with an inner diameter of 200 ~ 300 mm is usually used.
The large volume of landfill sewage regulation tank and the large amount of biogas generated by it may cause local methane accumulation if the gas guide facility is not properly designed or fails, causing a cover drum bag and an explosive safety hazard. When designing the old regulation pool of Shanghai Laogang garbage dump to be stamped, Due to insufficient consideration of biogas drainage capacity, the "drum pack" with a height of 2 to 3 M was once bulged; If the wind blows, resulting in negative pressure suction, it will increase the extent of the drum on the film, plus the impact of the wind on the drum, if the film's tensile strength is not enough, The wind may cause damage to the cover. Later, 4 to 6 air ducts with a diameter of 60 to 80 mm were added to the cover. The drum bag problem is one of the key problems to be studied when designing the floating cover of the sewage adjustment tank in the garbage landfill in the valley and coastal cities. Some measures should be taken to avoid the appearance of drum bags:
(1). As mentioned above, reasonable and flexible design film size, so that the film float smoothly, avoid crease, reduce wind impact on the film.
(2) Design a submembrane gas extraction system. This requires designers to estimate the biogas production and yield of sewage in the regulating tank, and also to estimate the evaporation of water at higher temperatures. Proper selection of pipe diameter, diameter of opening hole and opening rate of trachea and gas collection pipe. The biogas yield is estimated based on the specific conditions of the landfill, for example, the daily water intake of the regulating pool of the Tianziling garbage dump in Hangzhou is about 1500 T, producing biogas 750M per day, Biogas can be passively derived or extracted by the negative pressure of the fan.
(3) The working conditions of the various loads on the membrane(water storage weight, pressure weight pipe weight, film self-weight, pump weight, human and wind load, etc.) and the pressure of the gas under the membrane are considered in a comprehensive manner. Determine a gas pressure to maintain film stability. Set gas pressure release valve appropriately.
4. Thickness of geotechnical film
In the project, HDPE geotechnical film with a thickness of 2.0 to 2.5 mm can be used as the membrane of the floating cover. From the published literature, the thickness of HDPE geotechnical film selected for each project is mostly 2mm. In fact, according to the specific conditions of each project, The thickness of the film is selected by test and calculation. The design service life is 20 years. The surrounding environmental conditions and requirements are as follows:
1) Surface wind speed 129km/h, air temperature -34 ~ 43 °C;
2) The gas pressure under the cover may reach 50 Pa;
3) Gas transport pumps and pipelines can be installed under the cover;
4) The cover can be equipped with precipitation collection and emission equipment;
5) The film itself is easy to splice and maintain sealing;
6) Biogas volume: 104 ~ 119mm / H.
The design of the float cover should take into account the requirement that the film can float smoothly on the water surface from the bottom of the pool to the highest pool water level. The material performance indicators of the geotechnical film should take into account the local climate conditions, the upper membrane load and the submembrane pressure and various conditions of use. It is determined by the necessary tests and calculations. Considering the Environmental Protection anti-odour function of the floating mask and the anaerobic effect of the cover, the adjustment tank is designed as a large anaerobic digestion tank of leachate to give full play to its efficacy, thus greatly reducing the operation cost of the sewage treatment plant.

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