NOVELTY - A multifunctional large-size combined type methane tank comprises large-size methane generating pool having long cement pipe, and feeding and discharging ports provided with rubber pad and precast cover; methane storage pool; spare methane storage pool; cover plate; methane pipelines provided on the three pools and equipped with valves; concrete columns provided in the storage pools; water inlet and outlet switches; methane-pressure meter; pressure bag; methane switch; flexible connector and light aerating container.    USE - Multifunctional large-size combined type methane tank for use in village field, especially in the field which is far away from the house.    ADVANTAGE - The combined type methane tank can automatically store methane, aerate and automatically fill. With the methane tank, local materials can be used, the fed materials can be processed and discharged locally, the labor intensity is saved, and the reuse of renewable sources of energy is realized.    DETAILED DESCRIPTION - A multifunctional large-size combined type methane tank comprises a large-size methane generating pool built by using concrete or precast slab underground of the field. An arc precast slab is covered on the pool surface of the methane generating pool. Two ends of the pool surface are covered with big and small feeding and discharging port precast slabs. A rubber pad and a precast cover are arranged on the feeding and discharging port on the precast slabs. A methane pipe is fixed on the upper end of the big port precast slab. The pool bottom and the pool wall at the big port end are inclined. A long cement pipe is fixed on the pool wall, and the lower end of the long cement pipe is communicated with the pool bottom while the upper end is exposed on the ground. The arc precast slabs, the surfaces of the two feeding and discharging port precast slabs, the pool wall, the long cement pipe and the methane pipe are cased integrally by using a steel bar concrete. A methane storage pool and a spare methane storage pool are built at the position around the methane generating pool, 5 meters away in the same method. The bottoms of the methane storage pool and spare methane storage pool are respectively fixed with an upright concrete column. The upper ends of the pool walls of the methane storage pool and spare methane storage pool are respectively fixed with two or one methane pipe. A cover plate is used to cover the pool surface, and a small hole is provided in the middle of the cover plate, through which the concrete column is inserted. The surface of the cover plate of the two methane storage pools, the top of the concrete column, the methane pipe and the pool wall are cased integrally using the steel bar concrete. A water inlet switch and a water outlet switch are fixed on part of the long pipe that is exposed on the ground. The upper ends of two methane pipes on the upper end of the methane storage pool are connected to three-way valves. One three-way valve is respectively connected to a methane switch and an exhaust one-way valve, and then to the methane pipe connecting the three-way valve to the methane generating pool. Two sides of the other three-way valve are connected to a pressure-limiting valve and a methane inlet one-way valve and then to the methane pipe provided on the spare methane storage pool. The upper end of the other three-way valve is connected to the ground. A methane-pressure meter, a pressure-limiting valve, an exhaust one-way valve, a line type pressure bag, the methane switch, a flexible connector and a light aerating container are connected on the ground. In operation, the raw materials are fed from the feeding and discharging port at two ends of the methane generating pool, the rubber pad and cover are covered, and the surfaces of the three pools are buried in soil to form the multifunctional large-size combined type methane tank.