NOVELTY - Mixture precipitating device for heating by waste heat comprises a mixing tank and at least two precipitating tanks, where the fully-enclosed separation walls are arranged the adjacent precipitating tanks and between the precipitating tanks and the mixing tank; one end of a top part of every separation wall is opened with an upper overflow port, the adjacent upper overflow ports are located at two ends of the same diagonal line, and the mixing tank is provided with a mixing blender and a feeding pump.    USE - The mixture precipitating device is useful for heating by waste heat (claimed).    ADVANTAGE - The device can not only improve the temperature and concentration of the concentrated slurry and be stronger in energy conservation, but also diminish the volume of a fermentation tank by one fold, can reduce capitalized cost, and can guarantee the later heating of the fermentation tank to be faster and higher.    DETAILED DESCRIPTION - Mixture precipitating device for heating by waste heat comprises a mixing tank and at least two precipitating tanks, where the fully-enclosed separation walls are arranged the adjacent precipitating tanks and between the precipitating tanks and the mixing tank; one end of a top part of every separation wall is opened with an upper overflow port, the adjacent upper overflow ports are located at two ends of the same diagonal line, and the mixing tank is provided with a mixing blender and a feeding pump; the mixture precipitating device further comprises a biogas slurry waste heat conducting tank which is in contact with a fully-enclosed heat conducting device and the mixing tank; a middle part at the top of the biogas slurry waste heat conducting tank is provided with a heat quantity uniform distributing blender, an inner part is equipped with a water sealer and a biogas slurry pump, the water sealer is communicated with an overflow pipe, and the other end of the overflow pipe is communicated with an overflow port in a fermentation tank; and the biogas slurry pump is arranged adjacent to the water sealer and is connected with a solid-liquid separator outside the biogas slurry waste heat conducting tank. An INDEPENDENT CLAIM is a method for using mixture precipitating device for heating by waste heat comprising: (a) feeding sewage from farms into the grit chamber, flowing the sewage into the mixing tank via a sewage grid, flowing sewage into the precipitating tanks one by one via the upper overflow port when the sewage in the mixing tank reaches the upper overflow port, and then precipitating the sewage in the mixing tank and the precipitating tanks; (b) precipitating process, separating sewage into clear water and fermentation solution, floating clear water upwards, sinking the fermentation solution, flowing the floating clear water to the next precipitating tank via the upper overflow port, blocking the sunk fermentation solution by the one way valves to prevent the fermentation solution from flowing back into the next precipitating tank, synchronously, blocking the fermentation solution precipitated at every dead angle at the bottom of the tank to the middle part at the bottom of the tank by the vertical inclined base in the outmost precipitating tank; the concentration of the fermentation solution in the mixing tank is highest and is reduced from the mixing tank to the outmost precipitating tank in sequence; (c) in precipitating process, sending the hot biogas slurry discharged by the fermentation tank to the water sealer by the overflow pipe, flowing the hot biogas slurry to the biogas waste heat conducting tank via the water sealer, conducting the heat quantity of the hot biogas slurry in the biogas waste heat conducting tank to the mixing tank by the heat conducting device, after conducting the heat quantity, pumping the hot biogas slurry after being cooled to the solid-liquid separator outside the biogas waste heat conducting tank so as to perform solid-liquid separation via the biogas slurry pump; the heat quantity uniform distributing blender mixes the hot biogas slurry entered into the biogas waste heat conducting tank by the water sealer and the primary cold biogas slurry in the biogas waste heat conducting tank so as to reach uniform heat quantity; and (d) after precipitating, starting the mixing blender and the feeding pump, pumping the fermentation solution in the mixing tank into the fermentation tank by the feeding pump, flowing the fermentation solution from the outmost precipitating tank to the mixing tank in process of extracting fermentation solution, and the one way valves at the bottom of the separation walls play a role in preventing the fermentation solution from flowing back while flowing the fermentation solution.