NOVELTY - Fermented methane liquid is mixed with flocculating agent and separated to obtain solid with phosphorus. The pH of supernatant is regulated and remaining phosphorus nutrient is recovered. The lime liquid supernatant is blown off, and nitrogen nutrient of methane liquid is recovered in absorption loop of dilute ammonium sulfate solution. Ammonium sulfate is obtained by centrifugal separation. The blown-off methane liquid is aerated and purified to purify methane of fermenting system, and pH value is regulated, and returned to the fermenting system.    USE - Recovery of composite nutrient of nitrogen and phosphorus from methane liquid, and primary methane purification.    DETAILED DESCRIPTION - Methane liquid is pressurized in a methane-liquid storage groove to enter a flocculation reaction tank, and uniformly mixed with a flocculating agent measured and added to a flocculating agent feeding system to enter a flocculation gun barrel. Solid component with phosphorus is separated, allowing to stand still for depositing flocculation precipitate in flocculation gunbarrel. The liquid supernatant is overflown to a lime reaction tank. The deposited flocculate with phosphorus is dewatered using a solid-liquid separation system to obtain a solid substance with phosphorus. The remaining phosphorus is recovered and separated by lime. Methane liquid which enters the lime reaction tank and lime from lime addition system are uniformly mixed, and the pH is regulated to 9-11. Phosphorus nutrient which is residual in methane liquid is recovered, and solid with calcium and phosphorus is obtained by dewatering. The liquid supernatant enters the lime liquid supernatant storage groove. Methane liquid of lime liquid supernatant storage groove enters the tower through the upper portion of absorption tower after being pressurized to contact with air which is boosted from the tower bottom, and heated by a heater and pressurized by an air heater on the tower plate. Ammonia of methane liquid is absorbed by air to enter scouring tower along air. The blown-off methane liquid enters an aeration groove. Dilute ammonium sulfate solution is sprayed from top of scouring tower to contact with air with ammonia boosted from the tower bottom in countercurrent to absorb and recover ammonia of air. Purified air is pressurized via the heater and draught fan for recycling use. The concentration of ammonium sulfate solution reaches a certain concentration, and separated via a centrifugal separation system to obtain ammonium sulfate. Pressurized methane of the front fermenting system is introduced into the aeration groove for aeration, and absorbed to remove carbon dioxide and hydrogen sulfide after thorough contact with gas liquid. The pH of methane liquid is reduced to neutral and returned to the front fermenting system for recycling use.