NOVELTY - A sterile and particle free permeate stream is separated by ultrafiltration from a biogas reactor (16) in which anaerobic fermentation of liquid waste takes place. The permeate stream is treated in an ammonia stripper at an elevated temperature to result in an ammonia fraction and nutrient salt fraction.    USE - For treating liquid organic waste particularly liquid manure from intensive production of pigs. The liquid waste includes waste water from households, industry or agriculture, including sewage from institution such as hospitals.    ADVANTAGE - By using the liquid waste treatment method advantageous results are obtained and methane enriched gas is produced.    DETAILED DESCRIPTION - The nutrient salt fraction is separated into a fertilizer concentrate fraction and a water fraction by reverse osmosis unit (42). The pH of the nutrient salt fraction is reduced to below about 7.0 before reverse osmosis. Fibers and particles having a size larger than about 1mm are separated from the waste liquid prior to fermentation in the biogas reactor. The bacteria and suspended organic material retained by ultrafiltration unit (22) are returned to the biogas reactor. Ultrafiltration is performed using a tubular ultrafiltration membrane. Ammonia stripping is performed at a temperature of about 40 degreesC preferably at least about 50 degreesC more preferably about 60 degreesC or 70 degreesC. The ammonia stripper is heated using the energy obtained by burning of biogas produced by the biogas reactor from flue gas. Ammonia stripping is performed using a combination of reduced pressure of about 0.25-0.75 bar e.g. about 0.5 bar e.g. and a temperature of about 65-85 degreesC e.g. about 70-80 degreesC. The liquid waste or manure is from livestock production. INDEPENDENT CLAIMS are also included for the following: (i) A plant for treatment of organic waste material in liquid form which has unit for returning bacteria and suspended organic matter retained by ultrafiltration unit to the biogas reactor. A compressor evaporator is installed to provide both reduced pressure and heat. The plant further has units for automatically monitoring the fermentation process and regulating the feed of untreated liquid waste into the biogas reactor. The fermentation process monitoring unit has section for automatically measuring volatile fatty acids, coupled to section for adjusting the organic matter content in the biogas reactor to a desired set point value based on measured values for volatile fatty acids. A unit for determining the amount of liquid in the reactor is coupled to a unit for adjusting the amount of liquid in the reactor to a desired set point value. The amount of liquid determining unit has a pressure gage located at or near the bottom of the reactor vessel, the pressure gage is coupled to a valve downstream of the ultrafiltration unit that automatically allows permeate to be returned to the reactor as necessary in order to maintain a desired level of liquid in the reactor. (ii) A method for producing biogas enriched in methane which involves subjecting liquid from biogas reactor to ultrafiltration to result in a permeate fraction enriched in CO2 and a methane enriched fraction. The methane enriched fraction is returned to the biogas reactor to result the biogas having an increased methane content. Ultrafiltration is performed using tubular ultrafiltration membrane with mean molecular weight cut off of about 10000-30000 Dalton, preferably 15000-25000 Dalton. The biogas mixture has about 70%-90% or 75%-80% by volume methane and at most about 10%-30% or 20%-25% by volume CO2.    DESCRIPTION OF DRAWING(S) - The figure shows the basic components of the treatment plant.    Biogas reactor; (16)    Ultrafiltration unit; (22)    Reverse osmosis unit (42)