Method for the production of organic fertilizer or organic raw material for, inter alia, the manufacture of yeast, based on ammonium and/or nitrate, said organic fertilizer or organic raw material having a nitrate concentration between 2 and 25% by weight, comprising the steps of: - providing an organic source of ammonia gas, in particular air contaminated with ammonia gas; - dissolving the ammonia gas in water to form an NH 3 solution in water; - oxidize the ammonium hydroxide in the NH 3 solution in water with air to nitrite by means of bacteria, and then oxidize the nitrite with air to nitrate by means of bacteria to form an acidic nitrate solution, characterized, in that the method comprises the steps: producing an acidic nitrate solution in a first bio-reactor by means of bacteria; discharging the acidic nitrate solution from the first bio-reactor to a second bio-reactor for a further conversion of nitrite to nitrate by means of bacteria; maintaining an O 2 saturation of at least 80% at the prevailing temperature in the first bio-reactor and the second bio-reactor; recirculating the content from the second bio-reactor to the first bio-reactor as an acidic nitrate solution recirculation stream; wherein an NH 3 solution in water is dosed into the acidic nitrate solution recirculation stream to form a feed stream having a pH greater than 6, said feed stream being supplied to the first bio-reactor, wherein the amount of NH3 solution in water supplied to the acidic nitrate solution recirculation stream is controlled by a control system dependent on the measured pH value of the acidic nitrate solution recycle stream and/or the measured pH value in the feed stream. disconnecting the first bio-reactor and the second bio-reactor; and discharging the nitrate solution of the second bio-reactor. - providing an organic source of ammonia gas, in particular air contaminated with ammonia gas; - dissolving the ammonia gas in water to form an NH 3 solution in water; - oxidize the ammonium hydroxide in the NH 3 solution in water with air to nitrite by means of bacteria, and then oxidize the nitrite with air to nitrate by means of bacteria to form an acidic nitrate solution, producing an acidic nitrate solution in a first bio-reactor by means of bacteria; discharging the acidic nitrate solution from the first bio-reactor to a second bio-reactor for a further conversion of nitrite to nitrate by means of bacteria; maintaining an O 2 saturation of at least 80% at the prevailing temperature in the first bio-reactor and the second bio-reactor; recirculating the content from the second bio-reactor to the first bio-reactor as an acidic nitrate solution recirculation stream; wherein an NH 3 solution in water is dosed into the acidic nitrate solution recirculation stream to form a feed stream having a pH greater than 6, said feed stream being supplied to the first bio-reactor, wherein the amount of NH3 solution in water supplied to the acidic nitrate solution recirculation stream is controlled by a control system dependent on the measured pH value of the acidic nitrate solution recycle stream and/or the measured pH value in the feed stream. disconnecting the first bio-reactor and the second bio-reactor; and discharging the nitrate solution of the second bio-reactor.