NOVELTY - Method for removing nitrite from a liquid, involves (a) providing nitrite-comprising liquid, preferably coming from the oxidation of ammonia in an aqueous solution preferably by micro-organisms, where the ammonia in aqueous solution comes more preferably from washing ammonia-comprising air, and (b) converting nitrite by using nitrite-converting substance, where an amount of the nitrite converting substance is added to the nitrite-comprising liquid.    USE - The method and system are useful for removing nitrite from a liquid which is useful for treating ammonia-comprising air coming from fertilizer processing, biogas plants, composting processes, livestock farming or industrial processes (all claimed).    ADVANTAGE - The improved method is more environmentally-friendly than existing processes as the residue consists mainly of air nitrogen, and the normal amounts of washing water to be discharged are significantly reduced and is less labor-intensive. The method can take place at room temperature, i.e. without additional heating or cooling of e.g. the washing liquid. The odor emission is greatly reduced. The treated liquid has a low nitrite concentration, as a result of which there is less inhibition of the biological activity.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) system (S1) for removing nitrite from a liquid, comprising a reactor having one or more inlets for nitrite-comprising liquid, an access for a nitrite-converting substance, and preferably one or more outlets for treated liquid, where each inlet is adapted for being connected to one or more additional systems in which a nitrite-comprising liquid can circulate;    (2) method for treating ammonia-comprising air, involves (i) washing the ammonia-comprising air in an aqueous liquid, (ii) at least partially converting ammonia dissolved in the liquid into nitrite, and (iii) removing nitrite from the liquid;    (3) nitrite-converting substance; and    (4) system suitable for treating ammonia-comprising air, comprising at least system (S1) and one or more air washers.