Biological elimination of ammoniacal N from a saline liq. involves (I) prepn. of a reaction medium, comprising (a) keeping sea water contg. natural bacterial strains at 20-30 deg.C and pH 6-8, (b) continuously aerating and stirring the water, (c) injecting, on the 1st day, an amt. of saline aq. liq. contg. ammoniacal N, to give a concn. of 5 mg N/l, (d) opt. causing nitrification of the medium, (e) slightly increasing, on the following days, the amts. of saline liq. contg. N, (f) when nitrification is established, maintaining aeration and stirring and stopping addn. of liq. contg. ammoniacal N, (g) when a major amt. of the N has been convertd to oxidised form, stopping aeration while maintaining stirring and injecting an amt. of organic matter such that the ratio of C (calc. as ethanol):N = 1-4, and continuing cessation of aeration and continuation of stirring as long as is necessary, and (h) repeating stages (e), (f) and (g) until the concn. of suspended matter is 2-4 g/l, and opt. with addn., at the start of, or during, (I) of zeolites of grain size 0-0.2 mm (0-0.1mm); and (II) a nitrification/denitrification cycle phase comprising (a) injecting, between a time (t0) and a time (t1), into the aerated, stirred liq., at 25-30 deg.C and pH 6-8, aq. saline liq. contg. NH4 to give a concn. of ammoniacal N in the medium below 40 mg/l, (b) continuing aeration and stirring to a time (t2), (c) stopping aeration while maintaining stirring and injecting, between (t2) and (t3), organic matter so that the ratio between C (calc. as methanol):ammoniacal N injected during the period (t0) to (t1) = 1-4, and (d) maintaining the non-oxidising conditions to a time (t4), where (t4-t2) is greater than (t2-t0), with the period (t4-t0) being one cycle.    USE/ADVANTAGE -   The brine may originate from washing zeolites charged with NH4, and the regenerated water is used again to wash zeolites. Other sources of the brine are the agro-food, pharmaceutical and chemical industries, and municipal and/or industrial discharge. Saline water can be treated, and accumulation of nitrates in the environment and atmos. pollution with NOx are reduced