A method for producing a carbonate-containing species-rich, nitrogen-containing species-free solution from a urea-rich solution is proposed. The method comprising the steps of: providing a first reservoir comprising a first mixture including urea and a catalyser comprising an enzymatic catalyser and/or a microorganism; allowing an enzymatic reaction catalysed by the catalyser to decompose urea, thereby obtaining a second mixture comprising nitrogen-containing species and carbonate-containing species; converting at least some of the nitrogen-containing species into gaseous nitrogen-containing species to obtain a third mixture comprising the gaseous nitrogen-containing species and the carbonate-containing species; filtering the third mixture by a gas- permeable filter, thereby separating at least some of the gaseous nitrogen-containing species from the carbonate-containing species while keeping the catalyser away from the gas-permeable filter; and collecting the so-obtained carbonate-containing species-rich, nitrogen-containing species-free solution.