Ammonia nitrogen (NH4-N) is found in aqueous solutions as ammonium ion (NH4+) or ammonia gas (NH3), depending on the pH and the temperature of the solution. It is a bioavailable form of the macronutrient nitrogen and found in many fertilisers. It is also the result of the decomposition of organic nitrogen in wastewaters. At high pH, NH3 is predominant. NH3 is volatile, colourless and has a pungent odour. If it escapes to the atmosphere, it can travel large distances and resettle with the rain, what can lead to overfertilisation (eutrophication) and acidification of ecosystem.