Nitrogen is a chemical element which can be found in fresh water and wastewater in many different forms. In wastewater for example mainly organic nitrogen which comes from the rejections of food can be found. This nitrogen is an essential nutrient and can lead to algal bloom, eutrophication and water pollution. In its reduced form (ammonium, NH4+ and ammonia NH3), it is transformed into nitrate, a process which consumes a lot of oxygen leading to oxygen depletion in the water. Nitrate (NO3-) is also a drinking water pollutant while nitrite (NO2-) is toxic to fish. Nitrite can be transformed to nitrate and nitrate can be transformed to gazeous nitrogen gas (N2) and released to the atmosphere (denitrification). Nitrogen is also an important constituent of fertilisers used for food production. Today, nitrogen used for food production is mainly mineral nitrogen which is mined consuming huge amounts of energy. By recycling nitrogen contained in wastewater, the pollution of ecosystems could be reduced and the energy consumption by nitrogen mining avoided.