Abstract: There are disclosed a process for removing impurities dissolved in water, and more particularly organic nitrogen compounds, nitrates, nitrites and ammonium compounds utilizing biological processes, comprising the steps of passing in the presence of a normal atmosphere the water to be purified in downwardly flowing manner through a system which ensures ammonification and nitrification, then passing said water in the presence of a nitrogen atmosphere through a system which physically expels oxygen, then passing said water once again in the presence of a nitrogen atmosphere through a system which biologically reduces nitrates, and finally after draining off the gases formed from the impurities, removing purified water and an apparatus for carrying out said process.