Waste water purification

Claim 1

A process for the purification of waste water rich in organic substances and containing substantial amounts of nitrogen compounds and sulfur compounds and optionally present recycled activated waste water sludge comprising subjecting the waste water and optionally present recycled activated waste water sludge to a first anaerobic fermentation step to produce an off gas containing gaseous methane, ammonia and hydrogen sulfide and an aqueous phase containing dissolved sulfide, recovering said gaseous hydrogen sulfide from said off gas produced in the anaerobic fermentation step and adding it to said aqueous phase from said anaerobic fermentation step, subjecting the said aqueous phase and an aqueous stream rich in nitrates, where the volume ratio of the aqueous stream rich in nitrates to said aqueous phase containing added hydrogen sulfide is at least 1:1, to an anaerobic biological denitrification step to produce nitrogen from nitrates in said aqueous stream rich in nitrates and biologically oxidizing the resulting aqueous effluent from said denitrification step by aeration to form nitrate ions from ammonia and carbon dioxide from the organic substances in the aqueous effluent, whereby the added gaseous hydrogen sulfide and the dissolved sulfide act as electron donors for the nitrate reduction during the denitrification step.