NOVELTY - Amount of sulfur compounds in a sulfur compounds contaminated wastewater stream is reduced, using a granular sludge treatment system comprising anaerobic microorganisms, which involves contacting the sulfur compounds contaminated wastewater stream with the anaerobic microorganisms; and oxidation of at least part of the sulfur compounds in the sulfur compounds contaminated wastewater stream by the anaerobic microorganisms, resulting in a sulfur compounds depleted wastewater stream.    USE - The method is useful for reducing amount of sulfur compounds in sulfur compounds contaminated wastewater stream using granular sludge treatment system comprising anaerobic microorganisms.    ADVANTAGE - The anaerobic granular sludge treatment system provides better recovery of the elemental sulfur or sulfate obtained by the oxidation of the sulfur compounds in the wastewater stream without disturbing the biological part of it. A first benefit of applying a granular sludge treatment process in combination with an aqueous nitrate solution in view of known suspended sludge treatment processes is that the sludge of the granular sludge treatment process is more compact. A further benefit is that there is an increased biological activity compared to suspended sludge systems due to the fact that more protection for the microorganisms is given in the granule. In view of known solid bed filters, no clogging occurs by applying the process. Further, there is increased biological activity compared to the solid bed system due to avoiding carrier material. A further benefit is that there is no precipitation of iron sulfides that can block filters downstream, which occurs when the effluent water is treated with a filter unit, like a sand filter bed, to keep back particles.    DETAILED DESCRIPTION - Amount of sulfur compounds in a sulfur compounds contaminated wastewater stream is reduced, using a granular sludge treatment system comprising anaerobic microorganisms, which involves contacting the sulfur compounds contaminated wastewater stream with the anaerobic microorganisms; and oxidation of at least part of the sulfur compounds in the sulfur compounds contaminated wastewater stream by the anaerobic microorganisms, resulting in a sulfur compounds depleted wastewater stream. The process further involves adding an aqueous nitrate solution to the sulfur compounds contaminated wastewater stream prior to being contacted with the anaerobic microorganisms, and enabling the anaerobic microorganisms to oxidize the sulfur compounds without the presence of oxygen. An INDEPENDENT CLAIM is included for granular sludge treatment system arranged to reduce the amount of sulfur compounds in a sulfur compounds contaminated wastewater stream, comprising anaerobic microorganisms capable of oxidizing the sulfur compounds, these anaerobic microorganisms being arranged to be contacted with the sulfur compounds and being arranged to oxidize at least part of the sulfur compounds in the sulfur compounds contaminated wastewater stream, resulting in a sulfur compounds depleted wastewater stream, where the granular sludge treatment system comprises a dosage system for adding an aqueous nitrate solution to the sulfur compounds contaminated wastewater stream before being contacted with the anaerobic microorganisms, and enabling the anaerobic microorganisms to oxidize the sulfur compounds without the presence of oxygen.