In all the streams before digestion, references show that the concentration of sulphate, sodium and chloride stays relatively constant. A well-documented source of these three elements is the intrusion of seawater or brackish groundwater in the sewer system. The concentration for these elements can be 5-10 times higher than the maximum range given if seawater is used as flushing water like in Hong-Kong . Sulphate and chloride are also commonly added in WWTPs as counter-ion of iron or aluminium (used to flocculate the sludge and remove phosphate), and present in industrial wastewater . of the dissolved sulphur is sulphide after thickening of PS and WAS according to . Then, dissolved sulphide can be eliminated by precipitation as FeS, provided enough iron is present or dosed to prevent H2S in the biogas.