Abstract: An aqueous ferrous sulfate waste stream is converted into sludge and improved water effluent by a preferably ambient indoor temperature and pressure flow process comprising: turbulently mixing said stream with a substantial excess of gaseous oxygen and a small excess of impalpable calcium carbonate in a reaction zone until the iron content of said stream is virtually all sludged; classifying resulting sludge into a coarser solids fraction (a portion of which is recycled to the sludging operation) and an aqueous suspension of finer solids; and separating finer solids from such aqueous suspension. As such streams usually contain free sulfuric acid, it is desirable to first precipitate from the stream a primary gypsum product by neutralization prior to such sludging operation.