Process and apparatus for continuously obtaining separate outflows of pure water and sterile sludge when starting with various aqueous slurries containing organic or oxidizable contaminants (as exemplified by domestic sewage or industrial waste) and first treating the flowing mixture with sulfur dioxide and iron under conditions of closely controlled acidity and in the absence of oxygen. The invention provides a multiple-stage, closed, operating system, a primary segment of which is thus characterized by a self-contained, continuously recycling oxygen-free gaseous flow comprising sulfur dioxide gas which moves through an iron reaction bed concurrent with the contaminant stream. Successive segments include continuously recycled oxygen-containing gas flow and closely monitored step-wise acidity control and utilization of free ions in consecutive neutralization and treating units, including introduction of barium ions by means of which potentially solid components are retained in solution or suspension until reaching the desired flocculation stage. The procedure eliminates such prior difficulties as progressive clogging of the iron reaction bed, calcium deposition in conduits, and premature precipitation as well as undesired retention of soluble components. The sequence includes units for automatic sludge removal, backflush cleansing of alternate filtering units during continued operation of the flow-treatment, use of a non-sparking ozone generator in association with particular air treatment lines, and final purification of exhaust air before venting so that the environment is in no way contaminated by the total operation. Overall assembly can be sized to handle composite waste/refuse of a large community or can be miniaturized so as to accommodate merely the output of a single flush toilet plus that of the garbage grinder of a small apartment or cabin cruiser, which basic unit can be portable and operated by a storage battery. A modified procedure is adapted to treating highly acidic feed stock such as sulfite wood pulp effluent. Sludge may have increased fertilizer value due to phosphate content withdrawn from aqueous phase.