Process and apparatus for the biological purification of wastewater

In a process for the biological purification of wastewater containing organic impurities, wherein the wastewater is aerated in a reactor, in the presence of activated sludge in part carried on a carrier material comprising particles of organic polymer compounds, with a gas containing molecular oxygen, and the wastewater activated sludge mixture thereafter being separated in a post-clarification stage into purified water and sludge with a partial stream of the thus-separated sludge being recycled into the reactor, the improvement comprising conducting the biological purification in the reactor in the presence of a carrier material comprising open cell macroporous particles having a specific gravity of 20-100 kg/m.sup.3, a volume of 0.1-125 cm.sup.3, a water absorption capacity of 30-70% after two minutes, a void ratio of 6-30 pores/cm, an air permeability of 150 to 350 1/dm.sup.2 /min at 10 mm water column differential pressure per 50 mm of thickness, and having open macropores of 0.1-3 mm, said reactor being a continuous flow-stirred tank with carrier particles present in the reactor in an amount comprising a volume proportion of 5-40% of the reactor volume to permit substantially free movement of the carrier particles suspended in the wastewater, to achieve decentralized growth of the organisms in the activated sludge on the particles and maintaining a solids concentration in the reactor of 6-15 g/l, sparging gas bubbles into the reactor from the bottom thereof of achieve circulation of the particles throughout the reactor, and recycling separated sludge from the post-clarification stage in an amount sufficient to maintain 0.9-9 kg/m.sup.3 of free activated sludge in the reactor.