Aerated ponds/lagoons are like facultative ponds in waste stabilization pond systems only that natural oxygenation, in order to achieve high rates of organic degradation and nutrient removal, is enhanced by mechanical air injection. Given that oxygenation is not dependent on algae activity and photosynthesis, ponds can be deeper and more suited for colder climates. There are two types of aerated ponds: (i) aerated facultative ponds or lagoons; and (ii) completely mixed aerated ponds or lagoons. For completely mixed aerated ponds or lagoons, the aerators used can either be surface mechanical mixers or subsurface diffusers. Completely mixed aerated lagoons (also called flow-through lagoons/completely-stirred tank reactor) act like activated sludge units without sludge return. Effluents from completely mixed ponds require a post-treatment in a sedimentation pond.