They are large and the residence time of the effluents within the reactor is not measured in hours but in weeks or months. The concentration of microbial biomass is low and oxygen is supplied by the photosynthetic activity of algae and by atmosphere reaeration. Several complex low-rate processes and reactions occur within these reactors. Pathogens are removed by singlet oxygen formed through photochemical activation of dissolved organic matter, high pH related to algal photosynthetic activity and settling. If the reactors are constructed smaller or very deep in order to reduce land requirements the whole system may become anaerobic.
Extensive reactors release no or very little sludge.
Intensive reactors such as activated sludge are designed to accelerate the degradation of organic matter. Sewage is put in close contact with a concentrated biomass of microorganisms and a large amount of dissolved oxygen is mechanically supplied to the reactor. Pathogens are also poorly removed in intensive reactors. This problem can not be easily overcome by chlorination, since chlorination of wastewater effluents presents several shortcomings. They lack buffer capacity due to the short residence time making the system sensitive to transient quality and quantity interruptions and thus reducing their reliability.