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.
Intensive reactors have no storage capacity.
Intensive reactors release large amounts of sludge whose treatment and disposal is difficult and expensive. Semi-intensive technologies represent an effort to combine the best characteristics of extensive and intensive technologies. Release no or very little sludge. The cost of a sewage treatment plant depends on numerous parameters and then varies a lot from project to project. The following table presents just a rule of thumb .