At a given point, the sewage is partially purified thanks to the BMP Inlet Protection. However, its still potentially dangerous and polluting. It is then sent to a subsequent tank called aeration, where the sewage is moved by special turbines immersed in the tank. The violent rotation of the turbines allows the passage of oxygen from the air to the liquid so as to guarantee the formation of anaerobic bacteria (bacterial flora).
The sewage then passes to another tank called final sedimentation, in which the active bacteria split the organic substances making them sedimentable, that is, separable from the sludge. Such substances, separating from the sludge, are deposited on the bottom of the tanks. The muds, so separated from the coarse substances, and from the organic sediments, return to being pure water that through special pipes is re-introduced into the waterways.
The sludge collected in the sedimentation process is then further treated “biologically” for two purposes: to produce energy and fertilizers. In fact, some plants are equipped with a plant for the production of biogas, which has the function of producing the energy necessary for the operation of the purificationor to produce the gas necessary for the operation of thermo-electric plants.
The fundamental element of a biogas production plant is the digester. This consists essentially of a large tank, generally in reinforced concrete or in steel with plastic covers, which have the purpose of retaining the gases that develop during the anaerobic fermentation process, ie in the absence of oxygen. In this tank, a fermentation process takes place in a very short time, of a phenomenon that in nature, in the subsoil, takes millions of years.
Fermentation takes place thanks to the presence of particular bacteria that feed on organic substance (carbon, nitrogen, etc.) and that produce, during digestion, the so-called biogas, composed of various elements, including methane, which is useful for feeding the engines that generate electricity and heat.
These sludges, thus treated, further dehydrated and dried, are ground and bagged so that they can be reused as natural fertilizers in agriculture. This rather complex procedure demonstrates that city sewage treatment plants can be very effective in keeping the environment clean and extremely useful for the production of substances that benefit other productive sectors, if they are well made and maintained.
The utilities cannot be connected to the main citizen collector, because they are too far away, due to a quota problem or other. In these cases, the utilities are obliged by law to acquire special private biological pits that for form part of BMP Inlet Protection.