The wastewater is collected and conveyed by collectors/manifolds to the purification plant. The first treatments to which it is subjected are physical, i.e. the screening phase, needed to remove the coarse material (pieces of plastic, wood, stones, paper, etc.) which could clog pipes and pumps. The coarse sludge is then washed, pressed and taken to the landfill.
In the ensuing removal and de-oiling phase, the sands are separated by natural sedimentation, and the separation and rising of oils and fats on the surface is favoured by air insufflation which, while ensuring a limited turbulence, also prevents the sedimentation of organic substances.
In the primary sedimentation tank, separation by gravity of sedimentable solids takes place. The sludge that accumulates at the bottom of the tank is pushed and collected with various systems and sent to subsequent treatments. At this point the mechanical treatments that have removed about 1/3 of the organic load terminate.
The elimination of dissolved substances and suspended solids takes place in the activated sludge oxidation tank, where a process occurs which is based on the metabolic action of microorganisms (bacteria) using the organic substances and the oxygen dissolved in the slurry for their metabolic activities.
In this way flakes consisting of colonies of bacteria that can be easily eliminated in the subsequent sedimentation phase form. These metabolic processes are favored by the presence of oxygen, which is supplied by insufflation of air from the bottom.
In the final sedimentation tank, the sludge flakes are separated from the aerated mixture. A part of the activated sludge is recirculated in the aeration tank and the exceeding part is sent to the subsequent treatment. At this point the water coming out of the final sedimentation can be defined clean and can therefore be returned, after the necessary analyses and controls, to the surface watercourse.
In addition to mechanical and biological processes, other treatments may also be needed which aim to limit nutrients such as nitrogen and phosphorus in the final discharge. The nitrogen removal occurs by bacterial species biological processes in the oxidation tanks, while for the elimination of phosphorus a chemical process is used which consists in the addition of a flocculant product (eg iron salts) during the purification process.
The sludges resulting from primary and secondary sedimentation are pumped into a pre-thickener, where the solids concentration is increased and consequently the sludge volume reduced. From the pre-thickener the sludge is sent to the digester, where it remains for about 20 days in an anoxic environment at a temperature of 35°C. Specialized bacteria reduce the organic substance and partially transform it into inorganic substances producing, as a result of their metabolism, a gas with a high methane content (biogas).
The gas can be accumulated in the gasometer and used as an energy source for the production of electricity and heating. The sludge, digested and almost odourless, is pumped into the post-thickener to further reduce humidity.
The sludge volume can be significantly reduced using mechanical dehydration by centrifugation or other techniques. The dehydrated sludge has a semi-solid consistency that allows easy application in agriculture, composting or landfill disposal.