Environment

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.

In Italy, the reference legislation is Legislative Decree 152 of 11 May 1999 which incorporates the community directive 91/271/CEE concerning the treatment of urban wastewater.

In addition to regulating the wastewater by setting the concentration limit values for the various substances contained therein, Legislative Decree 152/99 also reports the quality of the water body, providing for the development of monitoring activities and possibly the quantification of the environmental damage exerted by man.

The Ministerial Decree 198 is also to be recalled, entered into force on 18 September 2002, “Implementation procedures on the status of water quality, pursuant to art. 3, paragraph 7, Legislative Decree 11 May 1999, n. 152”. In particular, it provides for the transmission to APAT by the autonomous Regions and Provinces of the cognitive data, all the information and the reports concerning water quality.

On April 3, 2006, Legislative Decree 152/06 “Environmental Consolidation Act” came into force which, taking up what had already been introduced with the previous Legislative Decree 152/99, modifies the regulatory framework regarding water pollution, in particular as regards the definitions of:

Discharge of wastewater: (art. 74 letter ff, Legislative Decree 152/06): “Any entry made exclusively through a stable collection system that seamlessly connects the wastewater production cycle with the surface water receptor body, on the ground, in the subsoil and in the sewage system, regardless of their polluting nature, also subjected to a preventive purification treatment. The releases of water provided for in art. 114 are excluded”.

Urban wastewater: (art. 74 letter i, Legislative Decree 152/06): “domestic wastewater or a mixture of domestic wastewater, industrial wastewater, or run-off meteoric wastewater conveyed into sewage, even separated, and coming from agglomeration”.

Domestic wastewater: (art. 74, letter g, Legislative Decree 152/06): “wastewater from residential settlements and services and deriving mainly from human metabolism and domestic activities”.

Industrial wastewater: (art. 74, letter h, Legislative Decree 152/06): “any type of wastewater discharged from buildings or installations where commercial or goods production activities are carried out, other than domestic wastewater and run-off meteoric wastewater”.

Phyto-remediation is a natural technology for soil or water clean-up, which exploits the natural ability of vegetable organisms to extract inorganic compounds (heavy metals) and/or induce the degradation of organic compounds in contaminated soils or in contaminated wastewater.

The processes that lead to the remediation of the soil by means of plants are different and are classified as follows. Depending on the pollutant removal methods, various possible technologies can be distinguished:

Phyto-extraction: in the event that the contaminants (especially inorganics) are extracted from the root system and accumulated in large quantities in the plant tissue; the subsequent mowing of the epigeal vegetable portion allows the removal of the contaminant.

Phytodegradation: in the event that contaminants (above all organic) are degraded within the plant tissue thanks to the normal metabolic activities of the plant organism. In this case, a fundamental role is played by the action of plant enzymes.

Phytostimulation: in the case that radical exudates play a stimulating role in relation to the microorganisms of the rhizosphere that carry out biodegradation processes of pollutants, for example PCBs and PAHs.

Phytostabilization: in the event that the contaminants are immobilized inside the rhizosphere and therefore leaching is prevented.

Phytovolatilization: in the event that contaminants (organic compounds) are extracted from the root system, possibly transformed and released into the atmosphere as volatile compounds; this is the case of compounds based on Se, As, Hg, Trichlorethylene, Methyl tert-butyl ether.