Waste water treatment (WWT)

Chemical-physical treatment plants are plants in which, by dosing of reagents under appropriate pH conditions, wastewater which is generally not biodegradable due to high toxicity, often due to the presence of heavy metals or organic biocides.

This type of plant is normally used for the treatment of industrial waste from mechanical processing, metallurgical, galvanic, textile, chemical and pharmaceutical industries, as well as for the treatment of waste and temporary storage areas.

In the case of water with a high organic content, the chemical-physical treatment plant may be used upstream of biological plants as pre-treatment.

The classical structure of a chemical-physical plant involves the following steps:

• Insolubilization of pollutants by dosing specific reagents 
• Coagulation by the use of organic or inorganic coagulants
• Separation of solids by decantation
• Dehydration sludge
• Final filtration
• pH check before discharge

In view of the type of pollutant, additional treatments may be required before the insolubilising tank, such as simple acidification, reduction of hexavalent chromium  , oxidation of cyanides, chemical oxidation of surfactants and organic substances.

The correct dimensioning of a sewage treatment plant therefore requires a thorough knowledge of the production process, a proper division of the waste water and the appropriate choice of different treatments.

Biological plant

Plants of biological type are normally applied with waters with high BOD/COD in order to promote through increased biological oxidation and reduce their values so as to fall within the limits of discharge.

There are different types of plants and technologies, including:

• Biological sludge plants
• Membrane biological systems (MBR)
• Fluidized bed biomass plants (MBBR)
• Batch reactors (SBR)

All these systems, which differ from each other by the support on which the biomass grows or by the method of separation or by the continuity of the cycle, have in common the fact of exploiting an aerobic microbial population (biomass ), for oxidation by oxygenation of organic substances into simple substances such as CO2 and H 2O .

The total oxidation of organic substances to carbon dioxide and water is rare: this means that part of the biomass will have to be separated and managed as solid waste.

There may be other sections in the biological plant structure , such as anaerobic basins for nitrogen reduction or sludge digestion lines.