In a previous article published on our blog we talked about sewage sludge , explaining that it is the residues deriving from the cleaning of wastewater.
In the same article, we illustrated the methods of treating and reusing sewage sludge, particularly in the agricultural sector.
A recovery and reuse system is also envisaged for waste water , obviously after adequate purification treatment.
Before delving into wastewater recovery and reuse techniques, it is advisable to understand what is referred to when using this term.
WHAT IS WASTEWATER
According to what is reported in article 74 of Legislative Decree 152 of 03/04/2006, waste water is divided into 3 categories:
- domestic wastewater: wastewater from residential settlements and services deriving mainly from human metabolism and domestic activities;
- industrial waste water: any type of waste water discharged from buildings or plants where commercial or industrial activities take place;
- urban wastewater: domestic wastewater or the mixture of domestic wastewater, industrial wastewater or rainwater, conveyed into sewer networks.
These waters can be collected in different ways , also indicated by current legislation:
- sewers : sewer systems serving agglomerations, that is the area where the population lives and produces;
- surface waters : these are those present on the earth’s surface, such as road ditches, cables and canals, streams, artificial water bodies, rivers, lakes and the sea;
- the soil and the surface layers of the subsoil : in this case we refer to the uncultivated, agricultural or urban land surface;
- groundwater and subsoil : these are groundwater strata.
In short, the waters that we use for our human, domestic or industrial activities and the meteoric run-offs (rains that have not been absorbed or evaporated, so to speak), flow somewhere, until they reach the sea.
Containing contaminating substances, sometimes toxic and harmful, the wastewater must be purified in order to avoid pollution of the aquifers and our natural basins.
WHY WASTEWATER TREATMENT IS ESSENTIAL
With wastewater treatment refers to the process of removal of contaminants from water wastewater of urban or industrial origin, contaminated by organic and / or inorganic pollutants.
Wastewater cannot be reintroduced into the environment without a purification process , as the soil, sea, rivers and lakes are unable to receive a quantity of pollutants greater than its self-purification capacity.
The purification cycle removes pollutants from waste water, generating the so-called purification sludge , which in turn will have to undergo a series of treatments to be reused or disposed of correctly.
HOW WASTE WATER TREATMENT TAKES PLACE
The waste water treatment is carried out inside dedicated purification plants , inside which there are various equipment.
A purification plant usually has two processing lines: one dedicated to waste water and another to sludge.
As regards the purification of waste water , with which the raw sewage coming from the sewers is treated, it proceeds in three phases :
- pre-treatment: this is a process that allows the removal of part of the sedimentable organic substances (solid substances heavier than water) contained in the sewage and includes screening, sandblasting, degreasing and primary sedimentation;
- Oxidative Biological Treatment: The process of removing settleable and non-settleable organic matter in waste water. This includes aeration and secondary sedimentation;
- further treatments are all those treatments carried out upstream that allow obtaining a further refinement of the degree of purification.
Wanting to summarize and simplify the wastewater purification process, we could say that first a coarse removal of solid residues (plastic bags, waste, stones, and so on) is carried out, to then proceed gradually to an increasingly greater purification, passing from mechanical methods to more sophisticated chemical and biological technologies, aimed at removing substances that dissolve in water.
HOW TO REUSE PURIFIED WASTEWATER
The wastewater that has gone through the purification process represents a resource not to be wasted, especially if we consider that the availability of freshwater for human use decreases significantly over the years.
According to the Ministerial Decree of 2 May 2006, here is what is meant by the reuse of wastewater:
“Use of recovered waste water of certain quality for specific intended use, by means of a distribution network, in partial or total replacement of surface or underground water.”
Reuse of wastewater is only allowed for the following purposes:
- irrigation: irrigation of crops intended both for the production of food for human and animal consumption and for non-food purposes, as well as for the irrigation of green areas or for recreational or sporting activities;
- civil use: street washing in urban centers, supply of heating or cooling systems, supply of dual supply networks, separate from those of drinking water, with the exclusion of the direct use of this water in buildings for civil use, with the exception of drainage systems in toilets;
- industrial use: as fire-fighting, process, washing water, and for the thermal cycles of industrial processes, with the exclusion of uses that involve contact between the recovered wastewater and food or pharmaceutical and cosmetic products.
Purified wastewater cannot be used, under any circumstances, like drinking water and cannot come into contact with raw edible products.
Finally, more and more systems are spreading that allow, buildings, to recover and filter rainwater and gray water (i.e. coming from bathroom and kitchen water appliances) to use in bathroom drains, irrigate green areas, and other uses where drinking water is not needed.