Water purification: operation of the wastewater purification network

Population growth is accompanied by the development of human activities (industry, agriculture, domestic activities) with the consequence wastewater of greater production of polluting discharges which in particular degrade the quality of the water. However, our water resource is not inexhaustible. If our wastewater is not systematically cleaned before being released into the natural environment, it risks damaging the environment, which would then no longer be able to provide enough water, thus increasing the risk of scarcity. This is why cleaning up our wastewater is absolutely essential to preserve our natural resources.


There are three categories of wastewater, classified according to their use:

Domestic wastewater

It is, on the one hand, the grey water (or grey water) that we consume every day (mainly in the bathroom and the kitchen to wash, do the dishes, clean the laundry etc.), and on the other hand apart from the black water (or black water) which comes from the toilets carrying urine and faeces.

This domestic wastewater contains various pollutants that can become pathogens:

  • Blackwater produces nitrogen, phosphorus, ammonia as well as faecal germs.
  • Gray water produces heavy metals (cadmium, lead, arsenic, mercury, etc.) via cleaning products, drugs and even cosmetics.

Industrial and agricultural wastewater

They come from factories, manufacturing workshops and agricultural structures. Their characteristics depend on the use made of them. This wastewater contains in particular toxic products, solvents, heavy metals, organic micropollutants, hydrocarbons, etc.

Rainwater and runoff

When in contact with air or during runoff (on roofs or roads for example), this rainwater becomes loaded with impurities leading to a degradation of the quality of natural water resources. This type of wastewater may in particular contain industrial fumes, pesticides, used oil, heavy metals, fuels, etc.


Sanitation is a purification process that includes:

  • Wastewater collection
  • Purification (or depollution), i.e. wastewater treatment
  • The discharge of decontaminated water into the natural environment
  • The management of rainwater and runoff: in the event of heavy rains that could lead to flooding, this uncontrolled water which, by carrying many pollutants, can end up directly in the natural environment and deteriorate resources.

The 2 types of sanitation

When wastewater is collected, transported and then treated by the public sanitation service, this is referred to as collective sanitation (or all in the sewer). This mainly concerns areas where the habitat is dense. Wastewater is collected and then transported via a network of pipes to a wastewater treatment plant which will take care of pollution control.

In the case of households not connected to the public network of water services, sanitation is carried out autonomously, it is a non-collective sanitation. This mainly concerns scattered residential areas. These areas are then equipped with an autonomous pretreatment device as well as a treatment device that uses the purifying powers of the soil.

Treatments for collective sanitation

The purpose of pretreatment is to rid the water of elements which could prevent the following stages of treatment. It includes 3 operations:

  • Screening and sieving: this is a mechanical treatment that removes insoluble elements such as branches, plastics, etc.
  • De-sanding: this type of treatment makes it possible to extract by settling the sand which has mixed with runoff or resulting from the erosion of the pipes.
  • Degreasing aims to eliminate oils by bringing the grease up and then scraping it off the surface of the water.

The most widely used treatment is “activated sludge” treatment, which uses sludge from pre-treatment. The bacteria contained in this sludge are used as a treatment tool. A wastewater treatment plant does not produce drinking water because it can still contain pollutants and microbial residues.

Other treatments can be used in particular in smaller communities: lagooning, phyto-purification (reed beds).

The treated water then returns to its natural environment.


Regarding collective sanitation

The municipalities manage the water services. There are different management modes for public water and wastewater treatment services:

  • Direct management: the municipality or agglomeration takes charge of all the services
  • Delegated management: the municipality or agglomeration delegates all or part of the management to a specialized company, either in the form of a concession (in this case, the delegated company finances, builds, and operates the equipment) or by leasing ( in this case, the community remains the owner of the equipment and the delegated company operates and maintains this equipment).

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