Reasons for treating wastewater
There are two broad categories of wastewater, also known as effluent:
- Urban wastewater from homes, stores and small workshops, combined with rainwater.
- Industrial effluent* from factories.

Water treatment Station at Total Petrochemicals plant in Carling Saint-Avold

Purpose of treatment
The purpose of wastewater treatment is to protect life in streams and coastal areas and to make water usable downstream.
Part of the effluent* is treated directly on site, by consumers with autonomous systems such as septic tanks and seepage pits and by factories that have their own treatment plants. The rest is channeled through the sewer system to a municipal treatment plant (there are around 12,000 in France).

How does treatment work?
The effluent to be treated goes through a series of processes, each of which removes a number of contaminants. The principal stages are as follows:

- Phase 1: Screening, to eliminate large-scale waste.
- Phase 2: Grit removal (sand and heavy matter settles) and grease removal (floating oil and hydrocarbons are skimmed off the surface)
- Phase 3: Primary settling: suspended matter* slowly settles to the bottom of the tank and a raking mechanism moves the sludge to a discharge pipe
- Phase 4: Biological treatment: Increasing the oxygen content promotes the proliferation of microorganisms in the effluent that transform the organic contaminants* (carbon and nitrogen-based pollution) into water and carbon dioxide.
- Phase 5: Clarification, using sedimentation to separate purified water from sludge and secondary residue resulting from the breakdown of organic material.
- Phase 6: Testing: the treated effluent is tested periodically or continuously before being released into the environment.

What happens to the contaminants?
Part of the contaminants removed in treatment are destroyed. organic contaminants*, for example, are transformed into carbon dioxide and water by bacteria. Screen debris, sand and grease are often treated with household waste while sludge is either used as a fertilizer, landfilled or incinerated.

Why are there still contaminants in the water?
Not all wastewater is treated completely : households without treatment systems, leaks of sewer networks, old treatment plants, not equipped to handle heavily polluted rainwater, etc. Even treated water that meets the requisite standards contains a minimum amount of pollution since it would be too expensive-and perhaps impossible-to make it as clean as it was before use. For all these reasons, the quality of water in French rivers and streams is not perfect, even though it has improved over the past several years.