Waste water treatment

The wastewater treatment involves three stages, called primary, secondary and tertiary treatments.

Pre-treatments :

-    Removal of large waste
The wastewater passes though a screen. The screened wastes collected are then disposed to an incineration plant or to a landfill site.

-    Sand removal
Sand, grit, and stones need to be removed early in the process to avoid damage to pumps and other equipment in the remaining treatment stages. The removal of this material is made by sedimentation. The contents of the sand catcher may be sent to recovery operations or to landfill sites.

-    Oil and grease removal
The flotation is the most widely used technique to remove oil and grease in the wastewater. Its principle is based on the injection of thin air bubbles in the tank; oil and grease float to the surface and are then removed by scraping.

The removal of sand and oil and grease occurs most of the time in the same tank: the sand settles to the bottom while the oil and grease rise to the surface.

-    Settlement
Solid fraction contained in the effluent is settled in a specific tank.

Secondary treatment :

Secondary treatment is designed for the futher treat of the small organic particle contained in the liquid coming from the primary process.
This treatment can be done in two ways, biologically or chemically. Both techniques can be used in a complementary way to improve the efficiency of the treatment.

Tertiary treatment :

The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality before it is discharged to the receiving environment (sea, river, lake). Different tertiary treatment processes may be used as:
-    phosphate removal,
-    nitrogen removal,
-    disinfection.