Wastewater Treatment

Water is essential for many domestic and industrial activities. After use, this water contains numerous biological and chemical contaminants. This water, contaminated and rejected after use, is said to be wastewater.

Wastewater can contain several unwanted elements:

• sand and other suspended matter;

• pathogenic microorganisms that can cause disease (bacteria and parasites);

• decomposing organic waste;

• nutrients that stimulate excessive growth of algae and aquatic plants;

• various chemicals (cleaning products, solvents, hydrocarbons, drugs, etc.).

To reduce the risk of contamination and disruption of aquatic ecosystems, wastewater must be treated and decontaminated before it is returned to the environment. Wastewater treatment comprises a series of operations involving physical, chemical, and biological processes.

Physical pretreatment removes larger solid waste and insoluble matter. Firstly, during screening, the wastewater passes through a grid which retains large floating waste. The grit removal is then used to remove the sand and gravel from water through sedimentation in a tank. Finally, oil and grease produce a foam on the water surface. This foam is then removed with a scraper during the oil removal stage.

The water then undergoes a physico-chemical treatment, also known as primary treatment. This involves sedimentation to remove part of the suspended matter in the water. The addition of certain chemicals helps the sedimentation process by forming small flakes of material. These then sink to the bottom of the basin where they form a bed of primary sludge. They can then be removed from the water and recycled.

The remaining dissolved organic material undergoes biological treatment, also called secondary treatment. Air is first injected into aeration tanks so that the oxygen promotes the growth of microorganisms. These then break down some of the organic matter into carbon dioxide and water. Bacteria, subsequently, decompose the excess of organic matter in the biological filtration process. The solids settle to the bottom of the basin and form secondary sludge which is also treated, as was the primary sludge. During this stage of wastewater treatment, 75% to 95% of the organic matter is removed. Following this stage, the water is clean enough to be returned to the waterways, but it can also undergo additional treatments.

Complementary treatments are added to basic treatments when wastewater must be discharged into fragile ecosystems. Among these additional treatments, disinfection eliminates pathogenic microorganisms by adding chlorine or ozone or by the action of ultraviolet rays. Phosphate removal, on the other hand, allows excess phosphates to be precipitated in water by adding lime or iron chloride. Finally, filtration through a bed of sand and activated charcoal removes various chemical contaminants.