Key stages in the treatment of wastewater

Preliminary treatment

This stage removes materials from the raw wastewater that could damage our equipment. This screening involves removing: paper and plastics; grit removal; flow balancing; fat and grease removal.

Preliminary treatment

Primary treatment

In this stage wastewater flows through large primary sedimentation tanks with mechanical scrapers. These tanks use gravity to settle the solids. The solids are then transferred to sludge treatment facilities.

Primary treatment

Secondary treatment

This stage adds large amounts of air to degrade the biological content of the wastewater. Most wastewater treatment plants treat the settled wastewater liquor using aerobic processes.

Secondary treatment

Floc formation

We use bacteria and protozoa to consume soluble contaminants like sugars, fats and organic short-chain carbon molecules. A lot of the less soluble fractions are bound into clumps of fine particles that we call floc.

Floc formation

Final settlement

The wastewater is now passed through final settlement tanks. These tanks use gravity settlement and mechanical scrapers to remove the floc.

Final settlement

Tertiary treatment

Some wastewater treatment plants improve the wastewater quality further using tertiary treatment.

 

Tertiary treatment

Nutrient removal

To meet discharge licence conditions we reduce the levels of nutrients, such as nitrate and phosphorus. We do this through biological processes and chemical addition in the secondary and final settlement stages.

Nutrient removal

Quality control

Our quality control measures include regular laboratory testing of the final treated wastewater. This is to ensure the cleaned water has no detrimental effects on aquatic life or other water users.

Quality control

Drain to Sea

Wastewater is liquid waste discharged by homes, businesses, industry, agriculture and storm water from our roads. It must undergo a complex treatment process, that can take up to two days, before it is safe to release back into our water system.

Follow the journey from drain to sea

Wastewater treatment in wetlands

A constructed wetland is a type of sustainable wastewater treatment system designed to look and function like a natural wetland. They are created to treat wastewater in an environmentally-friendly way before allowing it to return safely to the water system.

Wetland treatment of wastewater

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