The purification process

The steps in the purification process

Step 1: Pre-treatment

The incoming wastewater is supplied or transported at such a height that it can flow naturally through the various components of the wastewater treatment plant. We refer to this untreated wastewater as influent. After treatment, the water is called effluent.

This influent contains many solids, such as plastic, wood, and paper. These materials cannot be biologically purified. All coarse particles are screened out of the water using coarse waste screens and, after being washed and dewatered, are transported to the landfill as solids.

The sand present in the influent is removed in a sand trap. Here, due to the low flow velocity, a mixture of sand and sludge settles to the bottom. Scrapers push this sediment to the discharge opening in the sand trap. The sand and sludge are then separated, after which the sand is discharged for reuse and the sludge is returned to the influent. Not every wastewater treatment plant has a sand trap.

In larger wastewater treatment plants, the water then enters large circular primary settling tanks. The water enters the center of the tank and then flows slowly to the outside of the tank. The settleable waste materials sink to the bottom in the form of sludge (primary sludge) and are eventually transported to the sludge digestion process. Light particles (fat) are discharged as a floating layer and ultimately also processed in the sludge digestion process.

Step 2: biological purification

Biological treatment is the actual start of wastewater treatment. After one or more pre-treatment steps, the influent enters the aeration tank. In this tank, bacteria, also known as activated sludge, purify the influent. With the help of large amounts of oxygen, they "eat" the dissolved organic waste. They obtain the oxygen through an aeration system that introduces air into the aeration tank. The purified influent then flows to a secondary settling tank for the final stage in the treatment process.

• The biological process described above is called the activated sludge system. During biological treatment, we create different conditions in which various types of beneficial bacteria convert almost all organic substances into settleable sludge and harmless gases. In addition, nitrogen and phosphate are removed from the water.
• Cultivating and maintaining a balanced bacterial mass is very important in this process. Due to the continuous supply of waste materials, the bacteria (activated sludge) multiply continuously, creating a surplus of bacteria. This surplus is ultimately transported to the central sludge dewatering facility.

Step 3: Post-treatment and discharge

From the aeration chamber, the biologically purified wastewater flows together with the bacteria (activated sludge) to the secondary settling tank.

The purified wastewater, together with the activated sludge, enters the center of the tank and then flows slowly to the outside of the tank. The activated sludge particles sink to the bottom and are returned to the aeration chamber for reuse. Any remaining light particles (fat) are removed as a floating layer and processed in the sludge digestion process.

When it leaves the wastewater treatment plant, the wastewater has been purified of almost all organic pollution. This water is of such high quality that it has no adverse effects on the plants and animals that live in and around the surface water. Any remaining biodegradable waste can be broken down naturally there. The purified wastewater is not used for the production of drinking water. 

Step 4: any additional provisions

At a number of wastewater treatment plants, we use metal salts to remove phosphates from the wastewater. This process forms insoluble phosphate compounds that settle with the activated sludge and remain at the wastewater treatment plant.