At the wastewater treatment plant

We wash ourselves, take showers, do our laundry, wash used dishes and go to the toilet several times a day. This produces wastewater. Since we can’t simply release this wastewater into the nearest stream or river, there are wastewater treatment plants all over the country. The water we pollute ends up here. One such treatment plant was built in the city of Coesfeld, near Münster in North Rhine-Westphalia. The city has about 36,000 inhabitants. The next diagram shows how a wastewater treatment plant works. All treatment plants in Germany look more or less like this, not just the one in Coesfeld. Dirty water enters on the left, and clean water flows out into a small river on the right. The treatment process has two main stages: mechanical treatment and biological treatment.

Every town or municipality has sewage pipes underground. Pressure pumps carry the wastewater to the treatment plant. At the plant, large screw pumps first have to lift the wastewater up. Coesfeld has three of them. Many people throw all sorts of things down the toilet that don’t belong there: cotton swabs, diapers, bandages, plastic items, and so on. That’s why every treatment plant begins with a screening building. Here, a mechanical screen removes coarse waste and large objects from the wastewater. It really stinks here! The screened-out material is collected in a container and then taken to a landfill.

During heavy rain, a lot of sand is washed into the underground sewer system. This sand and small stones can settle at the bottom of the grit chamber at the wastewater treatment plant. They sink under their own weight. The sand and stones are then vacuumed up from the bottom and removed. After the grit chamber, the wastewater flows into the primary settling tank. The water is still very cloudy at this stage. First, sludge settles at the bottom of this tank. Dirt and grease residue floating on the surface are also skimmed off in the primary settling tank.

Next comes the aeration tank. Tiny bacteria do their work here. To them, dirt particles are delicious treats. They simply eat them up. To help the bacteria thrive and multiply, air is constantly pumped into the first tank. Bacteria need oxygen.

The next aeration tank receives less air. Other bacteria living in the sludge remove further pollutants from the water. The third aeration tank is heavily aerated again. Here too, more pollutants are removed from the water. The biological treatment of the wastewater is now almost complete.

The water still looks pretty dirty, though. So it is sent to a settling tank for a while. Here, any remaining particles suspended in the water can slowly sink to the bottom. This makes the upper layers of water clearer and clearer. These settling tanks are shaped like funnels. The remaining pollutants sink to the bottom, while treated water flows out at the surface. In the final step, the water is sent through filter tanks filled with sand and other filter materials.

The water slowly seeps through these filters and becomes even cleaner. However, it is not yet suitable for drinking. It is now clean enough to be released into a stream or river without concern. In Coesfeld, that river is the Berkel. This small river continues through the towns of Gescher, Stadtlohn and Vreden, then crosses the Dutch border. It eventually flows into the Ijssel.

Sludge from the primary and secondary settling tanks is sent to the digester. It stays there for many days. Bacteria work on it again here, breaking it down at a temperature of 37°C into gas, water and solids.

These solids can be used as fertiliser in agriculture or burned. The gas produced in the digesters is collected in a large gas holder. It is used to generate electricity and produce heat.

Wastewater treatment plants are often quite large operations. About 15 people work at the Coesfeld wastewater treatment plant, keeping it running smoothly. A large control panel shows what is happening at the plant. Staff monitor and control the facility using computers. Water samples are regularly tested in a laboratory. They come from the wastewater arriving from the sewer system, as well as from the treated water discharged into the Berkel. Just like in Coesfeld, the wastewater you produce is treated at the wastewater treatment plant in your home town.