Municipal Wastewater Treatment Plants

Glossary of ETL Verfahrenstechnik GmbH

How It Works

A municipal wastewater treatment plant purifies wastewater using a combination of mechanical, biological, and chemical processes before the treated water is discharged into a receiving body of water, known as a recipient. These processes are essential for removing pollutants and protecting the environment.

Mechanical Treatment

The first stage of wastewater treatment is mechanical cleaning, where coarse solids are removed. Fine and coarse screens catch larger debris such as toilet paper, hygiene products, and food waste. At this stage, 30–50% of coarse material is already removed from the wastewater.

Grit and Grease Trap: In the grit and grease trap, the flow velocity of the wastewater is reduced so that heavy particles such as sand and gravel can settle and be suctioned off. Lighter substances like oils and fats float to the surface and are skimmed off.

Primary Clarifier: In the primary clarifier, the water flow is slowed down further to allow fine organic matter to settle. These materials either sink as "primary sludge" or float. The collected sludge is then pumped to the digester, where it is further processed as primary sludge.

Biological Treatment

The next step is biological treatment, in which bacteria and other microorganisms break down organic substances in the wastewater.

Aeration Tank: The aeration tank is the central component of biological treatment. Here, the wastewater is mixed with activated sludge—a mass of bacteria. These microorganisms break down the contaminants. Air is injected from below to provide oxygen and promote biological activity.

Anaerobic Tank: In this unventilated tank, phosphorus compounds are broken down by microorganisms that thrive without oxygen. The activated sludge process is the most commonly used method in municipal wastewater treatment plants.

Chemical Treatment

Chemical treatment is the final stage of wastewater purification. Remaining particles—particularly phosphorus—are removed from the wastewater to prevent eutrophication of water bodies. This is achieved by adding iron or aluminum salts, which react with phosphates to form insoluble compounds that settle as sludge and can be removed.

Secondary Clarifier

In the secondary clarifier, the activated sludge settles, and the purified wastewater is either discharged into the recipient or subjected to further chemical treatment. Excess sludge is transferred for additional sludge treatment.