4th treatment stage

The fourth treatment stage in wastewater treatment targets the removal of micro-pollutants and micro-contaminants. These micro-pollutants, also known as trace substances, are organic residues released from synthetic compounds. These include substances such as contrast agents used in radiology, pharmaceuticals, hormones, pesticides, industrial chemicals, and everyday products like cosmetics and household chemicals.

Although these substances are present in water only in very low concentrations (ng/l or µg/l), they can potentially pose risks to water quality and human health. Many micro-pollutants exceed the concentration limits set by the European Water Framework Directive. These contaminants typically enter surface waters through the sewage system and wastewater treatment plants, as they are not sufficiently removed by conventional mechanical and biological treatment processes.

The introduction of the fourth treatment stage in wastewater treatment plants aims to specifically target these micro-pollutants. In Switzerland, a water protection law has been in effect since early 2016, mandating the fourth treatment stage for large treatment plants or those located near especially vulnerable bodies of water. In Germany, the Federal Environment Agency called for the implementation of a fourth treatment stage in 2014, particularly for plants that discharge into sensitive waters. However, there is currently no legal framework comparable to that in Switzerland.

At present, the German federal states of North Rhine-Westphalia and Baden-Württemberg are leading the way in environmental protection and have established competence centers for micro-pollutants. These centers conduct scientific research, feasibility studies, support pilot projects, and assist with the large-scale implementation of the fourth treatment stage.

Two methods have proven particularly effective for the fourth treatment stage: adsorption using activated carbon and ozonation.

Activated Carbon Method
In the activated carbon method, activated carbon is added to the wastewater, binding micro-pollutants to its surface through adsorption. After a certain period, the loaded carbon is removed and either incinerated or regenerated. Depending on the grain size, a distinction is made between granular activated carbon (GAC) and powdered activated carbon (PAC).

Ozonation
Ozonation is an oxidative process in which ozone reacts with micro-pollutants, transforming them into non-toxic and biodegradable substances. Typically, ozonation is followed by a filtration step that removes both reaction products and any remaining toxic residues from the wastewater.

Measurement Technology for the Fourth Treatment Stage
Measurement technology plays a key role in the fourth treatment stage, as identifying hundreds of different substances is complex and costly. Substitute parameters such as the spectral absorption coefficient (SAC), which is most commonly used, provide indications of cleaning performance.