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Trace Substance Elimination – What is that?!

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Spurenstoffelimination – Was ist das?!
01Dec

With the help of modern measuring technology, a large number of substances man-made (anthropogenic) such as residues of pharmaceuticals or cosmetics can be detected in water today. Even if these so-called trace substances are only present in low concentrations (in the range of nanograms to micrograms per liter), various studies have shown that due to the ecotoxicological effects - i.e. the effects of substances on the living environment - there are still negative effects on the aquatic environment may occur. This also makes it much more difficult to achieve the goal of the European Water Framework Directive, to achieve good chemical and ecological status in surface waters.

There are various point and diffuse entry paths of anthropogenic trace substances into water bodies. Punctual inputs usually come from urban drainage (e.g. from sewage treatment plants or sewage system discharges) and have clearly defined discharge points. Their quantity and composition are easy to grasp. The diffuse inputs, on the other hand, reach the water bodies through run-off from the area of the catchment area, via the groundwater, precipitation or via the air (atmospheric deposition). Diffuse entries are therefore much more difficult to detect than isolated entries.
However, municipal sewage treatment plants are considered to be one of the main emitters for the entry of trace substances into water bodies, since trace substances can only be eliminated insufficiently or not at all with conventional cleaning processes. For this reason, selected municipal sewage treatment plants in Hesse are to be equipped with the so-called 4th cleaning stage, which can also be used to remove large amounts of trace substances.

The expansion of the sewage treatment plants with this further cleaning stage should take place in particular in particularly sensitive waters, e.g. Since the greatest effect on the aquatic environment is achieved with the expansion of larger sewage treatment plants (size class 4-5) and the cost-benefit ratio is the most favorable, corresponding sewage treatment plants are prioritised.

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procedure

There are various methods for removing trace substances in municipal sewage treatment plants that have already proven themselves in other federal states (in particular Baden-Württemberg and North Rhine-Westphalia) and Switzerland. A distinction is made between adsorptive and oxidative processes.

With adsorptive processes, the substances to be removed from the wastewater are attached to the surface of a solid (adsorbent). Activated charcoal is used as an adsorbent because it has a very high specific surface area and can therefore easily adsorb trace substances. Both powdered activated carbon (PAK) and granulated activated carbon (GAK) can be used. After the activated carbon has been loaded with the substances, it must either be reactivated or replaced/renewed to ensure that trace substances are continuously removed.

In the case of oxidative processes, the trace substances are chemically modified using an oxidizing agent (usually ozone). The oxidation creates new substances and transformation products with different properties and possibly different toxicological effects. The trace substances should be changed during ozonation to such an extent that the resulting products no longer have any harmful effects on the environment. However, since the ecotoxicity of the newly created substances is often not fully known, a downstream, biologically active stage is recommended after the oxidative process so that these are also eliminated.

Figure 2 provides an overview of the various processes that can be used in sewage treatment plants to eliminate trace substances.

The processes or the process combinations each have different advantages and disadvantages and are selected - mostly as part of a feasibility study - depending on local conditions, wastewater quality, objectives and other factors.

elimination performances

The elimination performance of the 4th purification stage in sewage treatment plants varies greatly in the case of anthropogenically caused trace substances, since each substance has different properties and, in addition, the different trace substance elimination processes have different efficiencies. In general, however, significant improvements in the entry of trace substances into the water are achieved with the help of a 4th cleaning stage. This fact becomes clear in Figure 3, in which the elimination performances of five sewage treatment plants are shown before and after expansion of an adsorption stage with powdered activated carbon. It is shown that after the expansion, about 60 % of 47 trace substances examined could be removed to at least 80 %, while before the expansion only about 25 % to 80 % could be eliminated.
Comparable results were also achieved using granulated activated carbon to eliminate trace substances, as shown by pilot projects at the Langen sewage treatment plant in the Hessian town of Ried.

outlook

The importance of expanding municipal sewage treatment plants with a further cleaning stage to eliminate trace substances is undisputed among experts in order to protect the aquatic environment from such pollution and to achieve good chemical and ecological status in surface waters in accordance with the European Water Framework Directive.

The gradual expansion of selected Hessian sewage treatment plants with the 4th cleaning stage is supported by the HLNUG as a technical and scientific environmental authority.

Especially with regard to demographic change and the associated increased consumption of pharmaceuticals as well as climate change, as a result of which more frequent dry weather runoff causes a higher proportion of wastewater in the watercourses, the state of Hesse will continue to face major challenges in the future. During this time, the levels in the body of water as part of the trace substance monitoring (https://www.hlnug.de/themen/wasser/fliessgewaesser/fliessgewaesser-chemie/spurenstoffe/arzneimittel) measured concentrations.