Municipal wastewater is made up of domestic and commercial wastewater. If the wastewater, which often contains pollutants, reaches the rivers untreated, it endangers the drinking water supply and the organisms living in the water. It is therefore absolutely necessary to clean the collected waste water in sewage treatment plants. In rural areas, reed beds are an alternative for treating wastewater. In the Federal Republic of Germany, around 8 billion m³ of waste water are produced every year, which is 128 l per inhabitant every day. The main contaminants in municipal wastewater are: suspended solids (sand, biological material, faeces) nutrients (nitrogen compounds and phosphates) degradable organic compounds (e.g. proteins, fats, surfactants) some critical heavy metal contents (Zn, Pb, Cd, Hg) some biologically active Substances (e.g. drugs, hormones) Waste water treatment A municipal sewage treatment plant has several cleaning stages. The main goal is to separate the solids, organic matter and nutrients from the wastewater. In the first stage, the mechanical cleaning, the solids are separated with the rake, which then go to a landfill or to incineration. The second stage is biological purification. Here, microorganisms break down most of the organic compounds in the activated sludge tank with aeration. The nutrients are also largely converted. In the secondary clarifier, the microorganisms settle as so-called sewage sludge. This will z. T. returned to the activated sludge tank. The multiplication of the microorganisms creates excess sludge, which is then used for sludge treatment. Biogas is obtained from the sludge in the digester. In biological cleaning, however, the phosphate is sometimes only insufficiently eliminated. In order to reduce the risk of water eutrophication due to phosphate, phosphate precipitation can be added or integrated as a third stage. Here, the phosphate is precipitated as poorly soluble iron(III) phosphate by adding iron salts. The excess sewage sludge that accumulates in biological treatment contains around 95% of water and must be drained before it can be used or disposed of at a special landfill. In principle, sewage sludge is a valuable organic fertilizer. But since it also z. B. heavy metals or poorly degradable toxic organic pollutants can be adsorbed, agricultural use is often not possible due to the high pollutant content according to the Sewage Sludge Ordinance. In Germany, only about 50% of the approximately 80 million tons of sewage sludge per year is used for agriculture, the rest is sent to landfills or incinerated. Another problem is that various substances are hardly degraded in sewage treatment plants, e.g. B. certain pharmaceuticals. This can become a risk, since they then have a negative effect on the organisms living in the water in the rivers or can get into the drinking water via the groundwater. This means that possible long-term effects cannot be foreseen. Drugs and other pharmaceuticals are z. T. excreted unchanged by humans and not completely broken down in the sewage treatment plant. For example, over 15 million women in Germany take the contraceptive pill. A large part of the 30 µg active ingredient (mostly ethinyl estradiol) per pill ends up unchanged in the waste water. There is hardly any degradation of this substance in the sewage treatment plant. The average concentration of ethinyl estradiol in Berlin sewage treatment works is 17 ng/l, but in some cases 2-4 times this concentration was found. Amounts of 1-5 ng/l have already been measured in the Rhine and Main. A quantity of 1 ng/l is a very low concentration, comparable to filling a basin with an area of 1 km² and a depth of 10 m with water and dissolving 4 sugar cubes (10 g) in it. But as little as 1 ng/L of this synthetic hormone can disrupt fish egg laying. Fish reared in sewage works feminize, ie many more females develop from the eggs than males, and a relatively large number of hermaphrodites also develop. This is an indication that certain substances in the wastewater treatment plant are or remain biologically active. After ingestion, many biologically active substances from medicines end up in our waters via the sewage. After ingestion, many biologically active substances from medicines end up in our waters via the sewage. Another example of substances that are difficult to break down are medications that are frequently administered, such as blood lipid reducers, painkillers or antibiotics. Some of these have already been found in sewage treatment plant wastewater, in groundwater and even in traces in drinking water. The measured concentrations are still well below those that can cause acute damage to the environment, but long-term effects cannot be ruled out. For example, the permanent emission of the smallest amounts of antibiotics can contribute to the formation of resistant germs. It is also not known whether these substances react with each other or whether they show a corresponding effect as a so-called cocktail. In addition to the contamination with suspended matter, various anions (e.g. nitrite, nitrate, phosphate ions) and heavy metals, industrial wastewater also has the problem of emulsified and oil-contaminated wastewater. Producers of this waste water are obliged to subject their liquid waste to physical and chemical treatment in waste water treatment plants. Only after reduction (e.g. by precipitation reactions or activated carbon filters) to the prescribed pollutant concentration in the waste water and the oil separation may the waste water be discharged into the municipal sewage system for cleaning in a waste water treatment plant (see above).