The majority of wastewater treatment in Wiesbaden takes place in the main Salzbachtal sewage treatment plant. On average, 50 million liters of wastewater from the districts of Auringen, Bierstadt, Dotzheim (partially), Erbenheim, Heßloch, Igstadt, Klarenthal, Kloppenheim, Medenbach, Naurod, Rambach and Sonnenberg are treated there every day. The second sewage treatment plant that is still in operation today is the sewage treatment plant in Biebrich. The rest of Wiesbaden's wastewater is treated there, an average of 18 million liters a day. The wastewater from Biebrich, Dotzheim (partially), Frauenstein, Amöneburg, Kastel, Kostheim and Schierstein is treated here. Breckenheim, Delkenheim and Nordenstadt, whose wastewater flows to the Flörsheim wastewater association, are an exception. All smaller sewage treatment plants that were built in the 20th century in various Wiesbaden districts and in places incorporated by Wiesbaden were shut down in 2001 or 2006.

A distinction is made between two stages of wastewater treatment: mechanical treatment, in which coarse pollutants are removed from the wastewater, and biological treatment, in which the carbon compounds, phosphates and ammonium nitrogen contained in the wastewater are largely broken down by bacteria.

Wiesbaden's main sewage treatment plant was built in 1884 in the Salzbachtal valley - for a catchment area corresponding to that of today's city center - as the first sewage treatment plant with a high purification capacity for its time. It consisted of three 30 m long and 10 m wide sedimentation tanks in which around 2,500 m³ of wastewater could be purified with milk of lime and the addition of compressed air. In later years, the degree of purification was improved by the use of different screens. Harp, rotary and string screens also removed coarse contaminants. In the last year of the Second World War, this screening plant was destroyed by bombs.

In the following years, the wastewater from Wiesbaden's city center flowed completely untreated into the Salzbach - and via this into the Rhine. In 1950, work began on the construction of a mechanical treatment plant with sludge digestion at the same location, which went into operation two years later. From 1952-62, the plant was expanded and improved step by step. The wastewater of 150,000 inhabitants could be treated in this sewage treatment plant. However, it was soon too small and completely overloaded. A new plant was therefore built in the 1970s: The mechanical treatment stage (1974), the sludge dewatering stage (1975), the digestion towers (1976) and the new biological treatment stage (1977) were put into operation one after the other. 22 years later, modernization of the main sewage treatment plant was once again on the agenda. In total, the plant was rebuilt over a period of eight years (1995-2003) - without interrupting operations.

For some years now, Wiesbaden has had one of the most modern sewage treatment plants in Europe. The sewage treatment plant in Biebrich was built back in 1905 in the "Rheinfeld" district as a screening plant for wastewater treatment. Several screens and sieves removed the coarse pollutants from the wastewater. The remaining pollutants could then be discharged into the river for many decades without any concerns due to the Rhine's high self-cleaning capacity. This changed abruptly after the Second World War. In 1963, the construction of a mechanical-biological sewage treatment plant began at a site not far from the old plant, which was extended in the 1970s. From 1999-2008, the Biebrich sewage treatment plant was modernized in two stages. First, the waterway was renewed. Since its completion (2001), the entire wastewater treatment process has taken place within one building. This virtually eliminates noise and odor nuisance. In August 2008, the new sludge path was put into operation after a four-year construction period. After the modernization of the Biebrich sewage treatment plant was completed, the two sewage treatment plants in Kostheim and Kastel, built in 1957/58 and 1968, were shut down. Since then, the wastewater from AKK has been pumped to Biebrich for treatment via pumping stations and sewers.

In 1976, the Naurod-Auringen wastewater association put the newly built sewage treatment plant in Auringen into operation. In 1977, as part of the regional reform, responsibility was transferred to the drainage company of the city of Wiesbaden. The sewage treatment plant was extended in 1984 and decommissioned in 2006. From 1970-2006, Medenbach's wastewater was treated in the sewage treatment plant built outside the town. Since 2006, the wastewater from the three suburbs has been fed into the main sewage treatment plant via pumping stations and channels for purification.