Mixed system

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A mixed system (mixed process) is a drainage system in drainage technology , in which all waste water ( dirty , foreign and rainwater ) is drained away in a common pipe. The alternative to this system is the separation system , in which the dirty water (with the external water) is discharged in separate channels.

Construction of mixed systems

Whereas in the past there was usually a compulsory connection for all rainwater, today large parts of the rainwater are managed locally in terms of quantity and quality. If the prerequisites for decentralized rainwater management are in place, there is only the need to introduce rainwater that needs cleaning into the mixing system and treat it in a sewage treatment plant. In spite of this, very large areas are usually connected in established mixed systems and thus have an effect on drainage.

The dimensioning of a mixed system takes place according to dimensioning approaches that take the local precipitation situation into account. The aim of dimensioning is to ensure adequate flood protection (DIN EN 752). For reasons of economy and general feasibility, it is necessary to limit the size of the channels. It is therefore possible that the sewer system will be overloaded if it rains heavier than the rated rains. In order to avoid this, discharge systems are arranged in the system, through which mixed water can get directly into the body of water. There are regionally different requirements for such systems.

The dimensioning of mixed water discharges is carried out either by considering the quantity, load or concentration of parameters such as COD, nitrogen or phosphorus. In the case of high water pollution, mixed water treatment systems, mostly rain overflow basins, are arranged. In the case of particularly sensitive waters, these are u. U. still equipped with retention systems or floor filters for further treatment.

Advantages and disadvantages of mixed systems

The advantage of the mixed water system is that only one sewer system has to be installed and operated in the drained area. This also applies in particular to the house connections, which are less complex to set up and for which the risk of incorrect connections (discharge of dirty water into rainwater sewers) is avoided. Further advantages are the lower space requirement and the lower investment costs.

The disadvantage is that the capacity of the sewage treatment plants must be designed to be much larger in order to be able to process foreign and rainwater at least to a limited extent. This problem is partially circumvented by the establishment of structures for mixed water relief (e.g. rain overflow basins , storage basins and canals), which are distributed throughout the sewer network.

Ideally, these basins should allow the flushing surge to pass and first direct the following, only slightly dirty water directly into the receiving water . However, due to the increase in wastewater since the systems were built, this is more and more often not the case, which means that a relatively large amount of contaminated wastewater in the mixed system is discharged into the watercourse without being treated.

Another disadvantage is that the channels must be dimensioned for the rainwater runoff , which can be more than a hundred times the wastewater runoff . That is why the sewers are not used to capacity during periods without precipitation and deposits can form. On the other hand, the rainwater runoff ensures regular flushing of the sewers, especially with shallow sewer networks.

The water pollution caused by a mixed system is not necessarily higher than a separated system, provided that the wastewater is continuously cleaned in a sewage treatment plant and, in particular, dissolved substances such as. B. Heavy metals are retained. The rainwater runoff from roads in particular generates considerable amounts of dirt. Events can become problematic when easily degradable (oxygen-consuming) substances fall below the limit values ​​for oxygen in water. This danger is particularly acute in slow-flowing, oxygen-poor and warm, nutrient-rich waters.

Situation in Germany

While the lower operating and installation costs of mixing systems were often the decisive factor in the decision in the past, the trend today is towards separation systems, mainly due to the relatively high water pollution from untreated waste water in mixing systems. In 1991 the share of combined sewerage in the total municipal sewage system in Germany was 56% and by 1998 it had fallen to 51%. During the same period, the proportion of wastewater sewers in the total length of all drainage channels increased to 30%.

See also

Individual evidence

  1. Manuela Göbel: Fish suicide and aesthetic problems , Main-Post dated November 29, 2011, accessed on November 7, 2014.
  2. Manuela Göbel: The Kürnach is an "extremely damaged" brook , Main-Post from April 1, 2013, accessed on November 7, 2014.
  3. http://ecologic.eu/sites/files/publication/2015/1973_band4_0.pdf