Combined sewer

A mixed system (mixed method ) is a derivation system of wastewater treatment systems in which all waste water (dirt, foreign and rain water) are removed mixed in a common line. The alternative to this system is the separation system in which the waste water is discharged ( with the extraneous water ) in separate channels.

Construction of mixed systems

Stock earlier usually a compulsory connection for all rainwater, large parts of the precipitation water are cultivated in quantity and quality excessive respect locally. There is thus in the presence of conditions for decentralized stormwater management only the need to initiate cleaning need rain water fractions in the mixing system and to treat a wastewater treatment plant. Still, very large area ratios are grown in mixed systems usually connected and so outflows effectively.

The dimensions of a mixing system is based on measurement approaches that take into account the local precipitation situation. Objective of the dimensioning is to ensure adequate flood protection (DIN EN 752 ). For reasons of economy and general applicability, it is necessary to limit the size of the channels. Thus, it is possible that stronger than the rated rain overloading the sewage system occur. To avoid this, spillways are arranged in the system can go directly into waters of the mixing water. There are regional differences in requirements for such systems.

The design of combined sewer overflows is done either on quantity or volume of goods or concentration considerations of parameters such as COD, nitrogen and phosphorus. In the case of high water pollution are mixed water treatment plants, mostly rain overflow basins, respectively. In the case of particularly sensitive waters, these may be equipped with restraint systems or soil filters for further treatment.

Advantages and disadvantages of mixed systems

Advantage of the combined sewer system is that in the dewatered area only a duct system must be installed and operated. This is particularly true for the house connections, which are less costly to build and where also the risk of a false connection is avoided (introduction of wastewater into storm sewers ). As further advantages of the smaller footprint and lower investment costs shall be given.

The disadvantage is that the capacity of the treatment plants must be designed to be very much larger to handle more debt and rain water, at least to a limited degree can. This problem is partially circumvented by the establishment of structures for combined sewer overflows ( eg overflow basins, reservoirs and channels ), which are distributed throughout the channel network.

Ideally, these basins should allow the first flush to happen and only run the a descendant, little dirty water directly into the receiving water. However, this is often not the case, which means that in the mixing system relatively large number of polluted wastewater is discharged untreated into the water bodies.

Another disadvantage is that the channels for storm water runoff must be dimensioned, which may be more than one hundred times the wastewater flow. Therefore, the sewers are underutilized in periods without rainfall and form deposits. On the other side of the stormwater runoff makes, especially in shallow wastewater networks for periodic flushing of the channels.

The water pollution by a mixed system is not necessarily higher compared isolating system, since a larger part of the annual runoff is continuously purified on a sewage treatment plant and in particular solutes are retained as eg heavy metals. In particular, provide the runoff from streets and Metal roofs significant pollution loads. Problematic discharge events can be, there are less than those by easily degradable (oxygen -consuming ) substances with the limits for oxygen in the water. This is particularly problematic in slow-flowing, oxygen-poor and cold, nutrient- rich waters.

Visible contamination by sanitary products can be avoided by the provision of computing and screening plants.

Situation in Germany

While previously the lower operating and installation costs of mixed systems often were decisive for the decision against a separation system, the trend today, especially because of the relatively high water pollution by untreated waste water in mixing systems, towards separation system. 1957, the share of combined sewer to the entire municipal sewer system in Germany at 84.2 % until today (2001) the share has declined to 63.2 % ( difference to 100% is each separation system ). However, a decision for a particular system should always be taken case by case basis ( depending on topography, load capacity of the water, etc.).