Wastewater discharges mechanical

The major part of the biosphere is aerobic and consequently priority has been given to the study and assessment of biodegradability under aerobic conditions. Nevertheless, there are environmental compartments that can be permanently (e.g. anaerobic digesters) or temporarily anaerobic (e.g. river sediments and soils) and surfactants do reach these. The majority of surfactants entering the environment is exposed to and degraded under aerobic conditions. This is the predominant mechanism of removal even in cases of absence of wastewater treatment practices (direct discharge) and it is estimated that less than 20% of the total surfactant mass will potentially reach anaerobic environmental compartments [1]. Only in a few cases, however, will the presence of surfactants in these compartments be permanent. The presence of surfactants in anaerobic zones is not exclusively due to the lack of anaerobic degradation. Physico-chemical factors such as adsorption or precipitation play an important role as well as the poor bioavailability of surfactant derivatives (chemical speciation) in these situations. 

These processes can result in the removal of a quantity of pollution through the common sewer system from housing estates via discharge chambers into the recipient (expressed in the terms of BOD5) which equals the residual pollution in the run-off from a mechanical-biological wastewater treatment plant. Because the character of precipitation is discontinuous and varies with time, the immediate effect on the recipient is less favourable. In the case of more intensive rainfalls, within a short-time interval the loading of the recipient can be 50 to 100 times higher than that in dry periods. 

For the quantity equalization of wastewaters, tanks with constant discharge are available the quality equalization takes place in so-called levelling tanks which may be either stirred (pneumatically or mechanically), or through-flow (Fig. 3.46). 

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