A prerequisite for the operation and control of a sewerage system is a reliable measuring technique. In addition to the recording of water volumes and levels which is common practice in modern sewerage systems with stormwater overflow tanks (SWTs), recording the strain put on the system due to harmful substances is also very worthwhile for control purposes.
In case of heavy rain events, SWT capacity within the sewerage system is often exceeded. If this happens, the excess water flows into the nearest body of water. If the strain posed by the water within the sewerage system is known about, the control system can allow the overflow to occur wherever the water is under the least amount of strain. In this way the environment can be protected to a considerably greater extent than is the case when sewerage system control is carried out by purely hydraulic means.
Ammonium and ammonia are present in equilibrium in the water and the one can be converted into the other via various processes. The equilibrium is dependent both on pH value and temperature, as a result of which these two parameters also become important measurement points. Ammonium represents the main strain within the sewer and ammonia is highly toxic to fish, as a result of which it can also cause serious damage.
The greatest problems with taking measurements within the sewer are:
Fats which clog up the sensors
Extreme variations in concentrations
Poor access
Fluctuation in levels within the sewer
Sludge, sand and coarse dirt
Fouling with biofilms
Risk of explosion due to gases
For these reasons, taking measurements within the sewer is considerably more difficult and problematical than in a wastewater treatment plant.
