In some waste water treatment plants, heat has long been recovered from grey water and then fed into a district heating network. One such example is the City of Zurich's Werdhölzli waste water treatment plant. The use of waste water as a heat source will continue to gain in importance: the Verband Fernwärme Schweiz (Swiss District Heating Association) estimates that around 11 percent of the total potential renewable heating sources for district heating will come from waste water treatment plants.
Not under 10°C
In a study published in the journal “Water Research”, Bruno Hadengue and his co-authors evaluate the ideal point at which to recover heat from waste water on its journey from the plumbing installations in buildings to the waste water treatment plant. The question is significant, because overcooled water can have a negative impact on the biological breakdown processes in the waste water treatment plant. For this reason, heat recovery from waste water is only permitted in Switzerland if it is not cooled below 10°C. This is a restriction that may well prove to be a problem in winter.
Data from the Fehraltdorf waste water network
For his study, Bruno Hadengue developed a chain of models which can be deployed to carry out thermo-hydraulic simulations of households, domestic pipe connections and the public waste water network. Using actual data from the waste water network in Fehraltdorf, collected as part of the Eawag Urban Water Observatory (UWO) project, the researchers created a reference scenario to be used for comparison with the simulated scenarios. “Our aim is to evaluate the consequences of alternative energy technologies on the waste water system”, explains Bruno Hadengue. “In the long term we want to show exactly what happens at what point in the system when heat is recovered”.
The results obtained so far from the study are clear: Systems that recover heat within households, for example from a shower drain channel (e.g. the Joulia system, which is already available commercially), have an insignificant effect on the temperature of the waste water. On the other hand, systems installed directly in the sewer system do have an impact. Such heat recovery installations come into their own where the sewer system is close to a district heating network. This can avoid wasteful transit distances.