Heat recovery from untreated wastewater : A case study of heat recovery from sewerline to district heating network

Abstract:  High amounts of heat energy are today deposited into the urban wastewater system. The current society-wide development for energy efficiency has so far barely touched upon the area of wastewater heat conservation, which is why the share of total energy consumption from water use is increasing. Through this master thesis a case study was performed, assessing in particular the heat recovery potential from untreated wastewater in the common sewer line upstream from a wastewater treatment plant (Käppalaverket) for supply onto the local district heating network (Norrenergi) by the use of a heat pump solution. The current wastewater treatment process is using temperature dependant biological treatment for denitrifying the wastewater before it is disposed to the Baltic Sea, which poses limitations on upstream heat extraction. The purpose of the study was to assess the heat recovery potential and possibilities when using untreated wastewater compared to what is done traditionally using treated wastewater after the treatment plant. Furthermore a technological review was done over the area of heat recovery from untreated wastewater and also an evaluation of potential equipment and technology suppliers. Hydraulic modelling and thermodynamic simulations of the wastewater system were performed. Results showed that during a majority of the year approximately 4 MW of heat could be extracted while staying within conservative limits in regards to a minimum influent temperature as well as a maximum upstream temperature decrease. During wet season however, no or very limited heat could be recovered as the influent temperatures are already in a rather sensitive range in regards to the biological treatment process. At this level, through analysis of available equipment for heat recovery from untreated wastewater, a maximum heat amount of approximately 18 GWh per year could be supplied to the district heating network. Furthermore, it was found that reducing the amount of supplementary water in the system would be highly beneficial, both regarding HR potential but also for the treatment process in the plant. Also, if extensive HR performed by water consumers would occur, the model shows that this would probably have a negative effect on downstream temperature and the treatment process. Through this study it was concluded that even though the theoretically available heat in the system is very large, the practical heat recovery potential is very limited under current conditions. The strongest reason is the limitation posed by the temperature requirements of the influent wastewater. If also cooling is considered, the heat recovery prospects might be better due to the lower net energy extraction from the wastewater. Regarding the economic feasibility of an installation for heat recovery from untreated wastewater, the assessment made in this project showed that it may actually be comparable to projects using other types of waste heat. The results and conclusions from this study should not be considered as a green light, or as motivation, for performing any upstream heat recovery installations. Such projects must be done in consensus with local authorities and especially the wastewater treatment plant in question. Further analyses in this area is considered essential before exploring it further, such as assessing the transient behaviour of the surrounding rock walls when heat is recovered upstream. The model used in this study also needs confirmation through actual temperature measurements within the system, which do not exist at the moment. Furthermore, a complete life cycle analysis should be carried out for the entire urban water system, which should find an optimal way of where to use and to recover the energy.