The present paper deals with the classification of the suitability of combined sewers for the installation of heat exchangers and with assessment of the theoretical potential of wastewater in the sewer system for heating of buildings. A classification scheme involving criteria like theoretically available heat, sewer diameter, number of the heat exchanger parallel modules in the sewer cross-section, hydraulic conditions (hydraulic capacity of the sewer, pressurized flow), and potential fouling by biofilm growth was developed. First, individual sewers in the pilot catchment were assessed based on monitoring the flow characteristics and wastewater temperatures and on pipe flow modelling. Second, connectivity of the suitable and partly suitable sewers was examined with respect to the length necessary for the installation of the heat exchanger with the minimum required power of 100 kW. For the continuous sewer sections, the maximum potential power was calculated. The presented approach is generally applicable, however, for other heat exchanger types and other climatic and economic conditions, values of the suitability criteria for the heat exchanger installation must be adapted.
 Lundie S, Peters GM, Beavis PC. Life cycle assesment for sustainable metropolitan water systems planning. Environ Sci Technol. 2004;38:3465-73. DOI: 10.1021/es034206m.
 Elías-Maxil JA, van der Hoek JP, Hofman J, Rietveld L. Energy in the urban water cycle: Actions to reduce the total expenditure of fossil fuels with emphasis on heat reclamation from urban water. Renew Sust Energy Rev. 2014;30:808-820. DOI: 10.1016/j.rser.2013.10.007.
 Racoviceanu AI, Karney BW, Kennedy CA, Colombo AF. Life-cycle energy use and greenhouse gas emissions inventory for water treatment systems. J Infrastruct Syst. 2007;13:261-70. DOI: 10.1061/(ASCE)1076-0342(2007)13:4(261).
 Cheng CL. Study of the inter-relationship between water use and energy conservation for a building. Energy Build. 2002;34:261-266. DOI: 10.1016/S0378-7788(01)00097-4.
 Daigger GT. Evolving urban water and residuals management paradigms: Water reclamation and reuse, decentralization, and resource recovery. Water Environ Res. 2009;81:809-823. DOI: 10.2175/193864708790893378.
 Meggers F, Leibundgut H. The potential of wastewater heat and exergy: Decentralized high-temperature recovery with a heat pump. Energy Build. 2011;43:879-886. DOI: 10.1016/j.enbuild.2010.12.008.
 Baek NC, Shin UC, Yoon JH. A study on the design and analysis of a heat pump heating system using wastewater as a heat source. Solar Energy. 2005;78:427-440. DOI: 10.1016/j.solener.2004.07.009.
 De Paepe M, Theuns E, Lenaers S, Van Loon J. Heat recovery system for dishwashers. Appl Therm Eng. 2003;23:743-756. DOI: 10.1016/S1359-4311(03)00016-4.
 Kahraman A, Çelebi A. Investigation of the performance of a heat pump using waste water as a heat source. Energies. 2009;2:697-713. DOI: 10.3390/en20300697.
 Wanner O. Wärmenutzung aus abwasser (Heat recovery from sewer systems). Schlussbericht Projekt Nr. 44177. Bern: Bundesamt Energie BFE. http://www.energieforschung.ch/; 2004.
 DWA. DWA - M114: Energie aus Abwasser - Wärme- und Lageenergie (Energy from Wastewater - Thermal and Potential Energy). DWA Hennef. http://www.dwa.de; 2009.
 Zogg M. History of heat pumps. Swiss contributions and international milestones. 9th Int IEA Heat Pump Conf. Zürich. Switzerland. IEA 1-16. http://www.zogg-engineering.ch; 2008.
 Kim J, Kim J, Kim J, Yoo C, Moon I. A simultaneous optimization approach for the design of wastewater and heat exchange networks based on cost estimation. J. Cleaner Prod. 2009;17:162-171. DOI: 10.1016/j.jclepro.2008.04.005.
 Czarniecki D, Pisarev V, Dziopak J, Słyś D. Technical and economic analysis of the application of the wastewater heat recovery system in an apartment building. Wrocław: Ofic Wyd Politechniki Wrocławskiej; www.eko-dok.pl/2014/13.pdf; 2014.
 Wanner O, Panagiotidis V, Clavadetscher P, Siegrist H. Effect of heat recovery from raw wastewater on nitrification and nitrogen removal in activated sludge plants. Water Res. 2005;39:4725-4734. DOI: 10.1016/j.watres.2005.09.026.
 Hvitved-Jacobsen T, Vollertsen J, Nielsen PH. A process and model concept for microbial wastewater transformations in gravity sewers. Water Sci Technol. 1998;37(1):233-241. DOI: 10.1016/S0273-1223(97)00774-9.
 Łagód G, Sobczuk H, Suchorab Z, Widomski M. Flow parameters effects on aerobic biodegradation of pollutants in sewer system. Ecol Chem Eng A. 2011;18(7):865-876. YADDA: bwmeta1.element.baztech-article-BPG8-0061-0002.
 Łagód G, Sobczuk H, Suchorab Z, Widomski M. Advection-dispersion pollutant and dissolved oxygen transport as a part of sewage biodegradation model. Environ Prot Eng. 2009;35(3):305-317. http://epe.pwr.wroc.pl/2009/Lagod_3-2009.pdf.
 Wilderer PA, Cunningham A, Schnidler U. Hydrodynamics and shear stress: Report from the discussion session. Water Sci Technol. 1995;32(8):271-272. DOI: 10.1016/0273-1223(96)00038-8.
 Ahyerre M, Chebbo G, Saad M. Sources and erosion of organic solids in a combined sewer. Urban Water. 2000;2:305-315. DOI: 10.1016/S1462-0758(01)00012-7.
 Banasiak R, Verhoeven R, De Suttera R, Tait S. The erosion behavior of biologically active sewer sediment deposits: Observations from a laboratory study. Water Res. 2005;39:5221-5231. DOI: 10.1016/j.watres.2005.10.011.
 Pisano WC, Barsanti J, Joyce J, Sorensen H. Sewer and tank sediment flushing: Case studies. Report EPA/600/R-98/157. Cincinnati, US EPA. https://www.epa.gov/nscep; 1998.
 Guzmán K, La Motta E, McCorquodale J, Rojas S, Ermogenous M. Effect of biofilm formation on roughness coefficient and solids deposition in small-diameter PVC sewer pipes. J Environ Eng. (Reston, VA, U. S.). 2007;133(4):364-371. DOI: 10.1061/(ASCE)0733-9372(2007)133:4(364).
 Kay JM, Nedderman RM. An Introduction to Fluid Mechanics and Heat Transfer. Cambridge: Cambridge University Press; 1975.