Heat recovery from urban wastewater: Analysis of the variability of flow rate and temperature in the sewer of Bologna, Italy

Domestic wastewater were characterize by higher temperature because inside the buildings 60% of the water was heated by showers, tubs, sinks, dishwashers and clothes washers. This heat, nowadays lost into the sewer system, could be make the wastewater a carrier of heat. This heat can be reused, for the production of clean and regenerable thermal energy, through heat exchangers and heat pumps, for the conditioning and heating of buildings. There are several options to recover this heat embedded in the wastewater: within houses (small scale applications), from the sewer (medium scale applications), or at wastewater treatment plants (large scale applications). In this paper results of a monitoring period of six months in the sewer system of Bologna (Italy) have showed the variability of wastewater flow rate and temperature and their daily trend. The trend of the flow (ratio between the hourly flow and the medium daily flow) for the generic day is linked with the population, with coefficients that are in the range of 0.25 - 1.50 with peak values ranging between 1.30 and 1.50; the trend of the sewage temperature for the generic day has coefficients in a range between 0.90 and 1.05, and it is independent by the population. The amount of thermal energy that can be obtained from wastewater and the optimally design of heat recovery systems depend on knowledge of the flow rate and wastewater temperature. This study can be useful to map the potential thermal energy of sewage systems and to design the heat recovery systems. © 2013 The Authors.