Analysing wastewater temperature development in a sewer system as a basis for the evaluation of wastewater heat recovery potentials

Energy Recovery from Wastewater: A Study on Heating and Cooling of a Multipurpose Building with Sewage-Reclaimed Heat Energy

“…The temperature trend observed in the sewer was in accordance with the data reported by Cipolla and Maglionico [17], Wanner et al [23], and Kretschmer et al [19], respectively, in Bologna (Italy), Zurich (Switzerland), and in a 24,000-population town in Austria.…”

“…Based on the aforementioned data, it is possible to use Equation (1) (modified from the original equation proposed by Kretschmer et al [19]) to graphically depict the relationships between the wastewater flow rate diverted at the recovery site for heat exchange (QRS, L s −1 ), available heat potential (PRS, kW), and temperature difference or decrease due to heat recovery (ΔTRS, K), as shown in Figure 4: PRS = QRS · c · ΔTRS · ρ (1) where c is the specific heat capacity of wastewater (4.18 kJ kg −1 °C −1 [24]), and ρ represents the wastewater density (assumed to be 1000 kg m −3 ). The temperature trend observed in the sewer was in accordance with the data reported by Cipolla and Maglionico [17], Wanner et al [23], and Kretschmer et al [19], respectively, in Bologna (Italy), Zurich (Switzerland), and in a 24,000-population town in Austria.…”

“…Based on the aforementioned data, it is possible to use Equation (1) (modified from the original equation proposed by Kretschmer et al [19]) to graphically depict the relationships between the wastewater flow rate diverted at the recovery site for heat exchange (Q RS , L s −1 ), available heat potential (P RS , kW), and temperature difference or decrease due to heat recovery (∆T RS , K), as shown in Figure 4:…”

“…where c is the specific heat capacity of wastewater (4.18 kJ kg −1 • C −1 [24]), and ρ represents the wastewater density (assumed to be 1000 kg m −3 ). The change of temperature in the sewer immediately after the reintroduction of the heat-recovered wastewater (ΔTSEWER, °C) can be calculated using Equation (2) [19]:…”

“…A heat exchanger is installed in direct contact with the wastewater that serves as a heat source or sink, and is later connected to a heat pump and then to the heating and cooling system of a building situated in close proximity. Temperatures of civil wastewater may be more than 25-27 • C in domestic outflows [18], representing a significant thermal energy source [16]; at the inlet of water treatment plants or water resource recovery facilities (WRRFs), the temperature is far lower (15-25 • C or lower, depending on the climate) [16,19,20], and thus, its use for heat recovery may be less practical. Also, heat recovery at this stage should be carefully considered, as it might interfere with treatment processes: biological processes, including denitrification, are sensitive to wastewater temperature, as biological reaction rates depend on Arrhenius' law, and excessive heat extraction from sewage could harm their efficiency [21].…”