Assessing heat recovery potential from sewers

Written: Arttu Pitkänen & Kalervo Aho

Climate change has forced municipalities to invest in low-carbon energy resources. Especially waste heat must be collected as efficiently as possible. Waste heat means all the energy flows which are produced by the main process but are not utilized. The difficulty has been the temperature levels in waste heat flows which are too low to be used as such. Lately, low-temperature energy resources have been a hot topic due to heat pumps, which allow to prime the low temperature flows to higher temperatures. This allows utilizing a lot of new energy flows which haven’t been available earlier. One very good example is heat recovery from sewers, which could include up to 19 % of the whole building’s energy output.

Energy inputs and outputs of a building.

Fluidit took part in a multi-company project where the energy potential of the heat recovery from sewers was investigated. The entire project consists of 5 divisions where Fluidit completed the research on network modeling.  The target was to model the thermal behavior of the entire network under different conditions to make sure the wastewater treatment processes won’t be disturbed. The physical model allows exploring different future scenarios where the heat recovery from the sewers is executed and gives a better understanding of the current network behavior. The work was accomplished as part of the master’s thesis of Fluidit district energy expert Arttu Pitkänen. The simulator development was done by Markus Sunela, the CTO of Fluidit. In the study approach, heat recovery was installed at every wastewater source in the network.

Heat recovery is installed at each wastewater source of the network.

Heat recovery from sewers is a technologically new field, there aren’t existing tools for modeling the system behavior. That’s why Fluidit developed new software that upgraded the existing Fluidit Sewer -software with a thermodynamical solver. We wanted the simulator to be fast and easy to use and at the same time handle networks that are 1:1 size to real networks.

We started the software development by selecting the most relevant heat transfer processes in the sewer conduits. The challenge was the air in the sewer pipes which makes things a little more complicated. However, we managed to form the most relevant partial differential equations which model the heat transfer processes. Because we wanted to be exactly sure about the used parameters, we conducted Computational Fluid Dynamics (CFD) calculations to ensure the heat transfer coefficient between wastewater and air is realistic. We used CFD also to estimate the heat gradient in the soil. After the CFD calculations were conducted, we created a tool for predicting ground temperatures at a specific level in the ground, based on the ambient air temperature. The ground temperature was one of the most important factors affecting the temperature of the wastewater due to sensitivity analysis. The final part was to create heat loss coefficients for the conduits based on the conduit dimensions, material, and the heat gradient in the soil.

Schematical view of the heat and mass flows in the pipeline.

When all the parameters and equations for the simulation were clarified, the calibration of the simulator started. The wastewater temperature measurements in Turku and Helsinki area were used to make sure the simulator gives realistic results. Simulations with the larger models yielded good results. Past studies had made research with much smaller networks, that were one tenth of the sizes used in the research. Studied networks were 610 km and 470 kilometers in length. There was no other simulator on the market that could’ve simulated the networks.

The comparison of simulated and measured temperatures.

Uncertainty of the simulations could be reduced by inducing more measurement points of temperature for calibration and measuring the ground temperature near the pipelines.

The study was successfully completed at the beginning of 2022 and the major outcomes of the study were:

  1. Using heat recovery at the source (residential / block) has no perceivable effect on the functioning of the network in the modeled scenarios
  2. Heat recovery may increase costs at the WWTP processes during winter and spring times
  3. Heat recovery reduces the temperature 10-18 % at the wastewater treatment plant, depending on the time of the year and the network
  4. Temperature of the soil has the greatest effect on sewage temperature during winter times
  5. Change of 1 degree of Celsius in the inlet sewage changes the temperature at the WWTP by 0.44 degree of Celsius.
  6. In the studied networks of Turku and Helsinki, the most feasible time for heat recovery from the wastewater would be during summer and autumn.
  7. The most cost-efficient method for arranging heat recovery from wastewater would be to install the recovery to pumping stations or other nexus points of the network.
  8. Finding out the potential heat recovery points and the energy potential can easily be done with the developed simulation software by Fluidit

If you are interested in using Fluidit software in your heat recovery projects, please contact us for more details!

Find the link to Arttus’s full thesis on the Resources page.