A significant amount of renewable heat flows through Metro Vancouver’s sanitary sewers each day as warm sewage. This excess energy can be recovered and used to heat buildings and is an economical means of displacing natural gas and reducing greenhouse gas emissions.
Because of the increased interest in extracting sewage heat for space heating and cooling, Metro Vancouver retained Kerr Wood Leidal Associates to examine the viability and implications of allowing sewage heat recovery projects. Metro Vancouver manages sewerage, drainage, and treatment of liquid waste for most municipalities in the Greater Vancouver area.
The objectives were twofold. First, to develop an understanding of the technical implications of allowing access to the sewers for sewage heat recovery projects. Second, to develop the tools needed to evaluate sewage heat recovery requests in a regional context.
The question was: how much heat can you safely recover without endangering sewage treatment processes, both now and in the future? To answer this question, KWL developed a Heat-Seeking Sewer Model and applied it to the five wastewater treatment plant catchment areas of Metro Vancouver’s sewer network.
KWL’s modelling results show that there is plenty of heat in the sewer system. Up to 100 MW of recoverable heat is available across the region, enough to heat about 650 to 1,000 high-rise buildings.
Why a sewer model is a complex problem
Accurately assessing how much sewage heat is available, and the best locations to recover it, is a complex problem. The sewer system is a dynamic system where a myriad of variable influences affect sewage temperature. Population growth, changes in land use, water conservation measures, rainfall and groundwater infiltration – to name just a few factors – all affect the temperature of sewage. To assess whether a sewer heat recovery project is advisable, it is critical to understand what is happening at any given point in the system, and to know what is influencing changes to sewage temperature.
Until the Heat-Seeking Sewer Model was developed, no model existed that could calculate these discrete changes for a large sewage network with multiple sewage heat recovery projects. Also, no sewer heat recovery model existed that provides a GIS interface and mapping capabilities.
This model estimates the available energy capacity within the sanitary sewer system as a whole and in each pipe section. It also identifies key factors that may affect the energy capacity. For example, it shows downstream constraints such as the heat requirements of wastewater plants and existing sewage heat recovery projects.
The model calculates the sewage flow rate and associated sewage temperature at nodes within the network. The sewage heat capacity throughout the system is calculated by tracking the flows and temperatures of each component separately.
The modelling engine consists of two modules: (a) a hydraulic modelling engine that calculates the daily average flow at each node in the collection system, and (b) an advective temperature modelling engine that tracks sewage temperature and available energy by using a mass balance of the sewer flow components. The advective temperature modelling engine also tracks the total energy recovered from the system by sewage heat recovery projects.
Because too much heat recovery is harmful, two temperature constraints were applied: the sewage temperature could not drop below 6ºC; and the inflow temperature at a wastewater treatment plant was set to a minimum of 11ºC. The 11ºC parameter is important because biological treatment processes are adversely affected if the temperature of the sewage falls below 11°C. Metro Vancouver was vitally concerned that sewage treatment not be adversely affected by sewage heat recovery.
Benefits to Metro Vancouver
The project allows Metro Vancouver to permit – or prevent – sewage heat recovery projects at specific locations. It has enabled Metro Vancouver to allocate available resources in an effective manner. Because of its mapping capability, the model also enables energy planners to match sewer heat resources to potential development areas.
The model can be run and rerun to assess and reassess changing conditions in the system. This ability is crucial to determining the viability and advisability of proposed sewage heat recovery projects.
Although the model was applied to Metro Vancouver, it can be deployed in any sewer collection system, enabling the widespread implementation and management of these potential sources of energy. cce
Project name: Heat-Seeking Sewer Model, Metro Vancouver, B.C.
Award-winning firm (prime consultant): Kerr Wood Leidal Associates, Burnaby, B.C. (Jason Vine, P.Eng., Alex Charpentier, P.Eng., Nathan Chase, EIT, Karen Sutherland, P.Eng., Yuko Suda, P.Eng., Alan Tse, EIT, Mike Homenuke, P.Eng., Wayne Wong, P.Eng., Mike McDonald, B.Tech, Nissim Levy)
Owner/client: Metro Vancouver