Written by: Adam Frey
It is only relatively recently that the opportunity to tap into municipal wastewater pipes as a thermal supply has been considered in North America. The sewage collection system provides a stream of flowing liquid (i.e., sewage) with relatively stable temperatures. Similar to the function of boreholes in a ground-source heat pump (GSHP) system, the wastewater provides a medium for heat recovery or heat rejection (a source/sink) for building heating and cooling when coupled with a heat pump. There are multiple names used in the industry to describe the approach – the term wastewater energy transfer (WET) is accurate and convenient. The table below offers a comparison key design and deployment considerations of WET systems, closed-loop (boreholes with underground pipes) and open-loop (production wells from an aquifer) ground-source heat pump systems (GSHP). Some significant advantages of WET systems over GSHPs is that there is no need for deep drilling to access the source of heat energy, and because it is continuously flowing, there is no need to balance the annual heating load to prevent shifts in ground temperature.
|WET||Closed-Loop GSHP||Open-loop GSHP|
|Daily Variability||Maybe, if project uses full flow||None||None|
|Seasonal Restrictions||~ None||Heating=cooling||~ None|
|Geographic Distribution||Linear along pipes||Nearly anywhere||Local geology dependence|
|Feasibility Assessment||Acquire measured flow data||~ None||Hydrogeologist consulted and drill deep well|
WET projects are not common in North America, but they have been implemented and are quickly being realized in many municipalities across Canada and the United States. They are more prevalent in Europe where there are higher costs for natural gas. The oldest running facility in Canada is the Southeast False Creek Neighborhood Utility in Vancouver, first developed in 2010, and planned for expansion in 2022. It supplies heat in combination with natural gas boilers that service a large district energy loop. Other major facilities include American Geophysical Union (AGU) in Washington DC, the National Western Centre in Denver, as well as planned projects at the Toronto Western Hospital and the Cogswell development in Halifax.
The City of Ottawa Energy Evolution Program has set a goal for the City to achieve carbon neutrality by 2050 and for City’s corporate operations to achieve carbon neutrality by 2040. One key need within this plan is for buildings and infrastructure to move to zero carbon heating using a range of viable technologies including air source heat pumps, ground source heat pumps, district energy systems that in turn have low carbon thermal supplies, and waste heat recovery. Within these options, WET systems were identified as a potentially sizable new opportunity that the municipality in particular will hold a role.
J.L. Richards & Associates Limited was retained by the City of Ottawa to provide a scoping study of the resource potential and viable use cases for implementing below-ground thermal energy resources from Ottawa’s sewer system and aquifer systems. The following is a summary of the findings of the study.
There are four different types of WET systems:
- using the drainpipes within a building;
- using the building’s main sanitary drain;
- using municipal sewage collection systems; and
- using the wastewater treatment plant effluent.
The first two are typically used for domestic hot water preheating and heating, respectively, while the latter two can be used for space heating and cooling of facilities. Within our study, we examined WET systems that couple with municipal wastewater pipes.
Key Thermal Parameters of Wastewater Systems
Sewage flow and sewage temperature are two key parameters that must be investigated when considering the sewage collection system as a thermal resource.
Technologies used to recover or reject heat to sewage have minimum sewage flow requirements. Thus, it is important to understand the minimum sewage flow available at locations where sewage is being considered as a thermal resource. However, the design of municipal collection systems is typically focused on understanding what the average and peak flows are throughout the system, based on the sewage generation from a contributing population and the effects of inflow and infiltration due to runoff and groundwater. Typically, minimum flow rates are not a focus during sewage collection system design.
As the generation of sewage in the City is the result of human activities throughout the day, it is intuitive to expect that the flows within the piping system fluctuate throughout the day. In general, peak flows occur in the morning and late afternoon relating to residential pre- and post-work usage. Flows are also reasonably high throughout the day and drop off substantially at night. Flows from major facilities such as universities, large manufacturing sites, and hospitals can be significant and with their own unique patterns. Although the flows fluctuate throughout a day, human-generated sewage flows are very consistent on a weekly, monthly, and even seasonal basis. In general, elements that impact sewage flow volumes include:
- Potable water consumption;
- Industrial water use;
- Infiltration flows from rain, snow melt, and groundwater; and
- In the case of combined sewer pipes (sanitary plus stormwater), flow is greatly influenced by rain and snow melt.
In areas where the minimum sewage flows are short in duration (a few hours) and less than the desired flow rate for the heat load, there may be opportunities to buffer the minimum flows through the use of larger wet wells that can act as temporary storage. This buffering or storage would increase the square area requirements and costs of the system but could provide opportunities in areas with marginal sewage flow rates. The daily average sewage flow rates would need to exceed the WET system requirements.
It is important to establish the temperature profile (including minimum and maximum sewage temperature) at the location of interest. Understanding the temperature fluctuations in the sanitary sewer flows is important in determining (i) the amount of heat that can be extracted from or injected into the sewer lines without adversely influencing the operations of the system, and (ii) the efficiency of the heat pump system (generally described by the coefficient of performance).
Sewage temperatures vary on a daily basis in relation to amount of facility hot water use. They also fluctuate throughout the year. In general, elements that impact sewage temperature include the following:
- Potable water distributions temperatures;
- Ground temperatures encasing the piping system which fluctuate with depth;
- Air temperatures (both seasonal and daily changes);
- Air movement within the sewer system;
- Infiltration flows from rain, snow melt, and groundwater;
- Land Use; and
- Large facilities with high hot water usage, such as hospitals.
As sewage is comprised mostly of water it is assumed to have the same specific heat capacity as water, which is 4.18 kJ/kg·K. The heat capacity that can be extracted from the sewage at a given time requires only three inputs: the sewage flow rate, the temperature drop in the heat extraction process, and the specific heat capacity of sewage (the value for water is generally assumed to be valid for sewage).
The below graphs show the temperature, flow and corresponding heat capacity for three sites within Ottawa from November 2020 to April 2021. The flow and temperature data was obtained from submersible probes being used for ongoing University of Ottawa COVID research that is monitoring COVID levels in the wastewater. As you can see, temperature profiles across the different sites were very similar with daily flow patterns that show minimum flow during nighttime hours. We considered a maximum temperature drop of 5 degrees Celsius for the technology used below. Flow had a larger impact on heat capacity than temperature. The Acres location (shown in blue) has significantly higher flow rates and is scaled on the right-hand y-axis.
WET technology types
WET systems generally consist of a heat exchange processes combined with pumping loop for sewage. The technologies commonly available can be classified according to the location of the main heat exchanger used to extract waste heat from the sewage. These categories include:
- Technology 1 – Internal Sewer Pipe Heat Exchanger (inserted inside the pipe);
- Technology 2 – Integral Sewer Pipe Heat Exchanger (integral to the pipe wall);
- Technology 3 – External Heat Exchanger.
The first technology requires installation of heat exchanger plates and tubes directly inside the existing sewage collection pipes. Fluid (e.g., water, glycol, etc.) is pumped in a closed loop from a mechanical room or building to the submerged heat exchanger tubes where it absorbs heat from sewage flowing through the sewer pipe. An example of this system is the Therm-Liner system from the German company UHRIG Group.
The second technology requires the replacement of existing sewage pipes with customized sewer pipes that contain heat exchanger coils integral to the sewer pipe wall. Fluid is pumped in a closed loop from a mechanical room or building to the heat exchanger for heat recovery or rejection, similar to Technology 1. The key difference is that the heat exchanger coil is integral to the pipe wall or wrapped around the pipe, rather than submerged in sewage inside the sewer pipe. Two Germany-based companies were identified that manufacture and distribute custom sections of sewage collection piping for heat recovery: Frank PKS NZ Ltd. (PKS), and Rabtherm AG.
The biggest difference between Technology 3 and the other technologies reviewed is that sewage is being pumped directly to a heat exchange system external to sewage collection system. Sewage is diverted from the City’s sewage collection system to fill a separate sump pit or tank for short-term volume build-up. Sewage contained in the pit is filtered to remove large solids and pumped to a heat exchanger within a mechanical room or building to recover or reject heat. After passing through the heat exchanger, the pumped sewage is combined with filtered solids and discharged to the municipal sewer downstream of the point of diversion. JLR reviewed products from two manufacturers of this technology: Huber Technology Inc. and Sharc Energy Systems.
Within the study, four archetypes of applications were studied at locations such as a downtown collector, a pump station, an urban collector as well as a major trunk line. Building energy modelling was also used to estimate the load profile against the capacity of the lines at two of the locations to obtain a levelized cost of heat or LCOH.
For more on the outcomes of the WET study for the City of Ottawa please see a recording of the webinar as well as the slide deck with the links below.
Adam Frey is an Energy Systems EIT at J.L. Richards and Associates Ltd. with over 10 years’ experience in the solar and HVAC industry. He is an experienced sustainable energy systems designer skilled in both renewable heat and power systems with focused expertise in building energy modeling as well as modelling solar photovoltaic (PV) systems.