Wastewater Thermal Heat Recovery: How Public Utilities Can Prepare and Take Advantage Of This Upcoming Resource Recovery Opportunity

BACKGROUND
Thermal heat recovery from wastewater is an underutilized technology market in North America but this is anticipated to change quickly. Sewer or wastewater heat recovery is using the latent heat of wastewater as an energy source, most commonly for hot water supply or for heating and air conditioning. Increasing efficiencies of heat exchangers and heat pumps make this a viable and cost-effective possibility. Systems are in operation utilizing in-building heat pumps, heat exchangers installed within the sewer piping itself, pumped bypasses from the sewer, or installations 'within the fence' at water resource recovery facilities (WRRFs). See Figure 1 for a typical system illustration and Figure 2 for common technology solutions on the North American market. Since about two decades, several central European cities have started to systematically evaluate and implement project opportunities for using the year-round moderate temperatures of wastewater for heating or cooling of buildings, hot water generation, or the heating of anaerobic digesters. North American municipalities can now benefit from the planning and technical approaches that have proved successful overseas (Velsen & Benz 2013, Brunk & Seybold 2014, Eccuro, 2020; Berlin Wasser, 2019). First pioneering utilities have implemented projects in the US and Canada defining the kind of project applications and technology solutions proven successful to make project payback periods attractive (Kohl 2019, personal interviews with various US and Canadian utilities and districts). Several ongoing developments make it timely for utilities to become aware of wastewater heat recovery opportunities for building heating and cooling and consider encouraging and supporting such projects within their service areas.
RELEVANCE
Key developments making wastewater thermal heat recovery timely in the US and Canada include:
1) Effluent Temperature Limits. Several US states have started to implement temperature standards for cold water surface water bodies that will require some water resource recovery facilities (WRRF) to cool effluents during parts of the year prior to discharge in coming years. The authors of this abstract alone have been involved in preparing effluent cooling cost studies for more than five utilities in three US states (Colorado, California, Oregon) over the past two years. The default technology for cooling compliance are cooling towers which come with significant capital expenditures, operational challenges, additional O&M costs, and increased energy consumption. Thermal energy is starting to be recognized as a wastewater pollutant and source mitigation through energy recovery is one viable, and much more sustainable compliance strategy. However, implementation of projects takes time and needs to be planned for early and broadly to be impactful in effectively supporting compliance strategies.
2) GHG reduction goals. A growing number of US cities and states have committed to clearly defined GHG reduction targets and timelines. As such, first cities have taken the implementation steps to develop renewable heating and cooling plans for buildings (e.g., City and County of Denver, June 2021) or preparing legislation to ban natural gas from new buildings to advance electrification codes (see Figure 3). The commonly considered renewable energy alternatives to natural gas are solar, wind, and geothermal energy. All of these technologies have limitations when it comes to the implementation for larger residential, commercial, or public buildings due to large space requirements. Heat recovery from wastewater, however, is a feasible renewable alternative specifically for larger construction projects due access to wastewater locally produced or located in adjacent large sewer interceptors. Return of investments of wastewater heat recovery systems can be more attractive compared to traditional renewable energy sources currently considered by planners and developers.
STATE OF THE INDUSTRY
To date, a variety of heat recovery systems are in operation in the US and Canada. Figure 4 highlights a few different types of systems across the US that will give WEFTEC participants an overview of possible technology scales and applications. King County (Seattle, WA) and Metro Water Recovery (Denver, CO) are implementing utility-wide systematic administrative approaches for regional heat recovery. These utilities are driven primarily by regional sustainability mandates. Metro Water Recovery in Colorado is also driven by future regulatory concerns related to temperature discharge. Their collaboration with the National Western Center Complex in Denver is to date the largest sewer heat recovery project in the US, with a 4 megawatt heat recovery system estimated to offset 2,600 tons of CO2 per year. Smaller developer or facility led sewer heat initiatives are also operational. For example, in Washington, DC, the American Geophysical Union renovated its campus in 2015, to utilize pumped sewer heat recovery for heating, cooling, and hot water for a 7-story office building. Avon, Colorado, has an older heat exchanger system that uses wastewater effluent heat recovery for heating of the community swimming pool. Madison MSD, WI, uses effluent heat in a similar fashion to heat their administrative building. The Boulder Commons net-zero project in Boulder, Colorado, is a more recent developer-led mixed use project utilizing sewer heat recovery for all hot water supply needs.
HEAT RECOVERY PROGRAM IMPLEMENTATION CHALLENGES
This paper will discuss common challenges utilities face when establishing heat recovery programs within their service districts and helpful resources available. This summary draws upon the experience of other utilities leading such efforts over past years in the US, Canada, and Europe that the authors collaborated with over past years, and from two ongoing wastewater heat recovery projects Carollo is currently conducting for the City of Boulder, Colorado, and Metro Water Recovery, Colorado, respectively, that aim to define the potential for heat recovery and help establish successful community-wide wastewater heat recovery programs. Briefly, key challenges that utilities should be prepared for include the following.
1. Data and information gaps. a. This includes temperature data in sewer lines.
2. Lack of interdepartmental coordination and private partnerships a. Identification and successful implementation of projects often requires the close collaboration of various departments and private owners, developers, or funding partners.
3. Understanding to how use applicable environmental attributes a. I.e., thermal renewable energy credits, the legal basis, broker process, and market values
4. Lack of awareness and incentives a. Lack of integration with local ordinances, building codes, etc. b. Public communication, marketing (Figure 5)
5. Establishing the necessary administrative process a. Support staff b. Legal contracts c. Engineering workflows d. Financial agreements
IMPORTANCE
This paper raises awareness of the potential, benefits, and successful examples of wastewater thermal heat recovery installations and programs in North America. Because it is anticipated that significant capital investments will made in effluent cooling infrastructure and alternative energy sources for private and public buildings in the US starting in coming years, it is important that utilities are aware of environmentally and fiscally responsible alternatives, technology options and resources that can help establish successful heat recovery programs.