Putting Wastewater to Work: Tapping the Energy Flow Just Beneath Our Feet

Part Two in a Three Part Series

The nation’s capital has long been a leader in green architecture, with more LEED-certified office space per capita than any state in the union (39.83 gross square feet per resident)—a distinction that earned it the first ever LEED Platinum leadership certification from the U.S. Green Building Council. Now it’s marking another milestone by becoming the first city in America to begin heating and cooling that office space with an economical and reliable form of renewable energy just a few feet below the surface of its sidewalks.

As readers of the first of this three-part series of blog posts already know, that renewable energy is wastewater. Employing a technology widely used in Europe and Canada and introduced into the U.S. by SmithGroup, DC Water, the capital’s water and sewer authority, will convert heat from wastewater into energy that can be used to both heat and cool its new 151,000 SF headquarters on the banks of the Anacostia River. Read more about the history of the technology and how it works in the first installment of this series, “Putting Wastewater to Work: America’s Next Great Energy Source.”

In this follow-up, we elaborate on the research we did to determine the best version of this technology for our client, DC Water, and then present a cost analysis and comparison to establish what we consider to be a rock-solid business case for the adoption of what’s known by the catchy name of Sanitary Wastewater Energy Exchange (SWEE) technology.

Ground Truthing

While SWEE technology has a demonstrated record of success in Europe and Canada, any innovation applied for the first time in any locale requires that we first do our own due diligence. In 2014, when we started our homework, there were two manufacturers of offline sanitary wastewater energy exchange systems to choose from: Huber in Germany and SHARC Energy in Canada. Both systems achieve the same end but differ slightly in how they do it: SHARC uses a patented screened augur system to separate solid from liquid waste and a plate-and-frame heat exchanger to extract thermal energy from the latter, while Huber uses a screening and settling system for separation and a shell-and-tube exchanger for heat extraction.  

We further vetted the SHARC technology in a trip to Canada in November of 2014, to inspect manufacturing processes and quality control at SHARC Energy’s headquarters in Vancouver BC and to tour existing installations to talk directly with plant engineers and operators about the reliability of their systems and the maintenance requirements. That tour included visits to three of the first operating wastewater energy exchange systems in North America:

• The SAIL condominium project in Vancouver at the University of British Columbia, which captures waste heat from a multi-family residential project to generate domestic hot water. In operation since 2014, this SHARC Energy project has proven to be one of the most cost-effective heating systems to deploy commercially.
• The Gateway Theatre Project in Richmond, BC, which uses a SHARC system to draw wastewater from a lift station to create an ambient heating loop that, depending on the time of year, heats or cools a 50,000 sf multi-use theater.
• The Southeast False Creek Energy Center was built to provide all the heating for the Olympic Village at the 2010 Winter Olympics in Vancouver. Although not a SHARC installation, it is one of the first district-scale installations of wastewater energy exchange systems in the world.

Based on our research into the technology and the first-hand findings gathered on this trip, we felt confident that the SHARC system’s reliability and economy would help DC Water to realize its ambition of not only meeting the strict standards of LEED Platinum certification but pushing well beyond them.

Money Down the Drain

Whenever we wash our hands, take showers, do the dishes, or clean our clothes, to cite but a few of our daily activities, we’re throwing away reusable energy—energy we’ve already paid for.  How much energy? The EPA estimates that 385 billion kWh of waste heat energy goes down North American drains every year. That’s enough wasted energy to power more than 35,760 typical American homes, according to figures from the U.S. Energy Information Administration. SWEE technology—reliable, cost-effective and with a shorter payback period than other green alternatives—recaptures this energy and re-uses it for heating and cooling.

To get a better sense of what this means in financial terms for a single commercial building, our engineers took as their model a 150,000 SF office building in the Washington, DC Mid-Atlantic region and then calculated how much it would on average cost to both heat and cool it with: 1) a conventional HVAC system using a cooling tower and boilers; 2) a geothermal system and; 3) the SWEE system that ultimately was chosen for the new DC Water headquarters.

The conventional HVAC system with a cooling tower and boilers cost the least to purchase: $693,750, versus $840,000 for the SWEE and a whopping $1,425,000 for geothermal. After that, however, the race isn’t even close.

To calculate the size of a cooling system big enough to meet all of a given building’s year-round needs, engineers usually base their estimates on peak cooling load when demand is at its highest —e.g., the hottest day of the year in a fully occupied building. We took a more conservative approach, averaging out expected demand over the course of a year and then factoring in estimated maintenance costs for each of the systems.

Although the conventional system’s price tag was lowest, that advantage was quickly wiped out by its estimated maintenance costs of $18,750 per year compared to $5,250 per year for the SHARC’s SWEE (starting in year three after the expiration of a two-year warranty). Factoring those additional savings into the equation, we calculated the payback period for the SWEE to be about five years. Maintenance costs for geothermal are roughly comparable to the SWEE system, but its high initial purchase price translates into a payback period measured in decades, not years. Also, geothermal has only limited applicability in urban areas because few buildings have enough land for the required well space.

Finally, for those concerned about climate change, the SWEE system cuts our model building’s carbon emissions by more than 830 metric tons per year—which adds up to nearly 20,750 metric tons of CO2 not emitted into the atmosphere over the system’s 25-year service life. That is the equivalent of removing 182 cars from the roads for 25 years—just from the installation of one SWEE system in one office building.

Any building could tap into this energy flowing through city sewer mains with cooperation of the wastewater utility agency. Every million gallons of wastewater has the potential to produce a megawatt of cooling for a building. In a city like Washington, DC, producing on average 300 million gallons of wastewater per day, the cooling potential is 300 megawatts—enough to cool 62.5 million square feet of office space. Moreover, cooling 62.5 million square feet of office space with SWEE would save nearly 3.7 million metric tons of CO2 emissions annually (equivalent to a whopping 821,323 cars removed from the road!). Clearly, mining the energy that flows just beneath our feet is a major step forward in green building technology and could make significant improvements in the environment.

The third and final installment of this blog will cover the opening of the new DC Water headquarters and the report on the results of the first functioning SWEE system.