Written by Judhajit Chakraborty, Assoc. AIA, LEED, AP, WSP Built Ecology
In our quintessential I search for using renewables in almost all our projects, we mostly look up to the skies to the Sun God and sometimes to Wind. But we never looked underground to the humongous renewable resource that is actually is a bi-product of us – Sewage. Wastewater is an endless energy source which starts in waste water streams of residential, commercial and industrial buildings and resides in our municipal sewer mains. Now, that we know or have an idea of the abundance of sewage, what does it do? Well, the sewage water is high temperature water (roughly around (60°F – 68°F). So, for a major chunk, this heat which goes into waste can be recovered and used for HVAC heating and also for Domestic Hot Water heating. But wait, that doesn’t end here. During summer months, cooling is needed; the sewer main can be used as a heat rejection device thereby completely eliminating the
need of cooling towers in large buildings. Now, how cool is that for something we never cared about. Sewage water is also treated for non-potable water demands like toilet flushing and irrigation. Therefore, projects having serious energy and water goals should consider sewage as the Mantra to their design goals. And if the site has a sewer main in its vicinity, then BINGO, you have found gold.
In this article I will be mainly explaining about the Sewer Heat Recovery process.
The Sewer Heat Recovery system treats the sewer main as a large heat sink. The system includes filtration and pumping of sewer water through a heat exchanger, providing a clean, closed loop condenser water system which can be connected to both heat pumps for heating and chillers. By closed loop, it means that none of the sewage elements other than the heat gets transferred. This system enables the use of heat pumps for heating which drastically reduces the energy required to heat the building. Compared to traditional gas boilers or electric resistance heating, heat pumps are able to save over 50% of the energy consumed for heating. In addition this shifts the heatingmon to the electricity which can be offset by on-site PV panels which is again a renewable source. The other advantage is that these Heat Pumps are about 6 times more efficient than normal boilers (COP of 5 and above). Similarly for Domestic Hot Water, the same principle is used. It takes a lot less energy to heat 65°F water than 50°F water. During summer months, when there are cooling requirements, these systems can reverse their heat pumps and use the 65°F sewage to dump excess building heat and reduce the air conditioning costs. This is exactly similar to geothermal systems which use the earth’s temperature for heating and cooling. But geothermal systems are way more expensive than these systems.
There aren’t many players in this new field. International Wastewater systems
based in British Columbia, Canada are among the very first developers of sewer heat recovery heat pumps called the SHARC series. Their heat pumps were used in the first big wastewater heat recovery system in North America, in Vancouver where they provided more than 70% of the energy needs to the Olympic Village designed during
the 2010 Winter Olympics. This is now one of the world’s greenest communities which have become a residential and mixed use enclave after the games. True with every other high performance building systems, these systems are slowly making inroads in the US market. It is still a new born baby in the industry and only through more education, awareness and outreach that designers and developers can understand the amazing potential of the dirty water as a source of clean renewable energy. It is out there, we just have to understand and accept this endless and untapped resource of energy.