I have already written about extracting energy from sewage using microbial fuel cells. While this technology is not yet ready for use, potentially it could harness biological energy that naturally occurs anyway to produce as much continuous electrical output as dozens of nuclear power plants. It could turn sewage treatment plants from a municipal expense to a profit center.
I have just learned of a different technology that can use wastewater to heat and cool buildings before it ever gets to the treatment plant. Wastewater is inherently warmer than clean water piped into a building from the municipal water supply. It provides a kind of geothermal energy.
Sewage geothermal energy in theory
Just as bacteria in a compost pile heat it up as they break down the biomass, so do bacteria in sewer water. In addition, the sewage also contains hot water from showers, dishwashers, washing machines in residential buildings and all manner of industrial processes in industrial buildings. It is about 60º F. in the winter and 75º in the summer.
Any conventional heat pump can transfer that heat to the clean water pipes, while keeping the clean water and wastewater completely separate at all times. Up until recently, however, the solid matter in the wastewater would soon keep the heat pump from working. A Chinese company has recently invented an anti-block machine to filter it out.
Now there’s a switch. Much of Chinese industrial complex relies on stealing processes patented elsewhere, but a company called Jin Da Di invented this blocking device and a Philadelphia company called NovaThermal Energy has licensed it.
Sewage geothermal energy in practice
Note that the sewage is pumped directly from the municipal sewage main. It is not necessary to redesign or replace any part of the existing infrastructure. From there it goes to the anti-block machine, where the solid matter stays. Once the rest of the system has used this water, it flushes out the anti-block machine and returns the solid matter to the sewage main.
The filtered sewage can be either further heated or cooled as needed. NovaThermal’s diagram shows the heat pump supplying hot water to a building. Depending on how the wastewater gets routed through the system, it can also cool the building in the summer or heat it in the winter.
Imagine the savings for a building owner! The building will no longer require hot water heating tanks . It can replace any chemical coolants it uses for HVAC with hot and cold water. If it uses hot water heating and cooling anyway, it will no longer require some of the extra equipment it now has to maintain.
Overcoming the yuck factor
NovaThermal’s target market is any commercial building more than 100,000 square feet, so long as it has close access to a major sewage trunk line. So far, however, it has only built demonstration projects at sewage facilities themselves. That is partly explained by the fact that it is a fairly new company. Its CEO, Elinor Haider, also notes that some building owners are squeamish about the technology.
Let’s get real and keep certain facts in mind.
- All those buildings already have both clean water and waste water circulating throughout them.
- All those buildings are already located above or close to a sewage trunk line.
- A heat pump relies on exploiting the difference in heat between two completely distinct sources. Those two sources, whatever they are, can’t mingle in the heat pump, or it won’t work at all.
- All the water that comes from any municipal water treatment plant contains water previously used to flush toilets upstream.
Wastewater has tremendous potential as an infinitely renewable resource. The system that NovaThermal sells requires only minimal adjustment to existing sewage systems to save existing large buildings 30-60% on their HVAC costs.
Microbial fuel cells, if it is possible to make them work on a large enough scale, will be able to generate enough electricity to become a profit center for the municipality and maybe even make some coal-fired power plants unnecessary.
And nothing at all will smell any worse than it does now.
It’s high time to outgrow the yuck factor.
See NovaThermal Energy, tapping into a constant resource: sewage’s heat / by Andrew Maykuth (Philadelphia Inquirer)