Despite the potential for some bathroom humor, human waste and wastewater is no joke. In fact, German statistics show that in Germany alone 4,400 Gigawatt hours of electricity are used in treating wastewater every year.
That’s Gigawatt hours, which is the output of one million barrels of oil, or the output of an entire coal-fired power station for one year. And that’s just for one smallish country that is half the size of Texas. Imagine how much power we use in the U.S. for water treatment.
The Berlin Centre of Competence for Water initiated Project CARISMO (for CARbon IS MOney, of course) to determine how the organic substances in the wastewater could be used to power the treatment plant itself. From a volume perspective, the sludge contains enough chemical energy to generate a surplus of energy. However, current treatment methods used to render the contaminants harmless — which include feeding oxygen through them so they turn to carbon dioxide — means that all the chemical energy potential is lost.
In order to harvest the energy, the CARISMO team set up a pilot program that separated the sludge at the intake point and transferred it to a digester to create biogas, allowing energy capture without any effects on the filtration process. In fact, by reducing the amount of sludge going into the plant, the team found that energy consumption for water treatment was reduced by half. There is even the potential for surplus energy to be fed back into the grid. A phase II pilot will be conducted to gain more practical experience with the process.
Skeptics don’t deny the validity of the process, but some feel there are other simpler and less expensive solutions to reduce energy consumption. The CARISMO process required some additional direct and indirect energy consumption through infrastructural changes such as the rotating screens used, mechanical thickening of the sludge for the biodigester, and the addition/fabrication of more chemicals. However, the additional process steps increased the biogas yield by 80 percent over the conventional treatment. As a proof of concept, the project already offers some interesting potential for net positive energy for water treatment plants, and further work may help reduce costs even more.