Anaerobic Digestion Examples

Digesters can be pretty much any size. I've seen one as small as a car inner tube that was used to power a gas grill and heat a small greenhouse. Some of them can be larger, as you'll see below.

The pictures below are from a cooperatively-owned anaerobic digester in Lemvig, Denmark. I'm particularly fond of this because the entire setup is owned equally by about 25 farmers, and is a non-profit operation. All of the organic waste from the farms is transported to the digester, including leftover vegetation and various types of manure. The biogas is used to generate electricity in a turbine which is either sold to the grid or used in the digester, and the "waste" heat is used to run the anaerobic digester. The remaining heat is used for district heating for the town - it heats up water, which is then run through underground pipes to be used to heat homes. This type of generator is considered cogeneration, which means it is used to generate electricity and useful heat. Recall that most power plants are about 35% efficient because so much energy is wasted as heat. Believe it or not, this cogeneration system is over 90% efficient when you include all of the "waste" heat that is captured and used! All digestate is then returned to the local farms and used as organic fertilizers. It is truly a closed-loop system!

Figure 1.5. Digestion tanks in Lemvig, Denmark.

Credit: D. Kasper

Figure 1.6. Methane storage tank, Lemvig, Denmark. This is like a giant bladder that can expand as it fills up with methane.

Credit: D. Kasper

Figure 1.7. Gas generator, Lemvig, Denmark. This generator uses cogeneration. It generates electricity and captures most of the waste heat to be used for other purposes.

Credit: D. Kasper

Figure 1.8. This digestate is used as high-quality organic fertilizer for the local farms.

Credit: D. Kasper

The images below show details of a smaller installation in Kussnacht, Switzerland. This installation is run by a single farmer (Seppi), who collects organic waste from his farm, other local farms, and area restaurants. Like the one above, Seppi collects the biogas and uses it in a cogeneration system that is about 90% efficient (50% heat, 40% electricity, and 10% is wasted). He runs a 100 kW generator and uses the electricity on his farm and sells the leftover to the grid. The heat is used to run the digester, and to provide space and water heating to his farm. He uses some digestate on his farm and gives the rest back to local farmers for free.

It just so happened that at the time of our visit (I brought students there for a study abroad experience), his previous digester had burnt down due to a generator fire. The upshot of this is that we were able to see inside the digester he was building, which you will see below.

Figure 1.9. Digestion tank in Kussnacht, Switzerland. The tank is under construction and is the green building in the middle.

Credit: D. Kasper

Figure 1.10. Inside the digestion tank. Note the mixing blades that are used to keep the organic material from settling. This was mentioned in the video previously.

Credit: D. Kasper

Figure 1.11. Feedstock for anaerobic digestion. Seppi uses organic material from his farm and local farms, as well as material like this from local restaurants. The plastic is removed before it is put into the tank.

Credit: D. Kasper

Figure 1.12. Natural gas cogeneration system in Kussnacht, Switzerland. This system is about 90% efficient!

Credit: D. Kasper

Figure 1.13. Digestate that remains after anaerobic digestion. Both (solid on the left and liquid on the right) can be used as high-quality organic fertilizers.

Credit: D. Kasper

Figure 1.14. Just for good measure, I've included a photo of Seppi charging up the electric bus that we were riding around Switzerland on. Many places in Switzerland have car charging stations. We went from place to place, charging almost exclusively using renewable energy.

Credit: D. Kasper