How does AD work?

Anaerobic digestion is a process through which bacteria break down organic matter in the absence of oxygen. In this instance, “organic” means coming from or made of plants or animals. Anaerobic digestion happens in closed spaces. It can also occur in nature or e.g., landfills, but in AD facilities it is optimized, controlled and the biogas that is produced can be contained. The anaerobic digestion is aided by microorganisms, involving a series of steps such as hydrolysis, acidogenesis and methanogenesis. There are different types of digestion plants:

  • Wet system, the biomass is turned into a soup, it is digested in a stirred tank reactor, this is a continuous system
  • Semi-dry system, the biomass is like a sludge, it is digested in a plug-flow reactor, this is a continuous system
  • Dry system, biomass is solid, digested in a tunnel without diluting, this is a discontinuous system, or a batch process.

WTT mostly works with residual municipal solid waste (MSW) and sources separated organic waste. This material is already solid, therefore WTT chooses to use the dry system, as this is the optimal technology for the utilization of these organic residues and wastes. The absence of stirring makes the system very reliable, maintenance friendly and it can accept highly contaminated waste. For kitchen and commercial food waste, a wet system is more appropriate.

Dry digesters are tunnels where organic material is stacked up in piles by front end loaders. The organic material is then saturated with digestate to ensure complete bacteriological action.  Percolate liquid dripping down through the pile is collected and recirculated by spraying it back into the organic mass. This is the key for successful dry anaerobic digestion. For the AD process to be efficient, different parameters must be controlled. These include, temperature, oxygen concentration, humidity, and ph. Also, the waste composition is important. The setting of these parameters needs to be monitored closely. For example, the oxygen needs to stay below 2%, otherwise the anaerobic bacteria get deactivated. Also, the temperature has to be within a certain range to keep the right bacteria working. Higher temperatures usually speed up the process, but also lead to an unstable process. The residence in the AD tunnels is set to 3 weeks. Experience shows that this retention time optimizes the gas productivity and leaves sufficient organic load to the material to be actively participating to the following aerobic process steps.

What is made during the AD process?

Biogas is generated during the AD process. Biogas is mostly methane (CH4) and carbon dioxide (CO2), with very small amounts of water vapor and other gases. The carbon dioxide and other gases or contaminants can be removed, leaving only the methane. Methane is the primary component of natural gas. The material that is left after anaerobic digestions is called “digestate”. The digestate is a wet mixture that is rich in nutrients. This closes the cycle, as after processing it can be returned to the soil, with the nutrients it contains as a potent fertilizer with high humus content after fermentation. In addition to the fertilizing effects of nutrients available to the plants, the practical application shows an improvement in soil structure, including a steady increase in the proportion of humus in arable land. This humus structure also drastically improves the water capacity of soils.

How is biogas used?

Biogas is a renewable energy source that can be used in a variety of ways. It can be used to power engines, produce mechanical power, heat, or electricity. It can fuel boilers, furnaces, heating digesters and other spaces. It can run alternative fuel vehicles and supply homes and businesses through the natural gas pipeline. How biogas is used and how efficiently it is used depends on the quality. Removing compounds like CO2, water vapor and other trace gases increases the energy/value of the biogas.

AD technology: for a better future

If organic waste is not treated, it generates large amounts of methane as it decomposes. Methane is a powerful greenhouse gas that traps heat in the atmosphere more efficiently than carbon dioxide. In addition, landfill, is representative of the globally prevailing linear economic system and contradicts the principle of sustainability. AD is an ideal circular solution to create biogas from organic waste, which can be converted into renewable heat and power. It is the perfect example of upcycling of a low-grade product into a high value energy source.

Biogas is:

  • 100% renewable (no new carbon)
  • Permanently available
  • Transportable
  • Storable