Importance of Anaerobic Digestion for Climate Change

Anaerobic digestion, as defined by the United States Environmental Protection Agency (EPA), is a process through which bacteria break down organic matter—such as

  • Animal manure
  • Wastewater biosolids
  • Food wastes

In the absence of oxygen, anaerobic digestion processes are rising due to the need to reduce carbon emissions and climate change. Anaerobic digestion has also been discussed from several angles. It is a renewable energy source with another output, a natural crop fertilizer. pH and methane organisms also play an essential part.

Anaerobic digestion is commonly used in sewage treatment facilities for sludge treatment. You can also use it for high-grade industrial wastewater. In addition to the materials destined for the compost pile, this process is suitable for converting fats and other organic matter.

Anaerobic Digestion Processes

Anaerobic digestion for biogas production takes place in a sealed vessel called a reactor, which is designed and constructed in various shapes and sizes specific to the site and feedstock conditions.

These reactors contain complex microbial communities that break down (or digest) the waste and produce resultant Biogas and Digestate (the solid and liquid material end-products of the AD process) discharged from the digester.

We can combine many organic materials in one digester through co-digestion. Co-digested materials include

  • Manure
  • Food waste (i.e., processing, distribution, and consumer-generated materials);
  • Energy crops
  • Crop residues
  • Fats, Oils, and Greases (FOG); from restaurant grease traps and many other sources.

Co-digestion can increase biogas production from low-yielding or difficult-to-digest organic waste. See the figure below for more illustrations.

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Outputs from Anaerobic Digestion Processes

The anaerobic digestion process produces 2 valuable outputs.

Biogas

Biogas is composed of methane (CH4), which is the primary component of natural gas, at a relatively high percentage (50 to 75%), carbon dioxide (CO2), hydrogen sulfide (H2S), water vapour, and trace amounts of other gases.

The end product of anaerobic digestion is biogas, a combination of carbon dioxide, methane, and water. It can be used as a renewable energy source, and the solids produced by the process are used for dairy bedding and composting.

We can use the energy in biogas like natural gas to provide heat, generate electricity, and power cooling systems, among other uses. Biogas can also be purified by removing the inert or low-value constituents (CO2, water, H2S, and many more) to generate Renewable Natural Gas (RNG).

Biogas can be sold and injected into the natural gas distribution system, compressed and used as vehicle fuel, or processed further to generate alternative transportation fuel, energy products, or other advanced biochemicals and bioproducts.
Digestate

Digestate

Digestate is the residual material left after the digestion process. It is composed of liquid and solid portions. These are often separated and handled independently, as each has a value one can realize with varying degrees of post-processing.

With appropriate treatment, we can use both the solid and liquid portions of Digestate in many beneficial applications, such as

  • Animal bedding (solids)
  • Nutrient-rich fertilizer (liquids and solids)
  • A foundation material for bio-based products (e.g., bioplastics)
  • Organic-rich compost (solids) and simply as a soil amendment (solids)

The latter may include the farm spreading the Digestate on the field as fertilizer. Digestate products can be a source of revenue or cost savings and are often pursued to increase the financial and net-environmental benefit of an AD/biogas project.

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Conclusion

You may be surprised to learn that this composting involves the microbial process of hydrolysis. Hydrolysis involves breaking chains of organic compounds into simpler monomers that other bacteria can easily digest.

Hydrolysis breaks down compound organic molecules into simple sugars, amino acids, and fatty acids in anaerobic digestion. Research on anaerobic digestion has been ongoing for many years. This research is now reaching an unprecedented level as more people seek environmentally sustainable waste management options.

Anaerobic digestion has many applications, ranging from agricultural to water and food waste treatment. Still, there are several gaps in this technology. The vast potential for anaerobic digestion warrants continued research.

Anaerobic Digestion Cost Benefits

Cost-benefit analysis of anaerobic digester operations has to take into account several factors. Benefits include:

  • The potential sale of either the biogas itself or the use of biogas to generate electricity that can sell back to the grid
  • Resale of digested fiver as compost or liquid Digestate as high-quality fertilizer
  • The heat generated by the system is in the form of hot water circulated by a CHP system.

Capital costs items include

  • Lift station pumps
  • Mixing tanks
  • Digester tank
  • Piping for gas and hot water
  • Gas pumps
  • Flow meters
  • Safety features
  • Generators
  • Electrical wiring and controls
  • Power transmission lines
  • Design engineering
  • Onsite buildings for generators
  • Maintenance
  • Operations, and much more.

The payback period Capital costs divided by annual net benefits) of this capital investment can be between five and six years. Read the AgSTAR Project Development Handbook, which contains the compilation of the latest knowledge in the industry on best practices for anaerobic digestion and Biogas systems. It also includes information on anaerobic digestion applications and processes, benefits, challenges, feedstocks, products, economic and financial factors, and more.

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Importance of Anaerobic Digestion for Climate Change

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