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Turning Waste into Wealth: Anaerobic Digestion Project
Innovative Project Spotlight: Turning Waste into Wealth through Biomethane Production

In the pursuit of a greener and more sustainable future, Scotland is turning towards innovative waste management and renewable energy solutions ( The Scottish Government, 2024) . One promising project that is gaining attention for its potential to turn waste into wealth is the Anaerobic Digestion: Green Gas from Waste initiative. This project delves into the fundamentals of AD, its current applications in Scotland, its potential benefits, challenges, and its pivotal role in contributing to Scotland's ambitious climate targets.

This project focuses on the feasibility of producing biomethane from cow manure and distillery waste using advanced technology.

What is Anaerobic Digestion ?

Anaerobic digestion is a well-known technology that allows microbial conversion of biomass without oxygen into a valuable mixture of biogas and digestate. Biogas, primarily composed of methane and carbon dioxide, is a renewable energy source for generating electricity, heat, or transportation fuel (Hardegen et al,2018). Digestate, the nutrient-rich residual matter, finds application as a biofertilizer, boosting soil health (Lamorinara et al, 2022).

                                         Anaerobic Digestion Phases 

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Fig 1: Phases of Anaerobic Digestion .( Grippi et al, 2020)

Acording to (Hargeden et al , 2018), four main important phases occurrs during the anaerobic digestion process:

Hydrolysis 

Is the slowest step of the four. In this stage, bacteria transform complex organic materials into liquefied monomers and polymers. The stage is important because higher-mass organic molecules are transformed into basic structural building blocks and used by microorganisms as substrate/food source.The enzymes are exoenzymes ( cellusome, protease,) from many bacteria. protozoa and fungi.

Acidogenesis

Acidogenic bacteria convert the soluble organic monomers of sugars and amino acids to small organic compounds, such as chain acids, ketones, and alcohols. The degradation of amino acids leads to the production of ammonia.

Acetogenesis
 

In this stage, both long-chain fatty acids and volatile fatty acids and alcohols are transformed by acetogenic bacteria to H2, CO2, and acetic acid. During this stage, the BOD (biological oxygen demand) and COD (chemical oxygen demand) are both reduced, and the pH decreases. Hydrogen plays an important intermediary role in this process, as the reaction will only occur if the partial pressure is low enough to thermodynamically allow the conversion of all the acids. Hydrogen-scavenging bacteria lead to a lower partial pressure. Hydrogen concentration in a digester is an indicator of its health. 


Several bacteria contribute to acetogenesis including:
 

  • Syntrophobacter wolinii, propionate decomposer

  • Syntrophomonos wolfei, butyrate decomposer

  • Clostridium spp., peptococcus anaerobes, lactobacillus, and actinomyces are acid formers.

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Methanogenesis 


Methanogenic bacteria convert the hydrogen and acetic acid to methane and carbon dioxide. Methanogenesis is affected by conditions such as temperature, feed composition, and organic loading rate. 
The gaseous product, biogas, consists mainly of CH4 and CO2, but also contains several other gaseous “impurities” such as hydrogen sulfide, nitrogen, oxygen, and hydrogen.


Several bacterial contribute to methanogenesis, including:

 

  •  Methanobacterium, methanobacillus, methanococcus, and methanosarcina, etc ( Hargen et al, 2018).

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Types of Feedstock


Anaerobic digestion plants are incredibly versatile, able to process a wide range of organic feedstocks. According to the Scotish government (2019) in Scotland, commonly useded feedstocks include:
 

  • Food Waste: From households, restaurants, and food processing industries.

  • Sewage Sludge: A byproduct of wastewater treatment.

  • Industrial Wastes: From distilleries, breweries, and other sectors.

  • Agricultural Residues: Manure, crop residues, and energy crops.

Fig 2: How does anaerobic digestion work? ( US EPA, 2024)

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