FAQs

Frequently Asked Questions in Anaerobic Digestion

What is Biological Treatment?

Biological treatment covers the treatment of waste using biological techniques; the best known is composting, and anaerobic digestion is another.

The common factor with all biological treatment processes is the use of microbial life (e.g. bacteria and fungi) to break down waste products to a more useful by-product, be it ‘compost’, “biogas” or remediated soil. Land remediation also utilises the biological processes within plants to clean up contaminated soil.

What is the meaning of Biodegradable?

All waste which can be broken down or digested by any form of biological or microbiological activity is termed “biodegradable”.

What is the meaning of “putrescible” and “organic”

The terms “putrescible” and “organic” when associated with wastes are commonly used to describe food or plant wastes which easily or naturally biodegrade. This web site uses the term “biodegradable” to cover all wastes that can undergo a process of biodegradation.

What are the four Main Processes of Anaerobic Digestion?

The four main steps of the process are:-

1. Hydrolysis or liquefaction; in which the complex primary polymers of carbohydrates, lipids and proteins are solubilised by enzymes secreted by hydrolytic bacteria, thus converting the insoluble biological polymers to soluble organic compounds.
2. Acidogenesis; in which soluble organic compounds, including the products of the hydrolysis (soluble monomers) are fermented to various intermediate products such as short chain organic acids (called volatile fatty acids or VFAs) and alcohols.
3. Acetogenesis; in which the alcohols and volatile fatty acids are converted into acetic acid, carbon dioxide and hydrogen by acetogenic bacteria.
4. Methanogenesis, in which the methanogenic bacteria complete the process by converting acetic acid and hydrogen to methane and carbon dioxide.

Each of these steps is performed by groups of bacteria, which are grouped by their feeding (trophic) requirements (see above).

There are many other reactions, which make the process more complicated, such as the action of the sulphate-reducing bacteria or the homoacetogens. These organisms compete with the methanogens for some substrates, such as formate, as well as controlling the hydrogen balance of the system by producing acetic acid from hydrogen and carbon dioxide.

However, in general the groups of bacteria are mutually beneficial and interdependent, i.e. removing inhibitory products of other bacterial groups and/or producing substances for other bacterial groups. The important point is therefore, that the process is dependent on the correct balance of each of these groups of bacteria, because any one group will not operate alone.

For example, if the slower growing methanogenic bacteria are removed, then acetic (and other) acids will not be degraded and the system will become increasingly acidic. Eventually bacterial action is stopped when the pH reaches inhibitory levels for the acetogens and acidogens.

What are the best Temperatures for Anaerobic Digestion, and what are these ranges called?

Anaerobic digestion will operate over a wide range of temperatures. However, there are two temperature ranges where the digestion is most rapid, mesophilic (about 35°C) and thermophilic (about 55°C).

How much Biogas will my Organic Waste Yield?

This is related to the BioChemical Oxygen Demand (COD) of your waste.

Published information is available from: Wastewater Engineering, Treatment and Reuse, Metcalf and Eddy, McGraw Hill.

This gives (in Table 10-10) the following for completely mixed Anaerobic Suspended Growth reactors treating soluble COD:-

• Methane production at 35 degrees C = 0.4 m3/kg COD
• Density at 35 degrees C = 0.6346 kg/m3
• Content of gas = 60 to 70% methane, (typical 65%)
• Energy content = 50.1 kJ/g

We recommend that anyone seriously looking into AD processes should refer to this or another good reference book. A good process design book will also contain worked examples.

What class of substances cannot be broken down by Anaerobic Digestion?

The lignins are not broken down under anaerobic conditions, as they require an aerobic process.

Can Landfill Leachate be treated by Anaerobic Digestion?

The use of Anaerobic Digestion to treat landfill leachate is not normally a good choice. The reasons are explained here.

What are the Stages of Anaerobic Digestion?

There are four stages of anaerobic digestion: hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

What is Hydrolysis?

1. Hydrolysis is a chemical decomposition process that uses water to split chemical bonds of substances.
2. Hydrolysis is the decomposition of a substance by the insertion of water molecules between certain of its bonds. Hydrolysis literally means reaction with water.
3. Hydrolysis reverses the condensation of amino acids into proteins by the acid- or alkali-catalyzed breaking of the peptide bonds and the addition of water at the break.
4. Hydrolysis reactions involve breaking polymer molecules such as polypeptides (large proteins) or starches into their monomer forms (amino acids and glucose molecules respectively).
5. Hydrolysis is the basis for the manufacture of soap (saponification), whereby fats (glycerolesters of fatty acids) are split with aqueous solutions of sodium hydroxide to form the sodium salt of the fatty acid (soap), and the alcohol (glycerol – more commonly known as glycerine).
6. Hydrolysis can be catalyzed by enzymes, metal salts, acids, or bases. Food is digested by hydrolysis.

What is Acidogenesis?

Acidogenesis is the second stage of Anaerobic Digestion, and it is a biological reaction where simple monomers are converted into volatile fatty acids;

What is Acetogenesis

Acetogenesis; in which the alcohols and volatile fatty acids are converted into acetic acid, carbon dioxide and hydrogen by acetogenic bacteria.

It is the conversion of the volatile fatty acids produced by the previous stage’s acidogenesis activities into acetic acid, carbon dioxide, and hydrogen occurs in this stage.

Acetogenesis is where digestion products are converted into acetate, H2, CO2 and new cell-matter.