Anaerobic Digestion

In anaerobic digestion, biomass waste is digested by a mix of microbial species to biogas (mostly methane plus carbon dioxide), a fuel which can be used in electricity generation or, after treatment, used as a household and industrial heating fuel. An indigestible residue is left containing plant nutrients. The residue is referred to as the digestate, which serves as fertiliser. The biomass may be food waste, farm animal waste or plant waste, or combinations of these.

Anaerobic digestion, combustion of biogas and use of digestate as fertiliser are collectively carbon-neutral

Carbon recycling in anaerobic digestion is illustrated in figure 1 to show that the overall process of producing and burning the biogas and using the digestate is carbon-neutral.

Carbon recycling in anaerobic digestion
First, all the carbon in the biomass is derived from atmospheric CO2 in photosynthesis. Second, the carbon in the biomass ends up either in methane, CO2 or digestate. The latter contains essential plant nutrients (e.g. nitrate and phosphate) as well as fibrous undigested plant material that contains carbon compounds, wherein the carbon is derived from atmospheric CO2. Third, the burning of methane produces CO2. Of course, the rate of consumption of biomass cannot exceed that of methane production and combustion without the process becoming unsustainable (running out of feedstock). It is important that no leakage of methane occurs as it has a much greater greenhouse effect than CO2.

The anaerobic digestion process

The process takes place in a reaction vessel called an anaerobic digester, the size and design of which will vary according to the scale of the operation, how the products will be used and the nature of the biomass used as feedstock.

All the microbes involved are anaerobic, i.e. they do not use up oxygen during the processes that provide them with the energy for growth. (Aerobic bacteria would oxidise the biomass to produce mostly CO2 and water.)

There are four stages to anaerobic digestion as outlined in figure 2.

Four stages of anaerobic digestion
1. Hydrolysis – the breakdown of the large molecules in the biomass into smaller molecules. This consists of reactions with water (hence called hydrolysis).

(a) The repeating units of each molecule of polysaccharides (e.g. starch) are digested into their simple units – molecules of sugars (e.g. glucose).
(b) Protein molecules with repeating amino acid units are digested to amino acids.
(c) Fats are broken down to fatty acids and glycerol.

The reactions are catalysed by enzymes attached to the outside of the microbial cell walls.

2. Acidogenesis. The products from stage 1 are digested by a second group of microbes that digest them to a mixture of short-chain or volatile fatty acids (mainly formic acid (formate) acetic acid (acetate) propionic acid (propionate) and butyric acid (butyrate)) and alcohols (mainly methanol). (Volatile means they boil (turn into gases) at relatively low temperatures.) Some hydrogen (H2) and CO2 are produced.

Other by-products, including ammonia (NH3) and hydrogen sulphide (H2S), result from the microbial action on carbon compounds containing nitrogen and sulphur present in the biomass feedstock. The solid residue resulting from this stage contains fibrous plant materials such as lignin and cellulose that are indigestible to the microbes used. This is solid digestate.

3. Acetogenesis. Stage 2 products‑‑formic acid, propionic and butyric acid and methanol‑‑are converted to acetic acid, hydrogen and carbon dioxide by a third group of microbes.

4. Methanogenesis. As the name suggests, methane is produced from acetic acid via a fourth group of microbes. Carbon dioxide is also produced. Soluble nitrogen and phosphorus compounds (nitrates and phosphates), which are essential plant nutrients are left in the slurry that remains (soluble digestate).

Uses of the products of anaerobic digestion

Biogas typically consists of about 60% CH4 to 40% CO2 plus small amounts of various by-products (notably ammonia and hydrogen sulphide). After removal of by-products and CO2, the methane can be fed into the gas supply grid or used as a transportation fuel. Of course, the downside here is that purification and transportation of the fuel to where it is needed (via tanker or pipeline) are processes that produce CO2.

The digestate is used as a fertiliser.

Combining anaerobic digestion with other technologies

It may be more energy-efficient to utilise the methane at the same site of its generation‑‑as in the following proposal, which illustrates how a number of low-carbon technologies could be integrated to enhance the growth of crops (plants for food or biofuel or algae for biofuel).

The biogas (methane and CO2) can be burned. The burned methane produces CO2 and water; raw materials for plant photosynthesis. The combustion products (plus heat of combustion) could be piped to a greenhouse to enhance crop production, as shown in figure 3. The digestate could be used to fertilise the greenhouse soil.

Note that utilising the CO2 in this way potentially turns a carbon-neutral process into one that results in a net reduction in atmospheric CO2.

Enhanced plant growth using products of anaerobic digestion
Round-the-clock photosynthesis is possible through using some of the heat of combustion to boil water and drive a steam turbine to generate electricity, which in turn could be used for electric lighting of the greenhouse at night.

Additionally, photovoltaic cells could be fitted to the greenhouse or an adjacent building, so that sunlight during the day could generate electricity, compensating for that used at night in providing light for 24h plant growth.

Is there the potential within Lanchester Parish for such a development?
Searching the web may lead you to other ways to combine anaerobic digestion to other processes.

Useful links

Anaerobic Digestion at Newcastle University
Anaerobic digestion at Newton Aycliffe: JJ Warren ABP Ltd/National Industrial Symbiosis Programme collaboration
Anaerobic Digestion: The Official Information Portal on AD
Biogen
Centre for Alternative Technology
Renewable Energy Association

 

Alan Myers
1 December 2011