Anaerobic Digestion (Organic Waste)

Factsheet Block Body

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All biogas digesters are basically designed following the same process of anaerobic digestion. Anaerobic digestion is a four-stage process consisting of hydrolysis; fermentation (conversion of non-soluble organic biomass to soluble organic compounds); acidification (e conversion of soluble organic compounds to volatile fatty acids and CO2, followed by the conversion of volatile fatty acids to acetate and H2); and finally methane formation. The final product, biogas, is a mixture of methane (CH4), carbon dioxide (CO2) and other trace gases (see also anaerobic digestion, general factsheet).

The anaerobic digestion of organic waste. Source: HOLLIGER ()

 

There are many ways in which anaerobic digestion can occur. The simplest reactors are covered waste dumps, where anaerobic digestion can occur naturally in uncontrolled systems. As mentioned above, today there is a large range of different types and designs of anaerobic digester technologies for the treatment of organic waste available. Even though the process for all these technologies is always the same (i.e. anaerobic digestion), depending on the composition of the substrate and the volume of the waste stream, complexity of design, construction and operation vary strongly.

Example of a Reactor set-up for the mesophilic anaerobic digestion of municipal solid waste. Source: HOLLIGER ()

 

The main differences regarding the design of these technologies exist regarding the biogas potential of the substrate; dry and wet processes; mesophilic and thermophilic processes; mechanically mixed and no-mixed reactors, batch and continuous reactors and one-stage or multi-stage processes etc. In the following, a short overview on all these parameters is given (refer to anaerobic treatment, general factsheet).

Some reference values of biogas production potential of different substrates are given in the table below. The table also illustrates the potential of petrol equivalents saved. Conventional digesters apply a wet digestion process (total solids (TS) below 20%). Wet digestion has the advantage to provide good fluidity of the slurries and optimal contact of the microorganism and the waste. The new technology, dry digestion (TS > 20 %), allows high gas yields using less space, but the process has only recently been developed and is applied only at high-tech level. Mesophilic digestion is generally more stable even though gas yields are lower. Thermophilic processes produce more biogas in shorter time but require input energy and bear the risk of the production of toxic ammonia(ISAT/GTZ Vol. I). Batch systems are the lowest-tech of all systems and also the cheapest (VANDEVIVERE et al. ). Their major drawbacks are a large footprint and a lower biogas yield due to the impairment of the percolation process via channelling and clogging (VANDEVIVERE et al. ). Batch systems have a high potential for application in developing countries. Multi-stage processes separate the different stages of anaerobic digestion in order to achieve optimal growth conditions for the microorganism and to allow the heating of the methanogenic stage only (reducing required energy inputs). However, multi-stage systems are the most complex, and most expensive, of all systems biogas systems for solid waste (VANDEVIVERE et al. ).Low-tech small scale wet digesters generally do not provide any mechanical mixing as the gas bubbles rising through the sludge to the top of the reactor provide sufficient mixing. Large-scale, and in particular dry digestion processes require the installation of a mechanical stirring (or transporting device in the case of plug-flow reactors).

 

Methane per ton (t) dry waste (m3/t)

Tons of petrol equivalents per ton of dry waste (t/t)

Food waste

500

0.43

Paper

330

0.28

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Grass

310

0.26

Branches and leaves

110

0.09

Energetic potential of municipal solid waste (MSW). Source: HOLLIGER ()

 

Most anaerobic digesters for the treatment of organic solid waste provide a post treatment of the remaining sludge before it can be reused as soil amendment in agriculture. Aerobic composting, the post-treatment used most often, allows both nitrification and the degradation of lignin, which is not possible under anaerobic conditions (HOLLIGER ).

Below, some examples of small-scale and large-scale anaerobic digesters for the treatment of organic solid wastes are given.

An Introduction to Anaerobic Digesters

A biological process occurs to this mixture to produce methane gas, commonly known as biogas, along with an odor-reduced effluent. Microbes break down manure into biogas and a nutrient-rich effluent.

Benefits

Local

• Reduced odor, fly eggs, weed seeds in digested manure
• Revenues:
  • On farm power production sales
  • Tipping fees from other feed stocks (food wastes)

Regional

• Pathogen reduction of land applied nutrients
• Rural electric grid voltage support

Global

• Reduced methane emissions, a greenhouse gas
• Reduced dependency on foreign fossil fuels

Notes and Lessons Learned

• Anaerobic digestion "Does Not Reduce Nutrients in Manure"
• If consdering to build an anaerobic digester, select an experienced "Farm Digester" consultant

Some helpful terminology used in anaerobic digestion.

Safety Note

Biogas production systems have many hazardous components and characteristics. No design or project is complete without a safety plan.

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