The Biogas Digester Expert Technical Pages

The Biogas Digester Expert is here to provide information and resources for those seeking information about biogas digesters.

The term Biogas Digester is commonly used to describe what is more correctly called an Anerobic Digestion Plant. The term biogas digester may also be used more specifically to refer to the biogas reactor vessel which is the location in all Anaerobic Digestion Plants where the digestion, (which is the biological reaction in which anaerobic decomposition takes place), and is also known as a fermenter.

Whatever you call it, a Biogas Digester is a natural processing system for organic materials, and is a highly sustainable method by which biogas is created which can be used to provide a renewable energy source for an extremely wide range of uses.

Currently this web site concentrates on simple low cost rural community and farm based anaerobic digestion topics, but we will soon be adding sections for another breed of biogas digesters which are becoming increasingly popular, at the other end of the size and complexity range. These large digesters are the new generation of Anaerobic Digestion Plants which utilise feedstocks such as biofuel crops and segregated (even non-segregated municipal wastes, and food wastes.

Properly designed and used, a biogas digester mitigates a wide spectrum of environmental undesirables: it improves sanitation; it reduces greenhouse gas emissions; it reduces demand for wood and charcoal for cooking, and therefore helps preserve forested areas and natural vegetation; and it provides a high-quality organic fertilizer.

What is a Biogas Digester?

We recommend that, if you are a beginner and new to the concept of biogas digestion, you spend 4 minutes and 30 seconds to watch our in-house produced video below. This video will give you a rapid introduction to the biogas digester.

The video starts by providing some basic information on the process and the "renewable energy" classification given to these digesters. It then describes the process inputs (feedstocks) which are needed and the digested outputs. The video then ends with a series of typical images of biogas digesters from around the world. The video will also be useful to refresh the memory for those that have a little knowledge of digesters, but want a reminder of the principles involved.

Biogas

Biogas refers to a gas made from anaerobic digestion of agricultural and animal waste, food waste and sometimes also Municpal Solid Waste, plus and biofuel crops.  Design differences for the various plant types mainly depend on the type of organic waste to be used as raw material, the temperatures to be used in digestion and the materials available for construction.  One solution adopted is to use a double digester, consuming the waste in two stages, with the main part of the biogas (methane) being produced in the first stage and the second stage finishing the digestion at a slower rate, but still producing another 20 % or so of the total biogas.  Dependent on the waste material and operating temperature, a batch digester will start producing biogas after two to four weeks, slowly increase in production then drop off after three or four months.  In cold climates the digester will need heating, but the small quantity of gas "wasted" on heating the digester will be more than compensated for by the greatly increased biogas production.

Methane

Methane is the combustible component of biogas while the digestate slurry is a valued fertilizer.  Most people are not aware that as the world turns to using renewable energy, the one huge source that has barely been used up to now is biogas methane.  Cow dung gas for example is 55-65% methane, 30-35% carbon dioxide, with some hydrogen, nitrogen and other traces.

Monitoring Technology

The carbon dioxide and methane contained in biogas can best be measured with infrared technology, and special infrared sensors are available from specialist instrument manufacturers and suppliers.

Manure

Manure, either from human beings or from animals, is a major pollutant source in rural areas.  Anyone who has visited India, for example, will remember the acrid smell of burning manure.  The acrid smoke leads to endemic eye disease, and the drying manure is a perfect breeding ground for flies of all types.  The manure would also go a long way to improving the quality of the soil and hence increasing the harvest if these valuable minerals were returned to it instead of going up in smoke.

Anaerobic Digestion

Anaerobic digestion is one of the most common biological procedures in nature, as the name implies, it means to carry out digestion or breakdown in the absence of air.  Anaerobic decomposition will produce methane, carbon dioxide, some hydrogen and other gases in traces, very little heat and a final product with a higher nitrogen content than is produced by aerobic fermentation.  Anaerobic decomposition is a two-stage process as specific bacteria feed on certain organic materials.  Anaerobic digestion will occur best within a pH range of about 6 and 7.8.  Anaerobic breakdown of waste occurs at temperatures lying between 0°C and 69°C, but the action of the digesting bacteria will decrease sharply below 16°C.  Anaerobic digestion technologies provide air and water quality benefits including pathogen destruction; odor control; organic stability; greenhouse gas (methane) and hydrogen sulfide emission reductions; and some nutrient management benefits.

Slurry

The slurry is mixed in an airtight tank filled with the organic waste during the process. The tank needs facilities which enable it be emptied of digested slurry with some means of catching the produced gas.  Some method of stirring the slurry in a digester is always advantageous, if not essential.  In some designs the gas collector and storage comprises an upturned drum placed on the top of the tank, and fitting close to the inside of the circular tank walls. The sides of the drum extend down into the slurry to provide a seal. A tap and hose pipe is attached to the top of the upturned drum and the biogas is led off to the house and kitchen etc, for direct use, from there.

The effluent from the reactor, a dark slurry, is a nutrient-rich fertilizer which is useful for  agriculture and aquaculture, due to conservation of nitrogen during the anaerobic process. 

Digestion

Digestion retention time ranges from a couple of weeks to a couple of months depending on the feedstock and the digestion temperature.  Too much nitrogen will cause undigested matter to be left over at the end of digestion (which stops when the carbon has been consumed) and reduce the quality of the fertiliser produced by the biogas plant. 

Anaerobic digestion of organics in most types of simple plants will proceed best if the input material consists of roughly 8 % solids. Systems intended for the digestion of liquid or suspended solid waste (cow manure is a typical example of this variety) are mostly filled or emptied using pumps and pipe work.

The complete anaerobic digestion of cow manure has been quoted on a number of web sites as taking about 8 weeks at normally warm temperatures.  If a continuous feeding system is used, then it is essential to ensure that the digester is large enough to contain all the material that will be fed through in a whole digestion cycle. 

Avoiding Odours

The waste is fed into the digester via the inlet pipe and undergoes digestion in the digestion chamber.  Hydrogen sulfide (H2S, also called "rotten egg gas") can unfortunately be an unwanted product of anaerobic digestion if the process is not properly controlled. For this reason all those who install and use these systems must take care to understand the requirements of successful operation and diligently apply themselves to maintaining the process on a regular basis.. 

Operator Health Considerations

Biogas is a product of the anaerobic digestion or fermentation of biodegradable materials such as manure or sewage, municipal waste, and energy crops.  Even though the bio-digestion process naturally reduces the pathogen load, handling biogas feedstock and using biogas slurry as fertilizer does carry some risk of infection.