A biodigester is a simple system to solve the energy-environmental problem, as well as to carry out an adequate management of both human and animal waste.
In its simple form, it is a container (called a reactor) which is hermetically sealed and inside which organic material such as excrement and vegetable waste is deposited (except for citrus fruits since they acidify). Organic materials are fermented with a certain amount of water, producing methane gas and organic fertilizers rich in phosphorous, potassium and nitrogen. This system may also include a chamber for loading and leveling residual water before the reactor, a device to capture and store biogas, and hydropressure and post-treatment chambers (filter and stones, algae, drying, among others) at the reactor outlet. .
The biodigestion process occurs because there is a group of anaerobic bacterial microorganisms in the excrements that, when acting on the organic material, produce a mixture of gases (with a high methane content) which is called biogas. Biogas is an excellent fuel and the result of this process generates certain residues with a high degree of nutrient concentration which can be used as fertilizer and can be used fresh, since bad odors are eliminated by anaerobic treatment.
Advantage
In large cities, organic solid waste is a big problem since it is disposed of in sanitary landfills, which break the natural cycle of decomposition because it contaminates groundwater sources due to the washing of the soil by water filtration (leaching) and also because it favors the generation of pathogens.
Organic waste, when introduced into the biodigester, is decomposed so that the natural cycle is completed and organic waste is converted into fertilizer and biogas, which prevents methane gas from being exposed, since it is considered one of the main components of the greenhouse effect. .
The use of biogas can replace electricity, propane gas and diesel as an energy source in the production of electricity, heat or refrigeration. In the rural sector, biogas can be used as fuel in electric generation engines for self-consumption by the farm or for sale to others. It can also be used as fuel for forced air furnaces, adsorption heaters and refrigerators. Converting appliances to gas operation is easy.
Biogas production is permanent, although not always constant due to climatic phenomena.
Types
There are two general types of biodigesters: the Indian and the Chinese system. The Indian biodigester was developed in India after the second world war in the 1950s, it arose out of necessity as farmers needed fuel for tractors and heating for their homes. in winter, then when the war ended, fossil fuels were obtained again, so they left the biodigesters and returned to hydrocarbons. As India is poor in fuels, the KVICK project (Kaddi Village Industri Commission) was organized, from which the Indian digester came out and the name of the fuel obtained is known as biogas. This digester works at constant pressure and its operation is very easy since it was designed to be handled by peasants with very little preparation.
The Chinese biodigester was developed by observing the success of the Indian biodigester, the Chinese government adapted this technology to its own needs, since the problem in China was not energy but health. The Chinese got rid of human feces in rural areas and at the same time obtained organic fertilizer, with the biodigester bad odors are eliminated and at the same time gas is obtained for kitchens and lighting. The Chinese biodigester works with variable pressure since the objective is not to produce gas but the already processed organic fertilizer.
The second generation digester basically operates on two levels. In the lower part of it, a tunnel or labyrinth is built, which serves to temporarily retain all the materials that tend to float; With the internal divisions, the labyrinth is divided into a series of independent chambers that communicate with each other continuously. By means of inclined planes and thin slots in the ferro-cement plates that make up the roof of the labyrinth, the passage of gas and already hydrolyzed and degraded material is allowed.
Slowly digestible materials, which complete their anaerobic degradation cycle in more than 100 days, can do so in time with excrements that require much less time, between 15 and 20 days.
The third generation digester is the mixture of several digesters in one unit. The labyrinth is typical of the Plug or Bag system, with effective lengths of 20 to 30 meters, it is the simplest and most practical system of all the conventional type digesters; the different independent chambers (6 or more depending on the design) provide the advantages of single-charge digesters; At the end of the route and at the top, there is the last large chamber, which is equivalent to the Indú-type digester, with its floating bell, loading at the bottom and outlet of the effluent by overflow at the top. This type of digester in particular offers a double economic advantage, since on the one hand a single unit of the appropriate size is built to the needs instead of several smaller independent ones; and on the other hand, the labor cost necessary to periodically load and unload the single load units is eliminated.
What aspects should the design of a biodigester consider?
Seven variables that influence good performance must be determined. The first is the amount of capital you are willing to spend; second is the quantity and quality of biogas to be obtained; third is the type of raw material available; fourth, the size of the biodigester; fifth the characteristics of the land; sixth, the use that will be given to the organic fertilizer and lastly, the temperature of the place where it will be installed, either at the ambient or greenhouse level.
Within the previous technical factors, the organic residue, the composition according to the carbon and nitrogen ratio, the intensity of agitation in the mixture until the pH is 7-7.2 and the existence of batteries that form methane together are evaluated. with the temperature of the fermentation process: Psychrophilic (15-18 °C), Mesophilic (28-33 °C), Thermophilic (50-60 °C).
You can see more in these videos:
Biogas at home: