What are biodigesters?
A biodigester system utilizes organic waste, particularly animal and human excreta, to produce fertilizer and biogas. A biodigester consists of an airtight, high-density polyethylene container within which excreta diluted in water flow continuously and are fermented by microorganisms present in the waste. The fermentation process is anaerobic, i.e., it takes place without oxygen, and the bacteria responsible for decomposition are methanogenic (i.e., they produce methane, also known as biogas). The processed manure is an organic, pathogen-free fertilizer that is rich in nitrogen, phosphorus and potassium. The products are primarily for self-consumption on farms.
Biodigester systems implementation
Biodigester systems may be implemented in any rural or urban area with sufficient space and a sufficiently large number of animals to generate at least 100 kg of manure a day. They are particularly useful on family farms that have livestock as a source of organic matter, cultivation areas on farms where fertilizer can be used and living quarters that can use biogas. They can be implemented on farms that need to improve soil fertility or the quality of life of the producers if the conditions referred to above are in place. Permanent access to water is required.
Threats and Impacts of biodigester systems
By producing a nutrient-rich fertilizer, this system reduces the need for agricultural inputs. Adding manure to soils reduces their deterioration and increases their productivity. Soil to which organic fertilizers have been added is less vulnerable to pests, erosion and drought. The methane that is produced, rather than entering the atmosphere, is used for domestic activities (cooking, heating water), which, by converting it to carbon dioxide, decreases its global warming potential.
How To Implement Biodigester
- Prepare the site.
- Calculate the volume of manure produced.
- Select and purchase a biodigester that meets the volume requirement.
- Set up the biodigester.
- Design and construct a system to channel the excreta to the biodigester.
- Construct a system to channel the methane to the location where it will be used.
- Collect the organic fertilizer and leachates.
- Apply the fertilizer.
- Use the biogas.
Inputs and Costs
Purchase and set up of a 10 m3 anaerobic-biodigestion system capable of processing 100 kg/day of excreta from farm animals. The main expense is for the biodigester, the pipes and the cooking stove. The cost of labour for maintenance, which is considerable, is not included because it is assumed that this labour will be provided by the producers. Two days of training on system operation and maintenance are assumed.
|10 m3 biodigester for 5 heads of cattle||US$|
Economic and Ecosystemic Benefits of Biodigester
Under optimal conditions, some 3 to 4 l of fertilizer are produced per kg of excreta, and its systematic use restores poor and infertile soils and increases yields. For example, a controlled experiment in Brazil, which ran trials with various doses, found that a 60 m3/ha dose of effluent applied to lettuce crops surpassed the results of mineral fertilization in terms of height, number of leaves, diameter and fresh mass of the lettuce (Chiconato, 2013). For a 10 m3 system, assuming that chemical fertilizers are completely replaced with the effluent and that biogas is used for cooking, the potential savings is US$ 350 per month. The utilization of biogas diversifies or replaces energy sources for household consumption (1 m3 of biogas replaces 0.5 kg of LP gas). Ferrer and others (2009) report that the biogas produced by a 5 m3 system is sufficient to cook for three to four hours a day. This has positive effects for the health of the users and the ecosystem by replacing the burning of dung or firewood.
Limiting Factors of Biodigester
Proper operation requires an average temperature above 15C. In areas with lower temperatures, a greenhouse or a thermal insulation system must be constructed, since biogas production decreases in cold temperatures (Poggio and others, 2009). The site where the biogas is to be used must not be more than 150 m apart from the biodigester, because, beyond this distance, gas pressure decreases.
The biodigester must be used constantly; otherwise, a process of putrefaction sets in within the container. When this occurs, the container must be emptied and the system cleaned. The system’s efficiency increases when the biodigester is integrated into the farm and is connected to the latrines. It is important not to exceed the maximum recommended organic matter capacity, according to the design of the biodigester, to ensure that the manure remains in the container for a sufficient amount of time for the pathogens to be removed.
The excreta must be diluted in a 1:3 ratio, for which urine or water may be used. Any solids, along with any inorganic material, should be removed before the excreta are introduced into the digester. If the animals have been given antibiotics, at least four days should be allowed to go by before using the manure, because antibiotics can harm the bacteria inside the reactor. The biogas is used for stoves with conventional valves connected to a hose or pipe, without any type of pressure regulator.
Units to Monitor Project Progress: Biodigesters installed (number).
Unites to Monitor Measure's Impact: Fertilized area (ha); methane used (m3/year); fertilizer and biogas produced (l/month).
- Chiconato, D. and others (2013). “Resposta da alface à aplicação de biofertilizante sob dois níveis de irrigação”. Bioscience Journal, vol. 29, No. 2.
- Ferrer, I. and others (2009) Producción de biogás a partir de residuos orgánicos en biodigestores de bajo costo. Barcelona: Universidad Politécnica de Cataluña.
- Poggio, D. and others (2009). "Adaptación de biodigestores tubulares de plástico a climas fríos", Livestock Research for Rural Development, vol. 21., No. 9.