Biogas is a flammable gas produced by organic materials after it has been decomposed and fermented by anaerobic bacteria in tightly sealed environmental digesters under certain temperature, humidity, acidity and alkalinity conditions. The process in which biogas bacteria decompose organic materials to produce biogas is known as biogas fermentation. Manure-based biogas digesters refer to fermentation tanks which are used to treat animal manure including human waste via anaerobic fermentation. The methane concentration of biogas is around 60%, so the recovery and utilisation of biogas from digested slurry in a biogas digester will reduce CH4 emissions from just escaping from the manure. In addition, the biogas can be used to provide electricity or thermal energy and reduce CO2 emissions from fossil fuel (coal) displaced by biogas.
A biogas digester is composed of six parts: fermentation chamber, gas storage, inlet tube, outlet chamber, removable or sealed cover, and a gas pipe line (see in figure 1).
The mechanics of biogas generation can be described as follows:
- The captured gas is stored in the upper part of the digester tank (gas storage area), which is constructed in an arc shape. The generation of biogas will gradually increase the pressure in the stored area. When the volume of the captured gas is larger than the amount consumed, the pressure in the gas storage will increase and slurry will be pushed into the outlet chamber. If the amount of gas consumed exceeds gas availability, the slurry level drops and the fermented slurry flows back into fermentation chamber.
- The placement of the digester tank (underground fermentation) keeps the temperature in the tank relatively stable ensuring that the slurry can be fermented at adequate temperatures throughout the year without requiring additional heating.
- The bottom of the digester inclines from the material-feeding inlet to the material-outlet, allowing free flow of the slurry.
- The digester has been designed to allow the effluent to be removed without breaking the gas seal, taking the effluent liquid out through the outlet chamber.
As stated in the technology definition section above, biogas fermentation is a process in which certain bacteria decompose organic matter to produce methane. In order to obtain normal biogas fermentation and a fairly high gas yield, it is necessary to ensure the basic conditions required by the methane bacteria are met for them to carry out normal vital activity (including growth, development, multiplication, catabolism etc.).
Strict anaerobic environment
Microbes that play a major role in biogas fermentation are all strict anaerobes. In an aerobic environment, the decomposition of organic matter produces CO2; however, in an anaerobic environment, it results in CH4. A strict anaerobic environment is a vital factor in biogas fermentation. Therefore, it is essential to build a well-sealed, air-tight biogas digester (anaerobic digester) to ensure a strictly anaerobic environment for artificial biogas production and effective storage of the gas to prevent leakage or escape.
Sufficient and suitable raw materials for fermentation
Sufficient raw materials for biogas fermentation constitute the material basis for biogas production. The nutrients that methane bacteria draw from the raw materials are carbon (in the form of carbohydrates), nitrogen (such as found in protein, nitrite, and ammonium), inorganic salts, etc. Carbon provides energy, and nitrogen is used in the formation of cells. Biogas bacteria require a suitable carbon-nitrogen ratio (C:N). The suitable carbon-nitrogen ratio for rural biogas digesters should be 25~30:1. The carbon-nitrogen ratio changes with different raw materials, and one must bear that fact in mind when choosing a mix of raw materials for the digester.
Appropriate dry matter concentration
The appropriate dry matter concentration in the raw materials for biogas fermentation in rural areas should be 7%-9%. Within this range, a low concentration of raw materials may be selected in summer, while in winter a higher value is preferred.
Appropriate fermentation temperature
Biogas fermentation rates depend greatly on the temperature of the fermenting liquid in the digester. Temperature directly affects the digestion rate of the raw materials and gas yield. Biogas fermentation takes place within a wide temperature range (Xu Zengfu, 1981). The higher the temperature, the quicker the digestion of the raw materials will be, and the gas production rate will also become higher. Based on real fermentation conditions, we have identified the following three temperature ranges for fermentation:
- High temperature fermentation: 47°C~55°C.
- Medium temperature fermentation: 35°C ~38°C.
- Normal temperature fermentation: ambient air temperature of the four seasons.
Selecting the temperature range for bio-gas fermentation depends on the type, sources, and quantities of raw materials; the purposes and requirements of processing organic wastes; and their economic value. Most household biogas digesters are normal temperature fermentation.
Appropriate pH Value
The pH value of the fermenting liquid has an important impact on the biological activity of biogas bacteria. Normal biogas fermentation requires the pH value to be between 7 and 8. During the normal process of biogas fermentation in a rural digester, the pH value undergoes a naturally balanced process, in which it first drops from a high value to a low value, then rises again until it almost becomes a constant. This process is closely related to the dynamic balance of three periods of biogas fermentation. After feeding the biogas digester, the time that the pH value takes to reach its normal level depends on the temperature and the kinds and amounts of raw materials that are fed in.
- Reducing GHG emissions by reducing CH4 emissions from manure management and CO2 emissions from coal burning or other carbon based fuel source.
- Saving on energy costs for cooking and lighting by providing biogas which is clean energy.
- Fertiliser saving by applying the effluent from biogas digesters by replacing commercial fertiliser.
- Improving local environmental conditions in rural areas.
- Medium to high capital costs and the initial investment cost are the main constraints for installing a digester.
- Skilled and trained labour is required for the construction of a biogas digester.
- Requires availability of animal excrements for optimal biogas production.
- There are sometimes cultural prejudices against using gas from human waste.
Figure 2 shows the development trends of household biogas projects in China. By the end of 2008, the number of the overall household biogas digesters had reached 30.49 million. One can see that during the period of 1990-2008, the implementation of household biogas digesters increased 6.4 fold. Unfortunately, due to limited finances, most farmers have not been able to afford a biogas digester.
The biogas digesters have to be located in many cases in the remote rural areas, where farmers lack ready access to improved technologies and management methods. According to current experiences in China, the performance of some digesters are unstable, with varying levels of gas production. This is due to the lack of experience among the individual households, limited resources for biogas service support, and insufficient farmer training. Expertise is required to ensure that the digesters function properly, so maintenance and management of biogas digesters require adequate support services and trained staff, which is not available in rural areas.
Biogas technology can reduce emissions from farmyard manure, and its price ranges from US$12-40 per t CO2e saved. Biogas technology becomes suitable for mitigating GHG emissions if there are high amounts of organic inputs at a price of approximately US$12 per t CO2e saved (Wassmann and Pathak, 2007).
It is estimated that an 8 m3 household biogas tank can treat the manure from 4 to 6 pigs, yielding around 385 m3 biogas annually. It can save 847-1,200 kg of coal based on the calculation of effective heat equivalent. According to the methodology recommended by IPCC in 2006, if a household biogas digester treats the manure of 4 pigs, it can reduce GHG of 1.5~5.0 tonnes CO2e.
The cost of each household biogas digester (8-16 m3) ranges from US$500 to US$1,000 depending on the digester size. Most rural households within developing countries have low disposable income and weak financial capacity for making such a large investment. In addition, the household will continue to pay a biogas digester maintenance cost. By contrast, the current practice of deep-pit treatment method is by far considered the most attractive option for manure treatment given that it requires very limited additional investment and labour input.
Wassmann R and Pathak H. (2007): Introducing greenhouse gas mitigation as a development objective in rice-based agriculture: II. Cost- benefit assessment for different technologies, regions and scales. Agricultural Systems 94:826-840.
Xu Zengfu, (1981): Biogas technology. China Agriculture Press, Beijing.