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Extensive use of fuelwood and coal has helped to fuel China’s rapid economic growth but has also led to serious environmental damage. In Shaanxi Province (west China), soil erosion on the hillsides because of deforestation is widespread and the use of coal and wood on open fires produces significant pollution.
Shaanxi Mothers promotes the use of biogas plants which provide clean fuel for cooking and lighting, improve sanitation and hygiene and help prevent further environmental degradation.
The use of coal and wood on open fires produces significant pollution. Faced with these problems, the Chinese Government has restricted tree-felling and ordered the re-forestation of the terraced hillsides. Farmers are paid to plant trees and ensure that they survive, and are encouraged to keep pigs. The Shaanxi Mothers Environmental Protection Volunteer Association promotes the use of biogas plants connected to the pigsties. These plants provide clean fuel for cooking and lighting which replaces wood and coal, and they also improve sanitation and hygiene. The solid residue from the plants is also a valuable fertiliser.
Shaanxi Mothers is an NGO established by Mrs Wang Mingying in 1997, initially to raise money for tree planting projects through the collection of rubbish that could be recycled. It is run by a 59-person Board of Directors with a Standing Board to manage routine matters. In 2009, it had 1,200 association members and five employees. The work of Shaanxi Mothers is supported by local and provincial authorities, individuals, NGOs and donors.
In Shaanxi, most pigs are reared in pigsties. Pig waste, combined with human excrement from a toilet can be fed into a biogas plant. Bacteria in the plant decompose this waste matter under anaerobic conditions. The decomposition produces biogas, which typically consists of 60-70% methane and 30-35% carbon dioxide, and a semi-solid residue. The biogas is piped into the house and connected to a two-ring stove for cooking, a lamp for lighting and occasionally a water-heater for showers.
Annual cleaning and minor repairs to biogas plants are carried out every October and the pigsty is insulated at this time, which helps to keep both the pigs and the biogas digester warm in winter. However the biogas production falls off for one or two months during the coldest part of the winter.
US$1 = 8 RMB (China Yuan or Renminbi) (July 2006)
The price of the biogas plant varies between US$440 and US$560 (3,500 to 4,500 RMB), depending upon its size and features. Typically the user pays one third of the cost of the system in labour and cash, with one third from central and local government subsidies and the final third from the Shaanxi Mothers. In 2006, demand for biogas plants was such that not every family could receive a subsidy. In this situation, the village authorities decide who should receive financial support.
Experts from the Shaanxi Mothers train local technicians in each village on how to install, service and maintain the plants. The Shaanxi Mothers teach villagers how to use the biogas, maintain the plants and maximise the use of the slurry to increase crop yields. Plants should last for at least 15 years if they are used with care. Biogas appliances are made in several parts of China and can be ordered and delivered to Shaanxi.
The biogas digester is constructed in a pit which is excavated by a Shaanxi technician with assistance from a member of the household. The body of the digester is an underground cylindrical tank which is built very carefully from bricks and mortar by the technician. The fixed hemispherical dome, which acts as a gasholder, is made from concrete. The pigsty is built over the biogas digester, with the toilet in the house immediately adjacent to it, so that the pig and human waste can be sluiced directly into the plant. Most biogas digesters have a volume of 8m3 (some are 10 to 15m3).
Users must stir the slurry (decomposing waste) frequently, to ensure the bacteria reach all the material and to avoid very high concentrations of digestion products. The water which is used to sluice waste into the plants allows the slurry to flow easily, and maintains a suitable concentration of solids (about 7% in summer and 11% in winter). The acidity (pH) of the digester is monitored regularly to ensure that the concentration of volatile fatty acids does not become too high. For optimum results users must add cow dung periodically as it contains the right anaerobic bacteria. This usually has to be purchased from neighbours or trader.
Between 1999 and 2006, Shaanxi Mothers installed 1,294 biogas plants in rural farming households in the Shaanxi Province of China. With about four people per household on average, the plants bring benefits to about 5,200 people.
Less fuelwood and coal is needed for cooking and heating (although most people with biogas still prefer to cook bread dumplings in solid fuel ovens). Shaanxi Mothers estimates that a household which replaces fuelwood with biogas saves about 4.5 tonnes of wood per year.
Greenhouse gas savings have not been quantified, but are probably the equivalent of about 6 tonnes/year CO2 per biogas plant. This includes the direct avoidance of CO2 from coal and unsustainable wood use, and also the reduction in methane emission from pig manure. Thus the 1,294 biogas plants installed by 2006 saved about 8,000 tonnes/year CO2.
The use of biogas also greatly improves the quality of air in the home, reducing the concentration of carbon monoxide by about 74%, sulphur dioxide by 74% and dust by 77%, according to a survey carried out by the Shaanxi Mothers. In addition, the digester prevents effluent from seeping into the groundwater which reduces levels of pathogenic bacteria.
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The solid residue from the biogas plant makes useful fertiliser which is applied to fruit trees and vegetable plots. Biogas plants (or the pigs which supply manure to them) can also be used to dispose of fruit and vegetable waste from households.
Biogas is available at the point of use and is literally ‘on tap’. This saves time collecting wood and in preparing fires for cooking. Users enjoy being able to prepare a meal much more quickly using biogas and not having to endure a smoky kitchen. Women have used the time they save to work in the fields, make clothes for their children or spend time with friends and relatives.
The disposal of human and animal waste in a biogas plant can have a major impact on human health. Shaanxi Mothers records that the density of flies in houses with biogas is 64% lower than in houses without.
A biogas plant reduces household expenditure on fuel and fertiliser. One 8 to 10m3 biogas plant can supply a family of three to five people with enough gas for 90% of their daily fuel needs for 10 to 11 months of the year. This saves about US$75 per year (600 RMB) on fuel (coal or wood), US$40 per year (300 RMB) on fertiliser and US$20 per year (150 RMB) on electricity (through use of biogas lamps). Using the output residue from the biogas plant as fertiliser also increases household income by about US$250 per year (2,000 RMB), as a result of increased food production.
Taking all these together, biogas plant owners can increase their average disposable income by about US$380 per year (3,000 RMB), so the total cost of a biogas plant can be recovered in just over one year.
By 2009, with additional funding of US$ 185,000 (1.2 million RMB). Shaanxi Mothers had installed a further 973 biogas plants, bringing the total to 2267 plants in 41 villages. These plants save the equivalent of about 14,000 tonnes/year of CO2.
Shaanxi Mothers has developed its network of biogas service providers, training 236 people in 38 villages to help biogas households keep their systems in good working order. Additional training has also been given to 1,380 individual biogas households.
On the technological front, Shaanxi Mothers has developed cellars to collect rainwater in areas prone to drought in the Loess Plateau. By 2009, 358 rainwater collection cellars had been installed in three villages.
27 October 2022
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