The role of low-cost plastic tube biodigesters in integrated
farming systems in Vietnam: Part I
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Second FAO Electronic Conference on Tropical Feeds
Livestock Feed Resources within Integrated Farming Systems
THE ROLE OF LOW-COST PLASTIC TUBE BIODIGESTERS IN INTEGRATED FARMING SYSTEMS
IN VIETNAM (Part I)
Bui Xuan An
University Agriculture & Forestry, Thu Duc, Ho Chi Minh City
Vietnam
E-mail: an%sarec%ifs.plants@ox.ac.uk
INTRODUCTION
For the past 10 years or so, Vietnam has adopted modern farming techniques
that use imported agro-chemicals and fossil-fuel products in order to
increase exports of agricultural products and feed its population which has
grown to 75 million. The rising environmental problems and costly
socio-economic dependence on external inputs have alarmed certain leaders
and many of the population. Facing this situation, the use of
environmentally-friendly techniques at all levels of farming have had an
important role in rural development. Low cost plastic biodigesters make
efficient use of manure in the integrated farming system to produce gas for
cooking and effluent to fertilize ponds for fish, aquatic plants and crops,
bring advantages to the economy and to the environment. They have been
adapted from the "bag" digester or Taiwan model, simplified by using cheaper
polyethylene tubular film to replace the welded PVC sheet.
Many developing countries, such as Colombia, Ethiopia, Tanzania, Vietnam,
Cambodia and Bangladesh, have promoted the low-cost biodigester technology,
aiming at reducing the production cost by using local materials and
simplifying its installation and operation. Within three years, more than
1000 polyethylene digesters were installed in Vietnam, mainly paid for by
farmers. This report discusses the role of plastic biodigesters in
integrated farming systems in Vietnam and describes experience with the
introduction of biodigesters under local conditions.
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BIOGAS IN DEVELOPING COUNTRIES
After 1975, slogans such as "biogas for every household" led to the
construction of 1.6 million digesters per year in China, mainly concrete
fixed-dome digesters. Up to 1982, more than seven million digesters were
installed in China (Kristoferson and Bokhalders, 1991). In 1980, more than
50% of all digesters were not in use (Marchaim, 1992). The rapid development
of biogas in China received strong government support and sometimes
subsidies from local government and village government were up to 75%
(Gunnerson and Stuckey 1986). In recent years, the number of plants built
each year has fallen dramatically because of the reduction in subsidies with
a consequent switching from biogas to coal as a fuel. The biggest constraint
in the biogas programmes has been the price of the digesters. It was also
learned that the popularization of biogas would only be successful when the
direct benefits to the farmers were obvious.
In many respects, the same situation as in China prevailed in India where a
rapid biogas digester implementation policy exceeded the capabilities of
India's research and development organizations to produce reliable designs
and to optimize digester efficiency. As a result, earlier digesters in the
country were expensive and inefficient. This situation has been remedied
somewhat in recent years. According to Kristoferson and Bokhalders (1991),
new developments and designs are not incorporated as rapidly as they might,
and improved coordination and feedback will be required if development is to
be achieved. The poor performance of earlier biogas digesters can also be
attributed to poor backup services. This situation, which is still largely
prevalent, has led to a relatively high breakdown rate. Problems can be
classified as (a) design faults; (b) construction faults (c) difficulty of
financing; (d) operational problems due to incorrect feeding or poor
maintenance and (e) organizational problems arising from the differences of
approaches and lack of coordination.
Biogas production has been stimulated by popular publicity campaigns and
subsidized construction of biogas plants by central and local governments.
The floating cover design, introduced by the All-Indian Coordinate Biogas
Programme, is the most common system currently in use in India. This system
is more expensive than the fixed dome (Chinese) digester. Despite having the
world's second largest number of installed biogas digesters, the biogas
program has mainly concentrated on the expensive systems capable of being
installed only by the wealthier inhabitants in the rural areas (Kristoferson
and Bokhalders, 1991). India has placed far more emphasis on the survival of
small-scale farmers than ensuring their efficiency and growth in a
competitive environment through various policy instruments like the biogas
programme.
The situation is almost the same in many other developing countries, such as
the Philippines, Thailand, Nepal, Brazil. For example in Nepal, many authors
considered that, with the installation of more than thirteen thousand biogas
plants, the strategic plan and activity of biogas program implementation was
gaining more popularity and becoming a well developed example of technology
dissemination. The government has provided up to Rs 7000 for a plant built
in the lowlands and Rs 10000 in the hill areas (about 30-70% of the cost for
construction). According to a report from the Consolidated Management
Services Nepal, although biogas was introduced in Nepal about two decades
ago, the present infrastructure seems so weak that there is still the
dependency upon foreign countries for supply of some biogas accessories and
equipment. With subsidies of more than 50% of the cost of a family size
plant, many farmers who demanded biogas plants were more attracted to the
amount of available subsidies than by the utility of the plant as such. Many
newly-formed private companies were finding their business quite profitable
and a considerable part of the government subsidy was taken by these
companies as profit (Karki et al, 1994). Without subsidies the simple
pay-back period varied between 6 and 12 years in Nepal.
In many developing countries, frequent changes in government policies on
interest rates and subsidies have also had negative impacts on biogas
dissemination. These changes have disappointed the investors in long-term
biogas development. The progressive farmers who would like to have biogas
also become doubtful about their long-term biogas investments.
Biogas production was introduced into Vietnam more than 10 years ago as an
alternative source of energy to partially alleviate the problem of acute
energy shortage for household uses. Biodigesters of various origins and
designs were tested in rural areas under different national and
international development programmes, using household or farm wastes as
fermentation substrates. Indian-type, Chinese-type and ferro-cement-type
digesters were installed and evaluated in many provinces but concentrated in
urban areas (Thong et al, 1989; Khoi, 1989). However, few farmers used them
in practice.
The poor acceptability of these concrete digesters was mainly due to: (a)
high cost of the digesters; (b) difficulty in installing them; and (c)
difficulty in obtaining spare parts for replacement. A digester of a size
adequate for the fuel needs of an average family would normally cost VND 1.8
to 3.4 million (US$ 180 to 340) (Thong, 1989). This scale of investment is
considered unaffordable by the average farm family (An et al, 1994). In
addition, it would take about 2.5 to 3.5 years to pay back the initial
investment (Thong, 1989; Khoi et al, 1989). Besides, the replacement of
worn-out parts posed another technical problem, apart from the fact that
such spare parts are not always locally available. Khoi et al (1989)
reported that 33% of biodigesters installed in Cantho City had stopped
functioning while only 8 out of 17 of those set up in Quangnam-Danang
Province were still operable.
Vietnam is a nation with a low gross national product per capita, so getting
support for any kind of environmental program is difficult. Without the
support from the Vietnamese government or from overseas, the concrete
digester development is progressing slowly. Only the richest farmers in
rural or peri-urban areas can afford the construction of concrete digesters.
The development of concrete biogas digesters is therefore not sustainable in
rural areas. To disseminate the biogas fermentation technology in rural
areas, it is necessary to reduce the cost and use simple means of
construction.
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LOW-COST POLYETHYLENE TUBULAR DIGESTER
In the light of these constraints, many developing countries such as
Colombia, Ethiopia, Tanzania, Vietnam, Cambodia, Bangladesh have promoted
the polyethylene tubular digester technology, aimed at reducing the
production cost by using local materials and simplifying its installation
and operation. To this end it was decided to use a continuous-flow flexible
tube biodigester based on the "Taiwan" model and later simplified by Preston
and co-workers (An et al, 1994). The low-cost biodigester technology has
been well received by poor smallholder farmers in Vietnam for producing a
clean fuel to replace firewood. Within three years, more than 800
polyethylene digesters were installed in Vietnam, mainly paid for by farmers
(An and Preston, 1995).
Data on the design parameters and cost of digesters around Ho Chi Minh City
are presented in Table 1. The average length of the digesters was 10.2 m
with an estimated digesta volume of approximately 5.1 m^3 (length x 0.5
m^3). The material cost was slightly more than US$25 for a family digester.
Table 1: Mean values for some design parameters and cost of 194 digesters
installed around Ho Chi Minh City
Mean Range
Length (m) 10.2 4 - 30
Digester liquid volume (m^3) 5.1 2 - 15
Distance to kitchen (m) 23 8 - 71
Material cost (US$) 25.4 14 - 82
Time to first gas production (days) 17 1 - 60
Digesters in rural areas(%) 91
Floating digesters (%) 5
Source: An et al., 1996.
However, the biodigesters are still not fully integrated into the farming
system as there is only limited use of the by-product (the effluent) as
fertilizer for vegetables, fruit trees, fish and water plants (An et al
1994). The use of the effluent from biodigesters should be studied as a
resource for small scale farmers. The farmers always put questions about
quantities of manure fed to the digester, ratios between manure and water,
time of cooking, quantities of gas produced and the useful life of
biodigesters. The relevant data almost all comes from temperate countries
and from concrete biodigester plants.
Extension of the technology has had different successes in different
countries. It has been successful in Colombia, Vietnam and Cambodia but
there have been negative reports from other countries such as Bangladesh,
Nepal and Tanzania. The same technology was used but different results were
obtained. The difference is not only between countries but also in different
areas of a country (An et al, 1996). Many authors presented the advantages
of low cost and easy installation of the plastic digesters; meanwhile some
have been doubtful of life expectancy of the digester and the ability to
repair it.
It is necessary to study the constraints in each area carefully and seek
experiences from institutions with knowledge in this field. All institutions
and personnel who are involved in the biogas research and development should
be informed about experiences and results obtained elsewhere. The electronic
mail system is one of the most appropriate means to this end.
In most developing countries, when the subsidies from governments are
reduced, the number of plants built each year falls dramatically. The most
important problem in biogas programs in developing countries has been the
price of digester plants. For example, the price of a concrete digester
plant installed for an average family in Vietnam varied from 180 to 340 US$
(see above). Chinese designers tried to reduce the cost of red-mud digesters
to 25-30 US$/m^3 (Gunnerson and Stuckey 1986) but it was still high in
comparison with the polyethylene digesters (5 US$/m^3). This is obviously
one important feature which makes the polyethylene digesters attractive and
no farmer in the present study complained about the price.
Among the polyethylene digesters installed, 5% of them were floated in
ponds, adding an innovative feature to the development. According to Khoi et
al (1989), in the Mekong Delta where most land is low-lying, the application
of concrete digesters was very difficult especially when the water level
went up. The floating digesters solved this problem and, as they also
required little space, they were very well suited for use in low-lying
areas. More than 90% of the plants were installed in rural areas indicating
the good impact of the technology in these parts of Vietnam.
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INTRODUCTION OF BIOGAS TO SMALL FARMS IN THE THUAN AN DISTRICT
The effects of the introduction of digesters on small farms are presented in
Tables 2-5 (An et al 1996). Most of the farms with biodigesters belonged to
the medium-income group (sufficient food all year around). In this group
animal production is a very important component of their farming systems and
a sufficient number of animals is important for the dissemination of
biodigesters. The expense for the digester plant was paid back within
slightly more than 5 months, so most of the farmers found a great benefit
from installing digesters.
Table 2: Economic aspects of biogas introduction in 31 small farms in Thuan
An district, Vietnam
MEAN RANGE
Cooking time (hour) 4.4 1 - 9
Fuel saved in cooking (US$/month) 6.5 1.8 - 13.6
Biogas plant cost (US$/unit) 34.8 18 - 53
Number of pigs/farm 10.7 0 - 40
Payback time (month) 5.4 2 - 19
Source: An et al 1996.
Table 3: Farmers' participation and opinions on plastic biodigesters in
Thuan An district, Vietnam
ALTERNATIVES No.*
Getting first information from
Neighbours or relatives 32
Mass media 3
Payment of the digester plants
Farmers paid totally 33
Partially (demonstration) 2
Using slurry for Plants 3
Ponds 3
Nothing 31
Status of gas production Enough gas 26
Little gas 5
No gas 4
Advantages of biogas Saves money 34
Less pollution 35
Easy cooking 35
*No: Number of farmers
Source: An et al 1996
Table 4: Input and output of 31 digesters working at small farms around Ho
Chi Minh City, Vietnam
MEAN RANGE
Size of family 5.9 3 - 12
Manure loading (kg/d) 16 2 - 27
Ratio Water/manure 5.1 2.9 - 8.1
Loading rates (kg DM/m^3) 0.7 0.1 - 1.2
Temperature of loading (deg C) 26.4 25.7 - 28.5
Temperature of effluent (deg C) 27.0 26.0 - 29.1
pH of loading 6.7 6.4 - 7.1
pH of effluent 7.2 6.8 - 7.5
Gas production (l/unit/day) 1235 689 - 2237
Vol. Gas/capita (l/person/day) 223 68 - 377
Methane ratio (%) 56 45 - 62
COD of loading (g/litre) 35.6 22.4 - 46.0
COD of effluent (g/litre) 13.5 8.8 - 23.9
COD removal rate (%) 62 42 - 79
COD = Chemical Oxygen Demand
Source: An et al 1996.
Table 5: Effect of biodigestion on some microorganisms of manure in small
farms in Vietnam
MEAN RANGE
E. coli of loading (10^3cell/ml) 52,890 11,000 - 150,000
E. coli of effluent (10^3cell/ml) 75 2 - 450
Coliforms of loading (10^3cell/ml) 266,780 11,000 - 480,000
Coliforms of slurry (10^3cell/ml) 236 7 - 250
Source: An et al. unpublished.
Among 35 farmers interviewed, four of them were poor (not enough food in
certain months). The most important thing for them is food and they could
not afford a sufficient number of animals for feeding manure to the
digester. They wanted to borrow money to be able to raise animals. Four
farmers had no gas when the interview was carried out. Three of them did not
have animals because they found raising animals unprofitable if they had to
borrow money from local lenders at 5-10% monthly interest. This was an
important aspect, especially as resource-poor farmers cannot support the
digester installation and keep animals, although they know the advantages of
biogas.
The average DM percentage of manure was 25% and the loading rates ranged
from 0.1 to 1.2 kg DM/m^3 digester liquid volume.
Previously, animal manure was an environmental problem in villages in the
district, mainly in crowded and lowland areas where it caused pollution of
the air, water and soil. After installation of the digesters, all 35
families recognized better environmental conditions, less smell, fewer
flies, cleaner waste water, etc. Summarizing details of experiments
conducted with pig slurries, Pain et al. (1990) concluded that the digestion
reduced odour emission by between 70 and 74%. According to the women who
were responsible for food preparation, use of biogas meant that they could
attend to other work, while cooking. This is in contrast to the situation
when using solid fuels such as firewood which require much closer
supervision. The women stressed that they could now cook in a clean
environment, free of smoke. Their pots and pans were clean and they did not
have to spend time on tedious cleaning. They stated that they could cook all
food items on gas.
In the study, biodigestion decreased COD from 35610 mg/lit in the inlet to
13470 mg/l in the effluent, indicating a process efficiency of 62% (COD
removal rate). The digestion in biodigesters reduces the pathogens in waste
water so it prevents contamination from animal production. The volume of gas
per capita per day was about 200 litres, enough to cook three meals. The
loading rates were low and gas production could be improved by increasing
the amount of manure fed to the digesters. Beside cooking meals, five
farmers cooked animal feeds, three made wine, one made cakes and two
prepared tea and coffee in their cafeterias. This demonstrates that there
are several reasons for uptake, as discussed by Dolberg (1993).
An on-farm study on the use of slurry for some crops was carried out to
evaluate the effect of biodigesters in farm economics. The results were
presented in table 8. The crops were Lilium flower, elephant grass and sweet
potato. The use of slurry increased by 100% the benefit of biodigester
introduction in comparison with gas use only.
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FAO ELECTRONIC CONFERENCE:
LIVESTOCK FEED RESOURCES WITHIN INTEGRATED FARMING SYSTEMS
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