Compact Biogas System
Conventional biogassystems used organic wastes such as human or animalfaecaes, distillery effluents, or municipal solid waste.
These feedstock materials cannot be digested by the bacteria in the system.
These systems require 40-50 kg of feedstock and have a retention time of bout 40 days, to produce daily 1000 lit ofbiogas.
Minimum capacity of such systems is 2000 lit.
The gas is a mixture of CH4and CO2, containing about 40-50% CO2 by volume.
They generate daily 80-100 lit of effluent slurry.
Appropriate Rural Technology Institute (ARTI), developed during 2002-03 an innovative, compactbiogasplant, which uses starchy or sugary material as feedstock.
The newbiogasplant has the capacity of just 400 to 500 lit, requires just 2 kg starchy or sugary material to produce about 800 lit ofbiogas, and the reaction time is only 6 to 8 hours.
Special Features of the Compact Biogas System
The potential candidatefeedstocks, namely rain damaged or insect damaged grain, flour spilled on the floor of a flour mill,oilcakefrom non-edible oilseeds, seed of various tree species, non-edible rhizomes (banana,arums,dioscoreas), leftover food, spoiled and misshapen fruits, non-edible and wild fruits, spoilt fruit juice, etc. are readily available in rural areas.
1 kg of sugar or starch yields about 400litresofbiogas, within a period of 6 to hours. This quantity is enough for cooking one meal for 5 to 6 persons.
The effluent slurry generated daily by the plant is just a couple oflitres. It can be used as manure for plants growing around the house.
The compactbiogasplant, mass produced, would cost aboutRs. 3,500 (US$78). The smallest cattle-dung based domesticbiogasplant costs aboutRs. 12,000 (US$267).
Field Testing Objectives:
–To test the quantity and quality of biogas produced from different feedstock materials and their mixtures, under field conditions.
–To generate data tables of gas yield Vs feedstock used for the convenience of the users.
–Use an experimental biogas plant producing 25 lit biogas every 24 hours by consuming about 50 g feedstock.
–Study quantity and quality of biogas produced using a starchy and an oily feedstock separately.
–Study quantity and quality of biogas produced using mixtures of starchy and oily feedstock.
–Quantity of biogasestimated from the increase in the height of the floating drum.
–Quality of biogas deduced from chemical analysis.
It is not advisable to transport the biogas plant while in operation.
After installation, the plant requires about 4-5 days to stabilise.
Once stabilised, the plant routinely produced daily 25 lit biogasusing 50 g of a starchy feedstock.
Chemical analysis indicated that the gas contained <>
Sudden replacement of starchy feedstock by oily feedstock led to stopping of gas production.
Preliminary experiments involving mixtures of starchy and oily feedstock materials in different proportions indicate that biogas yield may be increased by using a proper combination of feedstock materials.
There is a need for generating extensive data tables ofbiogasyield and systemstabilisationperiods for various feedstock materials used individually and in mixtures.
An attractive feature of the compactbiogasplant is the exceptionally high methane content of the gas. This implies that the gas is useful not only as a clean cooking fuel but also as an excellent transportation fuel.
Due to simplicity of operation, easy access to feedstock materials, and user-favouringeconomics, the compact biogasplant has a potential torevolutionisethe household energy scenario in India by offering a more accessible alternative to LPG.
The dream of a ‘blue flame revolution’ –putting a blue flame in each and every rural kitchen in India -has become more realistic and achievable in the near future with this invention.
Several members asked me to provide more details about the compact
biogas plant being developed by us. I give below the latest status of
The biogas plant consists of two cylindrical vessels telescoping into
one another. The larger vessel, called the fermenter, has a total
internal volume of about 500 lit. A drum having diameter of 85 cm and
height of 85 cm would have the desired volume. The smaller vessel, which
telescopes into the larger one, serves as the gas-holder. The diameter
of the gas holder is about 2 cm smaller than that of the fermenter. The
fermenter vessel is provided with appropriate inlet and outlet pipes for
introducing the feedstock into it and for removal of spent slurry from
it. The gas holder is provided with a gas tap, through which the gas is
led to the burner. This system uses starchy or sugary material as
feedstock. 1kg of sugar or starch yields about 400 litres of methane,
within a period of 6 to 8 hours. This quantity is enough for cooking one
meal for 5 to 6 persons. The biogas produced by this system contains
theoretically about equal volumes of carbondioxide and methane, but in
reality, it turned out to have less than 5% carbondioxide. This
phenomenon is explained by the fact that carbon dioxide dissolves in the
water in the fermenter vessel and diffuses out of it through the 1 cm
gap between the fermenter and the gas holder. The gas produced by this
system has thus almost the same calorific value as LPG. It burns without
smoke or soot, producing an almost invisible bluish flame similar to
that of LPG.
Several prototypes, in operation for more than a year, have been
successfully tested using various feedstocks. The potential candidate
feedstocks, namely rain damaged or insect damaged grain, flour spilled
on the floor of a flour mill, oilcake from non-edible oilseeds, seed of
various tree specie
s, non-edible rhizomes (banana, arums, dioscoreas),
leftover food, spoiled and misshapen fruits, non-edible and wild fruits,
spoilt fruit juice, etc. are readily available in rural areas. This
system is much easier to operate than the dung based biogas plant,
because of the relatively small quantities of feedstock and effluent
slurry to be handled. The effluent slurry generated daily by the plant
is just a couple of litres. It can be used as manure for plants growing
around the house. The 500 litre biogas plant, mass produced from moulded
plastic drums, would cost about Rs. 3,500 (US$ 78). The smallest
cattle-dung based domestic biogas plant costs about Rs. 12,000 (US$267).
It requires daily 40kg dung, and owing to the retention period of almost
40 days, such plants have a minimum capacity of 2000 litres. They
generate daily 80 to 100 litres of effluent slurry. Daily handling of
such large quantities of feedstock and effluent is conside
red to be
arduous and bothersome by users.
Preliminary studies indicated that the amount of biogas produced and the
retention period varied from feedstock to feedstock and from season to
season. Also, when the feedstock was changed from one form to another,
the system took a few days to stabilise. Our studies also indicated that
the gas yield could be increased by using combinations of feedstock
materials. We are now looking at additives such as micronutrients,
nitrogen, phosphorous compounds etc., which might bacterial action and
yield more gas at a faster rate. Since the users would depend mainly
upon locally available feedstock, field trials are essential to
determine the retention periods and gas yield for different raw materials.
Many people in India, who read my article in a local neuspaper, copied
our design and have started to use this biogas plant in their
households. A schoolgirl submitted a working model of it in a statewi
science project competition and won the first prize in the state. A
company supplying science equipment to educational institute wants to
manufacture models (50 litre capacity) for supply to schools and colleges.
We have supplied 200 litre models to 10 voluntary agencies in different
regions for demonstrating this technology to villagers in their
respective areas. This model is meant for areas where the main diet is
rice. This model yields enough gas to operate a pressure cooker to cook
rice, beans, vegetables or meat for a family of five. In areas, where
the main diet of the people consists of unleavened flat bread, somewhat
like the tortilla, each piece of bread is made individually, and
therefore the stove has to be in operation for a longer time. In such
cases, we recommend the five hundred litre model.
Bio-Energy For Water Supply And Illumination
Basavanahalli near Tiptur in Tumkur district of Karnataka, comprises of 40 households with a population of 200 people and 180 cattle. The major problems faced by the villagers are drinking water and low voltage of electricity. The only source of drinking water is a borewell with a handpump. The women had to fetch water from a depth of 60 metres and carry it home by walking an average distance of 200 metres and this took a major share of their time and added to their drudgery.
In 1995, an initiative was taken by BAIF, Pune to establish a community biogas based electricity generation plant at Basavanahalli with the financial support from international Energy Initiative. The plant which has a maximum capacity of producing 20 cmt of gas by processing 500 kg of dung, was commissioned in August 1996. Biogas can be an important component of renewable energy supply providing energy and manure.
Biogas Plant-Operation and Maintenance
The villagers have formed a Gram Vikas Sabha to operate and maintain the biogas plant. All the families in the village are members of the Sabha and the executive committee consists of 10 members drawn from different sections of the community. Every house has been provided with a fan and a tube light. A youth has been trained and appointed by the Sabha operating the system. The water is supplied for one hour every morning and evening. Light is provided for 3 hours from 6 pm to 9 pm or on local demand. The timings for water and power supply is determined by the Sabha. Individuals have been provided with a passbook for recording entries on collection of cow dung and distribution of slurry. The Sabha meets regularly and discusses matters relating to the smooth operation of the system.
At present, gas distribution for cooking is not encouraged as the required management systems for gas distribution at the community level are yet to be perfected. The gas generated in the plant is fed into a 7.5 kva generator which operates with 20 percent diesel and 80 percent gas. The electricity so generated is used for pumping drinking water by operating a submersible pump. The electric supply line is drawn 1.0 m below the existing grid electricity lines using the same poles.
The slurry coming out of the biogas plant is dried on sand bed filters. This enables faster dung cake formation which will be easier to distribute among the farmers. The filtered water is again collected and recycled for mixing with fresh dung for feeding into the plant. For every kilogram of fresh dung contributed by the farmers, 850 gram of dried cakes are returned. The villagers who do not have land or cattle also contribute cow dung to the biogas plant by collecting dung from surrounding areas. The slurry in such cases is sold by the contributors to other interested farmers. There is very little loss of energy in community biogas based electricity generation and distribution system. The total cost of production and consumption is cheaper and highly decentralised.
Many social benefits though intangible accrue from the use of this plant. These are improved health and sanitation, reduction in women's drudgery, increased family labour efficiency. With surplus water supply for domestic use, sewage let out in open drains was envisaged. To avoid this pollution, the villagers are planning to grow coconut and banana in their backyards using this waster water.
The capital cost of establishing a unit capable of supplying electricity and water for 100 families is Rs. 5,00,000. The monthly expenses to run the plant works out at Rs. 1,500 which includes salary of the operator, minor repairs and diesel cost. This is met by collecting a monthly nominal fee of Rs. 20 from each family. It is also planned to generate more revenue by increasing the utilities by operating a flour mill and giving special supply to families during marriage or on special occasions. The viability of these plants will improve if the size of the village is large. A large number of families means more dung and less individual expenses leading to increased generation of electricity.
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Important Organisations working in Biogas
Shivasadan - Sangli
Gram Vikas - Orissa
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ARTI develops a novel biogas plant
Though remembered for its success in converting sugar-cane leaf trash and similar organic waste into charcoal briquettes, Dr Arvind Karve's Appropriate Rural Technology Institute [ARTI] in Phaltan near Pune, Maharashtra, is working on many other innovations as well. The charcoal briquette process won it the Ashden Award for 2002. Now here's news of ARTI's 'compact biogas plant'. It is small. It will accept a wide range of farm wastes—not just cow dung—as its input. It will digest them and yield gas in 6 hours.
The following note received from ARTI elaborates it further:
Appropriate Rural Technology Institute [ARTI], has been working on biomass based improved fuels and cooking systems for the last 8 years. ARTI received, in February 2002, the Ashden Award for Renewable Energy, 2002, for developing the technology of making charcoal briquettes from agricultural waste and the Sarai cooker [see note below], which uses these briquettes as fuel.
In January 2003, ARTI received from Shell Foundation, London, a grant of Rs.15 million, for commercialising improved biomass based fuels and cooking systems in India. This work was being conducted till 2002, by the Ministry of Non-Conventional Energy Sources, Government of India, under a government sponsored welfare programme called the National Programme on Improved Cookstoves [NPIC]. This programme was terminated by the Government of India, because it failed to have any impact at all.
Under the Shell funded project, ARTI distributes charcoal briquettes, Sarai cooker and various models of energy efficient cookstoves that were developed under NPIC, through small-scale commercial enterprises based on these technologies. The prices of the fuel and the devices have not been subsidised. The customers have to pay the full price. Under this programme, about 100 artisans have already started small rural enterprises under the guidance of ARTI.
About two years ago, ARTI developed, under the guidance of its President, Dr.A.D.Karve, a compact biogas system, which uses starch or sugar as feedstock. Just one kg of starch or sugar yields the same amount of methane as 40 kg of cattle dung. Whereas cattle dung requires about 40 days to get converted into gas, the starch/sugar based biogas plant delivers the gas in just 6 to 8 hours. Waste starch in the form of rain damaged grain, banana rhizomes, non-edible seeds of various tree species, oilcake of non-edible oilseeds, etc.Êis plentifully available in the rural areas. While the smallest traditional domestic biogas plant has a volume of about 2 cubic meters, the new compact biogas system is just as large as a household refrigerator.
This invention was publicised by Dr. Karve under the title "The Blue Flame Revolution". Papers based on this concept were also presented by him in international seminars held in October 2003 in Yogyakarta, Indonesia, and in January 2004 in Seattle, U.S.A. A short note on this technology was also published in the premier scientific journal "Science".
The smoke and soot generated by traditional cookstoves using traditional fuels like stalks of cotton or pigeonpea, maize cobs, dung cakes, etc. cause indoor air pollution in rural households. Because the improved fuels and improved cookstoves mitigate this problem, the United States Environmental Protection Agency [USEPA] had already accorded the status of a partner organisation to ARTI under its programme called "Partnership for Clean Indoor Air". Under the same programme, the USEPA has sanctioned to ARTI a grant amounting to about Rs. 6 million, for standardising and commercialising the compact biogas technology. The project, having a duration of two years, would be conducted under the leadership of Dr. A.D.Karve.
For those that may not know what a Sarai cooker is, here is a note on it by Dr Karve himself:
Sarai is a stainless steam cooker. It is a non-pressurised vessel, into which you put about 150 ml of water and then lower into it, a wire cage, which carries three cook-pots, one on top of another. The steam pot has a lid which is kept closed while the food is being cooked. The heat is provided by a charcoal burner, designed to hold just 100 g of charcoal or a single honeycomb briquette of 100 g. After the coal has caught fire, the pot assembly containing the food to be cooked is placed on the stove.
A hollow cylinder, also made of stainless steel, encloses the entire assembly. The surrounding annular gap is just 5 mm. Flue gases generated by the charcoal escape through this gap. In this way, the pot is heated on all sides, instead of just from the bottom. The boiling and evaporation test showed the efficiency of this gadget to be 70%. Beans, rice and vegetables for a family of 4 to 5 are cooked with just 100 g of charcoal briquettes. It takes between 45 minutes to an hour to cook the meal. At present we are advocating it for food that needs to be steamed or boiled. We have not attempted roasting or frying
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Thursday, October 07, 2004
Get your shit together - A story from China
Angela and Tooker travel the world by bike, foot, train and boat; the Greenspiration! web site, www.greenspiration.org, contains their reports and contact info i.e. greenspiration at web.ca
A NEW AGE FOR SEWAGE:
China Lights The Way
by Tooker Gomberg and Angela Bischoff
Yes, China has enormous environmental problems. Over 2/3 of industrial wastewater is completely untreated. Urban air is often so polluted that it reaches levels several times the national standards.
But China is also the land of ecological transportation: 50-90% of all urban trips are still made by bicycle. Recycling is widespread; paper and many other materials are rarely discarded.
But there is one arena, largely unknown in the west, where China especially excels. Dare we say it? China has got its shit together.
Legendary alchemists strove to turn lead into gold. These days in villages and farms across China, the peasants are doing even better, turning feces into fuel and fertilizer. The production of biogas is widespread throughout rural communities in China, where 70 percent of China's 1.2 billion people live.
We first caught a whiff of China's biogas revolution while surfing the Internet. Then, while cycling through the southern Chinese city of Yulin, serendipity introduced us to an English teacher who helped us track down a government official who filled us in.
We were sipping tea with Zhuo Youxing, President of Energy Work with the Office for the Countryside, explaining to him our Greenspiration Odyssey of travelling the world with our bikes, looking for inspiring ecological stories, and writing about them. He listened politely, and then told us a bit about China's biogas program. What we really wanted to do was to go see some biogas units in actual use. He wasn't too helpful.
As a last resort we pulled out a letter of introduction a friend had written, in Chinese on Friends of the Earth, Hong Kong letterhead. That was the magical ticket we needed. Within minutes we found ourselves in a chauffeured government car speeding off to visit village biogas digesters. Throughout the hour-long expedition the driver incessantly honked the horn, sending pedestrians and cyclists scurrying for cover, in a scene reminescent of a Keystone Cops movie.
Carsick and on edge, we arrived in Beilu County's Zhonghe village, a prosperous community thriving from the sale of lichee fruit. Here, biogas is in widespread use, transforming human and pig waste into gas for cooking, lighting, and heating water. Already 400,000 people in Guangxi province alone are using biogas, and in China five million households rely on biogas digesters.
The system we saw was simple. Each family houses the pigs and the outhouse in a concrete and brick building near their home. All the waste, including anything biodegradable, goes directly into the biodigester, an underground tank below the outhouse, where the materials break down without oxygen. The by-product of the process is an odourless and colourless mixture of gases, mostly methane, and a nutrient-rich slurry often used for fertilizing crops.
The gas rises in the tank and is funnelled into a plastic tube. The tube snakes out of the building, through the tree branches into the kitchen. In the kitchen the gas is used for cooking and lighting, and for hot water for bathing. Energy efficient cooking utensils, like a wok and pressure cooker, ensure that the methane gas isn't wasted.
A family of three or four, with four pigs, can produce enough gas for all their cooking, lighting and water heating.
There are substantial benefits from using biogas. If feces are not properly treated or composted they can become a breeding ground for disease-causing organisms, and also can pollute waterways. The biogas digestion process kills pathogens.
"In the past, especially in the countryside, farmers went up to the hills and cut the trees for fuel to cook. And now, after we installed biogas pools, they don't need to go up to the hills and cut trees. In this way we can protect the forest. Each family can save 12 kg of wood. By the end of the year they can save 4000 kg" Mr. Zhou told us.
And burning wood or straw causes smoke, a hazardous source of indoor air pollution. Not a problem when biogas is used.
China's biogas program began in 1958, but really gained momentum in the 1970's. Officials are aiming that by the year 2002 there will be 100,000 pools in Beilu County, more than half the households. The longer term goal is to have every household equipped with a unit.
The unit, technically called a fixed dome Chinese model biogas plant, or drumless digester, is odourless and fly-less. This design was first built in China in 1936. The air/water-tight underground tank is made of easily available and inexpensive materials, mainly concrete and brick. It is 1.7 metres deep, 2.2 metres diameter, and is expected to be maintenence-free for 30-40 years. The cost to the farmer is an affordable 1,600 yuan (about C$300). Today anyone who wants one just needs to call the special office, and a technician will be on the way; the farmer purchases the materials and hires the labour him/herself.
"We are building the biogas units to develop ecological farming," Mr. Zhou said. Also, farmers that can cleanly handle their pig effluent can then acquire more pigs. And since electricity is expensive, costing approximately C$0.16 per kilowatt hour, a biogas unit can save money. The farmer can then plant more lichee fruit trees.
Mr. Zhou proudly exclaimed, "First, we can solve the energy problem, and second we can reduce the damage to the forest. And then we will have good, ecological recycling ... Of course I am happy with the success of the project. I am serving the people."
Imagine the day when visitors to your home contribute fuel for cooking and lighting, just by using your loo. Imagine turning sewage waste into resource in our western cities and towns. Shit - even the alchemists would be impressed.
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Biogas in Dhanawas
Dhanawas village is about 50 km Southwest of Delhi. The village
situated in the Farrukhnagar block, is about 15 km from the
district headquarters Gurgaon, in the State of Haryana. There
are about 181 households with a total population of 1056. Tata
Energy Research Institute (TERI) has been working in the village
since 1984 and a number of technology interventions including
biogas, improved chulhas, solar lanterns, solar street lights,
biomass gasifier, biomass briquettes, etc. have been attempted.
Among all the interventions made in the village, biogas plant
has emerged as a most successful and infact, a competing
technology to LPG (growth of biogas plant was twice than that of
LPG during the last decade).
Time saving is the factor which scored highest and from the
feedback it was found that, the time saved by the use of biogas
for cooking, was spent in attending to other domestic chores or
The second ranking of institutional issues
reflected the confidence the people had with the implementing
agency. This confidence may be due to their observation of the
implementing agency on other activities or functionality of
installed biogas plants.
Smoke removal was perceived as one
important benefit ranked third which besides resulting in
improved health, reduced the cleaning of walls and whitewashing
Since, the biogas meet some large parts of the cooking in
the households, use of biomass fuels including dung cake has
reduced, though, not eliminated completely. Thus, it has become
possible to divert a large proportion of dung as fertilizer in
the fields. This alternate use of dung as fertilizer ranked
fourth among the factors.
The fifth ranking factor was a group
of other reasons where convenience of operation of biogas plants
over types of stoves and less or no maintenance problem along
with its relatively safer operation was acknowledged by the
Fuel saving that the researchers, policy makers, and
implementing agencies have paid maximum attention, did not score
well and ranked sixth according to the users.
The biogas light generally used as a standby in case of power failure is another perceived benefit by the villagers and as ranked seventh.
The subsidy involved, by the government (33%) and an additional
subsidy by TERI (another 33%) was identified as eighth important
factor, showing that it was not a limiting criterion. Less
effort in cleaning the vessels and kitchen and the plant as a
status symbol scored last in the ranking of factors.
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Wednesday, October 06, 2004
A system approach to biogas technology
A system approach to biogas technology
from "Biogas technology: a training manual for extension" (FAO/CMS, 1996)
Components of a biogas system
Biogas technology is a complete system in itself with its set objectives (cost effective production of energy and soil nutrients), factors such as microbes, plant design, construction materials, climate, chemical and microbial characteristics of inputs, and the inter-relationships among these factors. Brief discussions on each of these factors or subsystems are presented in this section.
This is the mixture of gas produced by methanogenic bacteria while acting upon biodegradable materials in an anaerobic condition. Biogas is mainly composed of 50 to 70 percent methane, 30 to 40 percent carbon dioxide (CO2) and low amount of other gases as shown in Table 1.
Table 1: Composition of biogas Substances Symbol Percentage
Methane CH4 50 - 70
Carbon Dioxide CO2 30 - 40
Hydrogen H2 5 - 10
Nitrogen N2 1 - 2
Water vapour H2O 0.3
Hydrogen Sulphide H2S Traces
Source: Yadav and Hesse
Biogas is about 20 percent lighter than air and has an ignition temperature in the range of 650 degrees to 750 degrees C. It is an odourless and colourless gas that burns with clear blue flame similar to that of LPG gas (Sathianathan, 1975). Its calorific value is 20 Mega Joules (MJ) per m3 and burns with 60 percent efficiency in a conventional biogas stove.
Methanogenic bacteria or methanogens
These are the bacteria that act upon organic materials and produce methane and other gases in the process of completing their life-cycle in an anaerobic condition. As living organisms, they tend to prefer certain conditions and are sensitive to micro-climate within the digester. There are many species of methanogens and their characteristics vary.
The different methane forming bacteria have many physiological properties in common, but they are heterogeneous in cellular morphology. Some are rods, some cocci, while others occur in clusters of cocci known as sarcine. The family of methanogens (Methanobacteriacea) is divided into following four genera on the basis of cytological differences (Alexander, 1961):
* A. Rod-shaped Bacteria
o (a) Non-sporulating, Methanobacterium
o (b) Sporulating, Methanobacillus
* B. Spherical
o (a) Sarcinae, Methanosarcina
o (b) Not in sarcinal groups, Methanococcus
A considerable level of scientific knowledge and skill is required to isolate methanogenic bacteria in pure culture and maintain them in a laboratory. Methanogenic bacteria develop slowly and are sensitive to a sudden change in physical and chemical conditions. For example, a sudden fall in the slurry temperature by even 2o C may significantly affect their growth and gas production rate (Lagrange, 1979).
The biodigester is a physical structure, commonly known as the biogas plant. Since various chemical and microbiological reactions take place in the biodigester, it is also known as bio-reactor or anaerobic reactor. The main function of this structure is to provide anaerobic condition within it. As a chamber, it should be air and water tight. It can be made of various construction materials and in different shape and size. Construction of this structure forms a major part of the investment cost. Some of the commonly used designs are discussed below.
Floating drum digester. Experiment on biogas technology in India began in 1937. In 1956, Jashu Bhai J Patel developed a design of floating drum biogas plant popularly known as Gobar Gas plant. In 1962, Patel's design was approved by the Khadi and Village Industries Commission (KVIC) of India and this design soon became popular in India and the world. In this design, the digester chamber is made of brick masonry in cement mortar. A mild steel drum is placed on top of the digester to collect the biogas produced from the digester. Thus, there are two separate structures for gas production and collection. With the introduction of fixed dome Chinese model plant, the floating drum plants became obsolete because of comparatively high investment and maintenance cost along with other design weaknesses. In Nepal, KVIC design plants have not been constructed since 1986.
Fixed dome digester. Fixed dome Chinese model biogas plant (also called drumless digester) was built in China as early as 1936. It consists of an underground brick masonry compartment (fermentation chamber) with a dome on the top for gas storage. In this design, the fermentation chamber and gas holder are combined as one unit. This design eliminates the use of costlier mild steel gas holder which is susceptible to corrosion. The life of fixed dome type plant is longer (from 20 to 50 years) compared to KVIC plant. Based on the principles of fixed dome model from China, Gobar Gas and Agricultural Equipment Development Company (GGC) of Nepal has developed a design and has been popularizing it since the last 17 years. The concrete dome is the main characteristic of GGC design.
Deenbandhu model. In an effort to further bring down the investment cost, Deenbandhu model was put forth in 1984 by the Action for Food Production (AFPRO), New Delhi. In India, this model proved 30 percent cheaper than Janata Model (also developed in India) which is the first fixed dome plant based on Chinese technology. It also proved to be about 45 percent cheaper than a KVIC plant of comparable size. Deenbandhu plants are made entirely of brick masonry work with a spherical shaped gas holder at the top and a concave bottom. The South Asian Partnership/Nepal (SAP/N), an INGO working in Nepal, has introduced Deenbandhu model plants in Bardiya district of Nepal. About 100 plants were constructed by SAP/N in the villages of Bardiya district in 1994. Preliminary studies carried out by BSP did not find any significant difference in the investment costs of GGC and the Deenbandhu design plants. Recently, Environmental Protection and Social Development Association (EPA), a NGO, has constructed modified Deenbandhu design plants in Bardiya district which is also approved by Biogas Support Programme (BSP). In addition to above designs developed particularly for household use in developing countries, there are other designs suitable for adoption in other specific conditions. Though they are not of much relevance to present conditions in Nepal, they could prove useful in the future. These designs are briefly described below for reference.
Bag digester. This design was developed in 1960s in Taiwan. It consists of a long cylinder made of PVC or red mud plastic. The bag digester was developed to solve the problems experienced with brick and metal digesters. A PVC bag digester was also tested in Nepal by GGC at Butwal from April to June 1986. The study concluded that the plastic bag biodigester could be successful only if PVC bag is easily available, pressure inside the digester is increased and welding facilities are easily available (Biogas Newsletter, No. 23, 1986). Such conditions are difficult to meet in most of the rural areas in developing countries.
Plug flow digester. The plug flow digester is similar to the bag digester. It consists of a trench (trench length has to be considerably greater than the width and depth) lined with concrete or an impermeable membrane. The reactor is covered with either a flexible cover gas holder anchored to the ground, concrete or galvanized iron (GI) top. The first documented use of this type of design was in South Africa in 1957.
Anaerobic filter. This type of digester was developed in the 1950's to use relatively dilute and soluble waste water with low level of suspended solids. It is one of the earliest and simplest type of design developed to reduce the reactor volume. It consists of a column filled with a packing medium. A great variety of non-biodegradable materials have been used as packing media for anaerobic filter reactors such as stones, plastic, coral, mussel shells, reeds, and bamboo rings. The methane forming bacteria form a film on the large surface of the packing medium and are not carried out of the digester with the effluent. For this reason, these reactors are also known as "fixed film" or "retained film" digesters (Bioenergy Systems Report, 1984).
Upflow anaerobic sludge blanket. This UASB design was developed in 1980 in the Netherlands. It is similar to the anaerobic filter in that it involves a high concentration of immobilized bacteria in the reactor. However, the UASB reactors contain no packing medium, instead, the methane forming bacteria are concentrated in the dense granules of sludge blanket which covers the lower part of the reactor. The feed liquid enters from the bottom of the reactor and biogas is produced while liquid flows up through the sludge blanket. Many full-scale UASB plants are in operation in Europe using waste water from sugar beet processing and other dilute wastes that contain mainly soluble carbohydrates (Bioenergy Systems Report, 1984). Such reactor has not been experimented in Nepal. There are also other designs of anaerobic reactors which are of less interest in the context of Nepal due to their limited utility. Reduction in investment cost using alternative construction materials has been one of the main driving forces in the development of new designs. In an effort to achieve this objective, use of bamboo, plastics and other such cheap construction materials have also been tried with varying degree of success (Cortsen, Lassen and Neilsen, 1995; Beteta, 1995). However, all such reported success stories are yet to take the form of implementation programmes in a mass scale.
The main factors that influence the selection of a particular design or model of a biogas plant are as follows:
Economic. An ideal plant should be as low-cost as possible (in terms of the production cost per unit volume of biogas) both to the user as well as to the society. At present, with subsidy, the cost of a plant to the society is higher than to an individual user.
Simple design. The design should be simple not only for construction but also for operation and maintenance (O&M). This is an important consideration especially in a country like Nepal where the rate of literacy is low and the availability of skilled human resource is scarce.
Utilization of local materials. Use of easily available local materials should be emphasized in the construction of a biogas plant. This is an important consideration, particularly in the context of Nepal where transportation system is not yet adequately developed.
Durability. Construction of a biogas plant requires certain degree of specialized skill which may not be easily available. A plant of short life could also be cost effective but such a plant may not be reconstructed once its useful life ends. Especially in situation where people are yet to be motivated for the adoption of this technology and the necessary skill and materials are not readily available, it is necessary to construct plants that are more durable although this may require a higher initial investment.
Suitable for the type of inputs. The design should be compatible with the type of inputs that would be used. If plant materials such as rice straw, maize straw or similar agricultural wastes are to be used, then the batch feeding design or discontinuous system should be used instead of a design for continuous or semi-continuous feeding.
Frequency of using inputs and outputs. Selection of a particular design and size of its various components also depend on how frequently the user can feed the system and utilize the gas.
Inputs and their characteristics
Any biodegradable organic material can be used as inputs for processing inside the biodigester. However, for economic and technical reasons, some materials are more preferred as inputs than others. If the inputs are costly or have to be purchased, then the economic benefits of outputs such as gas and slurry will become low. Also, if easily available biodegradable wastes are used as inputs, then the benefits could be of two folds: (a) economic value of biogas and its slurry; and (b) environmental cost avoided in dealing with the biodegradable waste in some other ways such as disposal in landfill.
One of the main attractions of biogas technology is its ability to generate biogas out of organic wastes that are abundant and freely available. In case of Nepal, it is the cattle dung that is most commonly used as an input mainly because of its availability. The potential gas production from some animal dung is given in Table 2.
Table 2. Gas Production potential of various types of dung Types of Dung Gas Production Per Kg Dung (m3)
Cattle (cows and buffaloes) 0.023 - 0.040
Pig 0.040 - 0.059
Poultry (Chickens) 0.065 - 0.116
Human 0.020 - 0.028
Source: Updated Guidebook on Biogas Development, 1984
In addition to the animal and human wastes, plant materials can also be used to produce biogas and bio-manure. For example, one kg of pre-treated crop waste and water hyacinth have the potential of producing 0.037 and 0.045 m3 of biogas, respectively. Since different organic materials have different bio-chemical characteristics, their potential for gas production also varies. Two or more of such materials can be used together provided that some basic requirements for gas production or for normal growth of methanogens are met. Some characteristics of these inputs which have significant impact on the level of gas production are described below.
C/N Ratio. The relationship between the amount of carbon and nitrogen present in organic materials is expressed in terms of the Carbon/Nitrogen (C/N) ratio. A C/N ratio ranging from 20 to 30 is considered optimum for anaerobic digestion. If the C/N ratio is very high, the nitrogen will be consumed rapidly by methanogens for meeting their protein requirements and will no longer react on the left over carbon content of the material. As a result, gas production will be low. On the other hand, if the C/N ratio is very low, nitrogen will be liberated and accumulated in the form of ammonia (NH4). NH4 will increase the pH value of the content in the digester. A pH higher than 8.5 will start showing toxic effect on methanogen population.
Animal waste, particularly cattle dung, has an average C/N ratio of about 24. The plant materials such as straw and sawdust contain a higher percentage of carbon. The human excreta has a C/N ratio as low as 8. C/N ratio of some of the commonly used materials are presented in Table 3 (Karki and Dixit, 1984).
Table 3. C/N Ratio of some organic materials Raw Materials C/N Ratio
Duck dung 8
Human excreta 8
Chicken dung 10
Goat dung 12
Pig dung 18
Sheep dung 19
Cow dung/ Buffalo dung 24
Water hyacinth 25
Elephant dung 43
Straw (maize) 60
Straw (rice) 70
Straw (wheat) 90
Saw dust above 200
Materials with high C/N ratio could be mixed with those of low C/N ratio to bring the average ratio of the composite input to a desirable level. In China, as a means to balance C/N ratio, it is customary to load rice straw at the bottom of the digester upon which latrine waste is discharged. Similarly, at Machan Wildlife Resort located in Chitawan district of Nepal, feeding the digester with elephant dung in conjunction with human waste enabled to balance C/N ratio for smooth production of biogas (Karki, Gautam and Karki, 1994).
Dilution and consistency of inputs. Before feeding the digester, the excreta, especially fresh cattle dung, has to be mixed with water at the ratio of 1:1 on a unit volume basis (i.e. same volume of water for a given volume of dung). However, if the dung is in dry form, the quantity of water has to be increased accordingly to arrive at the desired consistency of the inputs (e.g. ratio could vary from 1:1.25 to even 1:2). The dilution should be made to maintain the total solids from 7 to 10 percent. If the dung is too diluted, the solid particles will settle down into the digester and if it is too thick, the particles impede the flow of gas formed at the lower part of digester. In both cases, gas production will be less than optimum. A survey made by BSP reveals that the farmers often over dilute the slurry.
For thorough mixing of the cow dung and water (slurry), GGC has devised a Slurry Mixture Machine that can be fitted in the inlet of a digester. It is also necessary to remove inert materials such as stones from the inlet before feeding the slurry into the digester. Otherwise, the effective volume of the digester will decrease.
Volatile solids. The weight of organic solids burned off when heated to about 538 degrees C is defined as volatile solids. The biogas production potential of different organic materials, given in Table 2, can also be calculated on the basis of their volatile solid content. The higher the volatile solid content in a unit volume of fresh dung, the higher the gas production. For example, a kg of volatile solids in cow dung would yield about 0.25 m3 biogas (Sathianathan, 1975).
Digestion refers to various reactions and interactions that take place among the methanogens, non-methanogens and substrates fed into the digester as inputs. This is a complex physio-chemical and biological process involving different factors and stages of change. This process of digestion (methanization) is summarized below in its simple form. The breaking down of inputs that are complex organic materials is achieved through three stages as described below:
Stage 1: Hydrolysis. The waste materials of plant and animal origins consist mainly of carbohydrates, lipids, proteins and inorganic materials. Large molecular complex substances are solubilized into simpler ones with the help of extracellular enzyme released by the bacteria. This stage is also known as polymer breakdown stage. For example, the cellulose consisting of polymerized glucose is broken down to dimeric, and then to monomeric sugar molecules (glucose) by cellulolytic bacteria.
Stage 2: Acidification:. The monomer such as glucose which is produced in Stage 1 is fermented under anaerobic condition into various acids with the help of enzymes produced by the acid forming bacteria. At this stage, the acid-forming bacteria break down molecules of six atoms of carbon (glucose) into molecules of less atoms of carbon (acids) which are in a more reduced state than glucose. The principal acids produced in this process are acetic acid, propionic acid, butyric acid and ethanol.
Stage 3: Methanization:. The principle acids produced in Stage 2 are processed by methanogenic bacteria to produce methane. The reactions that takes place in the process of methane production is called Methanization and is expressed by the following equations (Karki and Dixit, 1984).
Acetic acid --> CH4
Methane + CO2
Ethanol + CO2
Carbon dioxide --> CH4
Methane + 2CH3COOH
Carbon dioxide + 4H2
Hydrogen --> CH4
Methane + 2H2O
The above equations show that many products, by-products and intermediate products are produced in the process of digestion of inputs in an anaerobic condition before the final product (methane) is produced. Obviously, there are many facilitating and inhibiting factors that play their role in the process. Some of these factors are discussed below.
pH value. The optimum biogas production is achieved when the pH value of input mixture in the digester is between 6 and 7. The pH in a biogas digester is also a function of the retention time. In the initial period of fermentation, as large amounts of organic acids are produced by acid forming bacteria, the pH inside the digester can decrease to below 5. This inhibits or even stops the digestion or fermentation process. Methanogenic bacteria are very sensitive to pH and do not thrive below a value of 6.5. Later, as the digestion process continues, concentration of NH4 increases due to digestion of nitrogen which can increase the pH value to above 8. When the methane production level is stabilized, the pH range remains buffered between 7.2 to 8.2.
Temperature. The methanogens are inactive in extreme high and low temperatures. The optimum temperature is 35 degrees C. When the ambient temperature goes down to 10 degrees C, gas production virtually stops. Satisfactory gas production takes place in the mesophilic range, between 25 degrees to 30 degrees C. Proper insulation of digester helps to increase gas production in the cold season. When the ambient temperature is 30 degrees C or less, the average temperature within the dome remains about 4 degrees C above the ambient temperature (Lund, Andersen and Torry-Smith, 1996).
Loading rate. Loading rate is the amount of raw materials fed per unit volume of digester capacity per day. In Nepalese conditions, about 6 kg of dung per m3 volume of digester is recommended in case of a cow dung plant (BSP, 1992). If the plant is overfed, acids will accumulate and methane production will be inhibited. Similarly, if the plant is underfed, the gas production will also be low.
Retention time. Retention time (also known as detention time) is the average period that a given quantity of input remains in the digester to be acted upon by the methanogens. In a cow dung plant, the retention time is calculated by dividing the total volume of the digester by the volume of inputs added daily. Considering the climatic conditions of Nepal, a retention time of 50 to 60 days seems desirable. Thus, a digester should have a volume of 50 to 60 times the slurry added daily. But for a night soil biogas digester, a longer retention time (70-80 days) is needed so that the pathogens present in human faeces are destroyed. The retention time is also dependent on the temperature and upto 35 degrees C, the higher the temperature, the lower the retention time (Lagrange, 1979).
Toxicity. Mineral ions, heavy metals and the detergents are some of the toxic materials that inhibit the normal growth of pathogens in the digester. Small quantity of mineral ions (e.g. sodium, potassium, calcium, magnesium, ammonium and sulphur) also stimulates the growth of bacteria, while very heavy concentration of these ions will have toxic effect. For example, presence of NH4 from 50 to 200 mg/l stimulates the growth of microbes, whereas its concentration above 1,500 mg/l produces toxicity. Similarly, heavy metals such as copper, nickel, chromium, zinc, lead, etc. in small quantities are essential for the growth of bacteria but their higher concentration has toxic effects. Likewise, detergents including soap, antibiotics, organic solvents, etc. inhibit the activities of methane producing bacteria and addition of these substances in the digester should be avoided. Although there is a long list of the substances that produce toxicity on bacterial growth, the inhibiting levels of some of the major ones are given in Table 4.
Table 4: Toxic level of various inhibitors Inhibitors Inhibiting Concentration
Sulphate (SO4- - ) 5,000 ppm
Sodium Chloride or Common salt (NaCl) 40,000 ppm
Nitrate (Calculated as N) 0.05 mg/ml
Copper (Cu++ ) 100 mg/l
Chromium (Cr+++ ) 200 mg/l
Nickel (Ni+++ ) 200 - 500 mg/l
Sodium (Na+ ) 3,500 - 5,500 mg/l
Potassium (K+ ) 2,500 - 4,500 mg/l
Calcium (Ca++ ) 2,500 - 4,500 mg/l
Magnesium (Mg++ ) 1,000 - 1,500 mg/l
Manganese (Mn++ ) Above 1,500 mg/l
Source: The Biogas Technology in China, BRTC, China (1989)
This is the residue of inputs that comes out from the outlet after the substrate is acted upon by the methonogenic bacteria in an anaerobic condition inside the digester. After extraction of biogas (energy), the slurry (also known as effluent) comes out of digester as by-product of the anaerobic digestion system. It is an almost pathogen-free stabilized manure that can be used to maintain soil fertility and enhance crop production. Slurry is found in different forms inside the digester as mentioned below:
- a light rather solid fraction, mainly fibrous material, which float on the top forming the scum; - a very liquid and watery fraction remaining in the middle layer of the digester; - a viscous fraction below which is the real slurry or sludge; and - heavy solids, mainly sand and soils that deposit at the bottom.
There is less separation in the slurry if the feed materials are homogenous. Appropriate ratio of urine, water and excrement and intensive mixing before feeding the digester leads to homogeneous slurry.
Use of biogas
Of the outputs of biogas, the gas is valued for its use as a source of energy and the slurry for its fertilizing properties (soil nutrients). Energy content of biogas can also be transformed into various other forms such as mechanical energy (for running machines) and heat energy (for cooking and lighting) depending on the need and availability of the technology. Some of the common uses of biogas are : cooking, lighting, refrigeration and running internal combustion engine.
Implications of biogas system
Biogas technology is best suited to convert the organic waste from agriculture, livestock, industries, municipalities and other human activities into energy and manure. The use of energy and manure can lead to better environment, health, and other socio-economic gains is shown in the Chart below.
Alexander, M. (1961) "Introduction to Soil Microbiology". John Wiley & Sons, Inc. pp 227-231.
Beteta, T. (1995) "Experiences with Plastic Tube Biodigesters in Colombia". Universidad Nacional Agraria, Managua, Nicaragua.
Bioenergy Systems Report : "Innovations in Biogas Systems and Technology" (1984). Bio-energy Systems and Technology Project of the USAID.
Biogas and Natural Resources Management (BNRM) Newsletter. Issue No 1 to 51, 1978-1996.
Cortsen, L., M. Lassen and H. K. Nielsen (1995) "Small Scale Biogas Digesters in Turiani, Nronga and Amani, Tanzania". University of Aarhus, Denmark.
Gunnerson, C. G. and D. V. Stuckey (1986) "Integrated Resource Recovery-Anaerobic Digestion-Principles and Practices for Biogas systems". World Bank Technical Paper No. 49.
Karki, A. B. and K. Dixit (1984) "Biogas Fieldbook". Sahayogi Press, Kathmandu, Nepal.
Karki, A. B., K. M. Gautam and A. Karki (1994) "Biogas Installation from Elephant Dung at Machan Wildlife Resort, Chitwan, Nepal". Biogas Newsletter, Issue No. 45.
Lagrange, B. (1979) "Biomethane 2: Principles - Techniques Utilization". EDISUD, La Calade, 13100 Aix-en-Provence, France.
Lund, M. S., S. S. Andersen and M. Torry-Smith (1996) "Building of a Flexibility Bag Biogas Digester in Tanzania". Student Report. Technical University of Denmark, Copenhagen.
Ni Ji-Qin and E. J. Nyns (1993) "Biomethanization : A Developing Technology in Latin America". Catholic University of Louvain, Belgium. pp 67-68.
Optner, S. L. (1997) "System Analysis". Penguin Books Ltd, Harmondsworth, Middlesex, England. Sathianathan, M. A. (1975) "Biogas Achievements and Challenges". Association of Voluntary Agencies of Rural Development, New Delhi, India.
Singh, J. B, R. Myles and A. Dhussa (1987) "Manual on Deenbandhu Biogas Plant". Tata McGraw Hill Publishing Company Limited, India.
The Biogas Technology in China (1989) Chengdu Biogas Research Institute, Chengdu, China.
Updated Guidebook on Biogas Development-Energy Resources Development Series (1984), No. 27. United Nations. New York, USA.
Yadava, L. S. and P. R. Hesse (1981) "The Development and Use of Biogas Technology in Rural Areas of Asia" (A Status Report 1981). Improving Soil Fertility through Organic Recycling, FAO/UNDP Regional Project RAS/75/004, Project Field Document No. 10.
FAO, Kathmandu (1996) "Report on the Meeting for the Development of a National Biogas Policy Framework and Celebration of the 10,000th Biogas Plant Construction with BSP Support". FAO, Kathmandu, 7 February 1996.
Gautam, K. M. (1996) Country Paper on Biogas in Nepal. Paper presented at International Conference on Biomass Energy Systems organized by Tata Energy Research Institute, British Council Division and British High Commission. New Delhi, India. 26-27 February 1996.
Karki, A. B. (1985) "Construction of Biodigesters at Farmers' Level". FAO Office for Latin America and the Caribbean, Santiago, Chile.
Karki, A. B. (1984) "Biogas Training Course for Rural Development Workers of Kingdom of Lesotho". FAO, Rome, Italy.
Karki, A. B., K. M. Gautam and A. Karki (1994) "Biogas for Sustainable Development in Nepal". Paper presented at Second International Conference on Science and Technology for Poverty Alleviation organized by Royal Nepal Academy for Science and Technology (RONAST), Kathmandu, Nepal. 8-11 June 1994.
The Eighth Five Year Plan 1992-1997. National Planning Commission, Nepal
UNDP/ESCAP/FAO/CHINA (1983) "Biogas Technology". The Asian-Pacific Regional Biogas Research and Training Centre, Chengdu, China.
United Mission to Nepal (1985) "Biogas : Challenges and Experience from Nepal". Vol. I.
United Mission to Nepal (1985) "Biogas : Challenges and Experience from Nepal".
Benefits of Biogas
Technologies » Biogas
Renewables in India
Policies of the Government
Ongoing projects/ programmes
Benefits of biogas
Biogas technology makes optimal utilization
of the valuable natural resource of dung;
it provides nearly three times more
useful energy that dung directly burnt,
and also produces
As a cooking fuel, it is cheap and
extremely convenient. Based on the
effective heat produced, a 2cu m biogas
plant could replace, in a month, fuel
equivalent of 26 kg if LPG (nearly
two standard cylinders), or 37 litres
of kerosene, or 88kg of charcoal, or 210 kg
of fuelwood, or 740 kg of animal dung.
In terms of cost, biogas is cheaper, on a life cycle basis, than conventional biomass fuels (dung, fuelwood, crop wastes, etc.) as well as LPG, and is only fractionally more expensive than kerosene; the commercial fuels like kerosene and LPG, however, have severe supply constraints in the rural areas.
Benefits of biogas
To the housewife, a biogas is easy to use and saves time in the kitchen; biogas stove has an efficiency of about 55% which is comparable to that of an LPG stove. Cooking on biogas is free from smoke and soot, and can substantially reduce the health problems, which are otherwise quite common in most rural areas in India where biomass is the chief source of fuel . The use of biogas is helpful to improve the quality of life in household.
However, the use of biogas is by no means confined to cooking alone. It can be used, through a specially designed mantle, for lightning, too. Further, biogas can partially replace diesel to run IC (internal combustion) engines for water pumping; small industries like floor mill, saw mill, oil mill etc. This would not only reduce dependence on diesel, but also help in reducing carbon pollutants which adversely affect the atmosphere. Dual – fuel engines (80% biogas and 20% diesel) are now commercially manufactured in India. Biogas can be similarly used to produce electricity, though this has not been attempted on a large scale in the country so far. Nevertheless, the versatility of biogas is its greatest advantage as a source of energy for the rural areas.
While biogas has multiple benefits at the individual family level, it also has several qualitative and quantitative benefits at the societal level. Firstly, a shift to biogas from traditional biomass fuels results in less dependence on natural resources such as forests, in less dependence on natural resources such as forests, checking their indiscriminate and unsustainable exploitation. Since dung is collected systematically when used in biogas, environment can be kept clean and hygienic.
The other advantage is that, unlike centralized systems such as thermal power plants and fertilizer factories, which entail huge capital investments and need elaborate distribution networks, biogas plants are decentralized systems which can be installed even in remote areas with very low investments.
Potential of biogas
In India, the dissemination of large–scale biogas plants has began in the mid-seventies and the process has become consolidated with the advent of the National Project on Biogas Development (NPBD) in 1981, which has been continuing since. Against the estimated potential of 12 million biogas plants, 2.9 million family type and 2700 community, institutional and nightsoil-based plants have been set up till December 1999. This is estimated to have helped in a saving of 3 million tonnes of fuelwood per year and manure containing nitrogen equivalent to 0.7 million tonnes of urea.
However, in terms of total dung that is available in the country, the potential is much more. The bovine population in India is 260 millions. As adult bovine produces an average of 10 kg of dung per day. Since grazing is a common practice in India, all the dung produced cannot be collected. If it is assumed that 75% of the dung is collected, nearly 2 millions tonnes of dung would be available everyday. At 25 kg per one cubic metre, this dung can feed as many as 40 millions biogas plants of 2 cubic metre capacity, which can be considered the ultimate potential for biogas technology.
But even this high potential of biogas is based on animal dung only. However, all organic matter can technically be used to generate methane; if the scientific experiments that are going on in the country under the patronage of MNES to develop alternative feedstocks (such as water hyacinth, kitchen waste, and poultry waste) come to fruition, potential for biogas generation could be virtually unlimited. It can be mentioned in this context that human waste is an excellent source of biogas which would enhance the potential; substantially. With such high potential, which can be routed to hitherto unemphasized applications of shaft power and electricity generation, biogas can make a significant contribution to the development of small industries and agriculture, and thus to the overall advancement of the rural areas.
Sqb.gif (830 bytes)Biogas plant models
Biogas: a source of rural employment.
Bio Gas works
Around the world, pollution of the air and water from municipal, industrial and agricultural operations continues to grow. Governments and industries are constantly on the lookout for technologies that will allow for more efficient and cost-effective waste treatment. One technology that can successfully treat the organic fraction of wastes is anaerobic digestion (AD). When used in a fully-engineered system, AD not only provides pollution prevention, but also allows for sustainable energy, compost and nutrient recovery. Thus, AD can convert a disposal problem into a profit center. As the technology continues to mature, AD is becoming a key method for both waste reduction and recovery of a renewable fuel and other valuable co-products.
This web site provides an outline of the status of AD as the most promising method of treating the organic fraction of animal manure, municipal solid waste (MSW), and other wastes. It also summarizes policy issues which influence the deployment of AD technology, facility design concepts, the energy, economic and environmental issues relating to AD, and the comparison of alternative treatment processes.
For more click here - www.biogasworks.com
India's low-tech energy success.
World Watch, Nov-Dec 1995 v8 n6 p21(3)
India's low-tech energy success.
Abstract: The Indian government introduced large-scale biogas production in 1981 through its National Project on Biogas Development. Having been in use in India for close to a century, biogas is produced by extracting chemical energy from organic materials in a sealed container called a digester. A mixture of methane and nutrient-rich slurry is then produced. Methane serves as the combustible component of biogas while slurry is a valued fertilizer. Meanwhile, the Indian government must push biogas production by redesigning rural energy subsidies and awarding the planning to the local level.
Full Text: COPYRIGHT Worldwatch Institute 1995
How 2 million power plants are turning cow dung into electric power and cooking fuel - and ending up with even better fertilizer than manure.
Cow dung may not be the first thing that comes to mind when you think of state-of-the-art energy technology. Yet in the tiny village of Pura in south India, this humble waste material is providing people with basic amenities formerly in short supply: electric light, pumped water, and clean cooking fuel. An ingeniously simple process that converts dung into a flammable gas, called biogas, has greatly improved daily life for Pura's 485 inhabitants - and for over 10 million other rural Indians.
Biogas, as its name suggests, is produced by extracting chemical energy from organic materials. This process takes place in a sealed container known as a biogas digester. The digester is usually a squat, cement cylinder two to four meters in diameter, with a duct in the side that allows the dung or other organic wastes to be fed in, along with water. In ambient temperatures of 25 to 35 degrees centigrade, the material soon begins to ferment. This produces a mixture of gases, primarily methane and carbon dioxide, and a nutrient-rich slurry. The gas is drawn out through a valve at the top of the digester, and the slurry is drained off into settling troughs at its base. .
Methane is the combustible component of biogas. It is piped into homes to be used as a cooking fuel, or used to fire a diesel engine to generate electricity, as in the case of Pura. The slurry is such an excellent fertilizer that it's often more highly valued than the gas - biogas plants are often called "biofertilizer plants."
Biogas dates as far back as the 16th century, when it was used for heating bath-water in Persia, and it has been used in India for almost a hundred years. In 1981, however, the Indian government launched biogas production on a large scale, by embarking on a "National Project on Biogas Development." Close to 2 million digesters have been constructed in India since then, and although the program has had its share of problems, it has made substantial progress. Most plants are located in rural, agrarian parts of the country and are designed to serve the cooking needs of a single household of four to seven people. Over 1,150 large "community" plants - like the one in Pura - have also been installed. These are operated by an entire village, and the cooking fuel or electricity is shared by the community.
Ostensibly 84 percent of Indian villages are connected to the electrical grid, but only 27 percent of their inhabitants actually had access to power in 1991, according to R.K. Pachauri of the Tata Energy Research Institute in New Delhi. That means 435 million people, more than half of India's population, lack electricity. And 80 percent of rural India faces difficulties in obtaining sufficient cooking fuel. Biogas bypasses these shortages and transmission problems by providing a decentralized and locally-controlled fuel supply from a readily available material.
Generating biogas also makes sense in the Indian cultural context. All products of the cow, including dung (or "gobar" in Hindi) are considered purifying agents by Hindus, according to O.P. Joshi, a sociologist at the H.C. Mathur Institute of Public Administration in Rajasthan. In the classical Indian epic, the Mahabharata, says Joshi, "gobar is described as the living place of Lakshmi, the goddess of wealth." Traditionally in India, dung is collected and fashioned into dung-cakes, to be burned directly as fuel or composted for fertilizer. Dung accounts for over 21 percent of total rural energy use in India, and as much as 40 percent in certain states.
Usually, dung used for one purpose is lost to the other, but biogas provides a means to both ends. It exploits the caloric content of the waste, while retaining the nutrients as fertilizer - and on both counts, it is more efficient than traditional methods. Direct burning only captures about 11 percent of the dung's energy value, but biogas generation has a 45 to 60 percent efficiency. In other words, biogas captures approximately 5 times as much energy as does direct burning. And the by-product slurry has twice the nitrogen content of composted dung because open-air composting allows much of the nitrogen to escape in the form of volatile compounds. The slurry also releases its nutrients more readily than composted dung. And unlike decomposing dung, it is odorless and does not attract flies or mosquitoes. Farmers in Pura say it actually repels termites, and inhibits weed growth.
In addition to the slurry's nutrient recycling function, the gas itself has important environmental benefits. It offers an ecologically sustainable alternative to fuel wood, which currently provides over half of India's rural household energy. Biogas can help check deforestation; in the 1980s, for instance, when biogas technology was introduced into villages near the Gir Lion Sanctuary in Gujarat, woodcutting within the Sanctuary dropped substantially. And since the conversion process in the digester is anaerobic (it occurs in the absence of oxygen), it destroys most of the pathogens present in dung and waste, thereby reducing the potential for infections like dysentery and enteritis.
The burning of traditional fuels like dung cakes or wood releases high levels of carbon monoxide, suspended particulates, hydrocarbons, and often, contaminants like sulfur oxides. (Dung contains traces of hydrogen sulfide, which is converted to sulfur oxides on combustion.) Exposure to these fumes in unvented cooking spaces increases the risk of respiratory disease. According to a study sponsored by the World Health Organization, Indian women cooking over firewood were inhaling as much of the carcinogen benzopyrene - a combustion by-product of wood - as they would by smoking 20 packs of cigarettes a day. Because it is a gas, biogas burns much more efficiently than these solid fuels. It leaves very few contaminants, although it is true that biogas releases small quantities of sulfur oxides. Biogas offers perhaps the most environmentally benign method for tapping the solar energy stored in bio-mass. It's a renewable and decentralized alternative to the other methane-based fuel, natural gas, which is commonly used in cities.
THE ECONOMICS OF SELF-SUFFICIENCY
But biogas is more than just a renewable energy technology. As a comprehensive rural development tool, it allows villages to meet fundamental needs using local resources. It is a labor-intensive technology, and therefore a significant source of employment, especially for village women who collect the dung and sell the slurry, and for the rural laborers who construct and maintain the plants. Some family biogas digesters even support small-scale enterprises, by providing electricity for agricultural and cottage industries. Community digesters encourage collective responsibility and local participation in decision-making. The role that women play in the operation enhances their social standing. Biogas also helps ease the traditional burdens of women and girls, by reducing the amount of time they have to spend collecting fuel-wood. And with the advent of biogas-powered pumps, it also reduces the time spent fetching water.
A biogas digester with a 2 cubic meter capacity - enough to meet the cooking needs of a family of five - costs approximately $350. The costs of inputs are minimal assuming the household has a water supply and at least five cows - the minimum necessary to supply the digester. Yet despite government subsidies that run as high as 85 percent of total costs, start-up expenses can seem formidable to farmers, whose participation in the cash economy is often limited.
FINDING A STRATEGY THAT WORKS
India's biogas plants are generally sound investments, particularly the community digesters, which can achieve greater economies of scale. Yet the biogas program has not yet taken off as some experts expected it to. According to the Tata Institute's Pachauri, only two-thirds of the installed plants are actually functioning (although official figures have placed this figure at 89 percent), and the project is now faced with major funding cuts. Given the technical simplicity and the many advantages of biogas production, what went wrong?
Biogas production is limited by environmental conditions such as the need for warm temperatures and availability of water and dung. In cooler or drought-prone regions, or in villages with insufficient cattle, projects have failed. These and other problems relating to construction and maintenance have been compounded by institutional factors that obstruct most renewable energy projects. As an essentially decentralized energy strategy, biogas has suffered from a far too centralized and top-down planning approach. Across-the-board implementation policies have led to the construction of plants in areas unsuited to biogas production; plants that suffered from technical difficulties were abandoned when project technicians failed to follow up. And although the government introduced many financial incentives such as subsidies and tax benefits to encourage biogas use, conventional fuels like diesel, kerosene and LPG are also highly subsidized in rural areas, so there is little incentive to make the switch.
TAPPING INTO THE POTENTIAL
Restructuring rural energy subsidies, and bringing the planning down to the local level, would unlock an enormous energy potential. India has more cattle than any other country - 262 million head. (It is possible that this immense herd may eventually prove unsustainable, but for the present at least, rural India needs its cows. They do much of the plowing and transporting that is done by tractors and trucks in other parts of the world.) Given the amount of dung available, it has been estimated that biogas could provide cooking fuel to 52 percent of the Indian population during the part of the year when conditions are optimum, and for 25 percent of the population during the lean season, when dung production, water supply, and temperatures are low. According to Amulya Reddy of the International Energy Initiative in Bangalore, harnessing this total potential would conserve some 130 million tons of wood. In theory, that amount of wood could yield enough liquid or gaseous fuel to power every truck, bus, and irrigation pump in the country.
Many other developing nations, primarily in Asia but also in Africa and Latin America, have implemented biogas programs. The Chinese program is the largest. In 1991, it had 5 million digesters; these use human waste as well as animal dung. The United States and several western European countries are interested in the biogas potential of other sources of organic waste, mainly municipal solid waste. It's too early to say how much power this approach will eventually yield, but over the next few years, a number of large U.S. and European biogas plants are scheduled to go online.
The goal of India's biogas program is to construct digesters for the 12 million rural Indian households that have enough cattle to maintain a regular supply of dung. Depending on family size, this would mean a regular supply of fuel for 60 to 85 million people. In a largely agrarian nation where rural electrification is limited and commercial fuels make up only 11 percent of rural energy use, biogas could go a long way toward improving the energy and environmental future. And given the broad availability of dung, crop residues, and other organic wastes, biogas could do the same elsewhere. Along with other renewable technologies like photovoltaics, biogas could help form the foundations of a decentralized energy strategy in many developing countries.
Payal Sampat was a research intern at the Worldwatch Institute and is currently studying international environmental policy at Tufts University.