Report 1 of 4: A Pat Solution - Nepal

Introduction

Biogas is a mixture of methane and carbon dioxide. It is produced by the action of bacteria on vegetable/organic material in airless conditions which is why the process is also known as ‘anaerobic digestion’. The bacteria slowly digest the material (usually animal dung, human wastes and crop residues) and produce a gas which is roughly 60% methane and 40% carbon dioxide.

Biogas is an effective fuel for cooking and lighting. It makes a meaningful contribution to the energy supply and it saves fossil energy, such as coal, oil and gas. It also makes a significant ecological contribution to heat and electricity production. It is an environmentally friendly source of energy because it produces electricity and heat but still keeps carbon dioxide emissions neutral and emits no sulphur. As fossil based fuels become scarcer and more expensive and carbon dioxide emission levels become of greater concern, the benefits and potential of biogas as a source of energy supply are being increasingly recognised.

The Biogas Support Programme in Nepal

In Nepal, there is a seemingly endless demand for fuelwood which is used mainly for cooking. The pressure on forests has reached record levels and deforestation, soil erosion and landslides occur frequently. Recognising the urgent need to find an alternative source of fuel, the Nepalese Government, the Netherlands Development Organisation and local banks set up the Biogas Support Programme to promote household biogas plants using animal dung as an available and appropriate fuel. As a result, biogas is being produced in family units for domestic purposes, such as cooking and lighting.

When it is used in appropriate burners, biogas gives a clean, smokeless blue flame that is ideal for cooking. For lighting, biogas has to be used in specially designed lamps which give a light similar to kerosene pressure lamps.

Producing biogas

Biogas is generated by continuing the process which begins when a cow eats grass. Having broken up the grass with its teeth, bacteria living in the cow’s gut assist the digestion process by breaking the food down into simpler chemicals and gases. One of these is biogas. Bacteria excreted with the cow’s dung can continue to turn the chemicals into gas if the conditions are similar to those in the cow’s gut. Light and air have to be excluded and the dung has to be kept at a warm temperature, somewhere between 20ºC and 40ºC. The conditions can be met by building a hole in the ground which should be lined with brick or stone mortar to stop the dung mixture from leaking out.

Biogas plants

In Nepal, fixed dome plants (figure 1) have been used to collect the biogas produced. A fixed dome plant consists of four basic components: the inlet, the digester, the gas holder and the outlet or expansion chamber.

The inlet, equipped with a mixing device, is used to collect the dung during the day where it is mixed with an equal amount of water. After the dung and water have been mixed together, a plug at the bottom of the inlet is removed and the mixture, called slurry, flows through a pipe into the digester.

The digester is a flat bottomed, round chamber, covered with a dome shaped concrete gas holder. The fixed dome plant needs to be sealed properly in order to prevent any gas leakage. The bacteria thrive on the dung mixture in the digester and create biogas. The gas then rises and is stored in the gas holder before being released into a pipe. The slurry leaves through the outlet or expansion chamber and flows into the compost pit.

Digestion time ranges from a couple of weeks to a couple of months depending on the feedstock and the digestion temperature. The residual slurry is removed at the outlet and can be used as a fertiliser, increasing agricultural production, especially in vegetable growing.

Outputs

When the conditions are right, one kilogram of dung produces 40 litres of biogas. At full capacity, the biogas stoves use approximately 400 litres of gas per hour but the user can reduce the size of the flame with the gas tap. Studies show that the average biogas family have been saving between 140 and 180 kilograms of firewood per month. Compost can also be obtained from human waste although it takes about 75 people to produce the same amount of dung, in kilograms, as one buffalo.

Applications for biogas

Biogas has a variety of applications. Table 1 below shows some typical applications and for one cubic metre of biogas. Small-scale biogas digesters usually provide fuel for domestic lighting and cooking.
 

Application	1m3 biogas equivalent
Lighting Cooking Fuel replacement Shaft power Electricity generation	Equal to 60 -100 watt bulb for 6 hours 3 meals for a family of 5 - 6 0.7 kg of petrol Able to run a one horse power motor for 2 hours Able to generate 1.25 kilowatt hours of electricity

Table 1 - some biogas equivalents (Source: adapted from Kristoferson 1995).

Cost and maintenance

The cost of a domestic biogas plant is between US$300 and US$400 depending on the size of the plant. In Nepal, a small government grant has helped to encourage people to invest in the plants as well as the availability to borrow money from a special credit scheme devised by the banks.

Once a biogas plant has been installed, there is little expense because it costs nothing to fuel it. It also requires very little maintenance in order to operate effectively. The by-product can be used as a fertiliser for the produce being grown in the fields. It adds to production and lowers the dependency on chemical fertilisers thus increasing savings.

Advantages of biogas plants

The digestion of animal and human waste yields several benefits:

• the production of methane for use as a fuel

• the waste is reduced to slurry which has a high nutrient content which makes an ideal fertiliser; in some cases, this fertiliser is the main product from the digester and the biogas is merely a by-product

• during the digestion process bacteria in the manure are killed which is a great benefit to environmental health

• it is an effective form of power supply which is created from natural and renewable resources.

Intermediate Technology Development Group would like to thank the Biogas Support Programme for providing the original material on biogas technology.
 

Further reading available from ITDG Development Bookshop

A Chinese Biogas Manual in the countryside: Popularising technology
Edited by Ariane van Buren.
This book uses diagrams and pictures to show how the basic design of the biogas pit can be adapted for construction in different soils, from sandstone to sheer rock, which should encourage other developing countries to embark on their own biogas programmes. Reprinted in 1997.
ITP £8.95 1997 ISBN 0903031655

Improved Biogas Units for Developing Countries
Ludwig Sasse
Vieweg £12.95 1991 ISBN 3528020636

Running a Biogas Programme: A handbook
David Fulford
Describes the designs and uses of biogas plants, with technical appendices, for domestic and community plants. Likely economic and social effects of biogas programmes are described from experience, and advice given in the problems of management.
ITP £14.95 1998 ISBN 0946688494
 

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This document is an output from a project funded by the UK Department for International Development (DFID) for the benefit of developing countries.  The views expressed are not necessarily those of the DFID.

TVE/ Practical Action gratefully acknowledge support for the HANDS ON programmes from the UK's Department for International Development (DFID), the European Commission (EC), the UN Foundation and UNDP/The Equator Initiative in collaboration with the Government of Canada, IDRC, IUCN, BrasilConnects and the Nature Conservancy.