Compact digester for producing biogas from food waste

Organisation Award Category Technology Region Year
Appropriate Rural Technology Institute (ARTI) Food Biogas India 2006

First prize 2006 - Food category


The Appropriate Rural Technology Institute (ARTI) in Pune has developed a biogas plant which uses food waste rather than manure as feedstock and supplies biogas for cooking. The plant is sufficiently compact to be used by urban households, and over 700 are currently in use.

Pune is a relatively affluent city in south India, and many people use liquid petroleum gas (LPG) or kerosene for cooking. Waste food is often discarded at the side of the road, as in many cities, attracting stray dogs, flies and rats and creating a public health hazard. The ARTI compact biogas plant is made from two standard high-density polyethylene (HDPE) water tanks: the larger tank acts as the digester and the smaller one is inverted and placed into it to serve as a gas-holder. The plant safely digests kitchen waste, food waste or waste flour from mills, thus reducing the problem of waste disposal. A 1,000 litre plant produces sufficient biogas to at least halve the use of LPG or kerosene for cooking in a household, as well as a small amount of liquid effluent which can be used as fertiliser. ARTI has developed the technology, field tested it, and managed the supply of about 700 biogas plants in Maharashtra. Around 100 plants are now being installed every month.

The first-prize Ashden Award to ARTI recognises the enormous potential for using this compact biogas digester in towns and cities, both to supply cooking gas and to assist in the disposal of organic waste.

The organisation

ARTI is a charitable trust, founded in 1996 by a group of scientists, technologists and social workers. It currently employs ten staff and has 22 members. The biogas project, started in 2003, is one of over ten different projects run by ARTI. Many of its staff are involved in more than one project and so there is continuous cross-fertilisation of ideas.

ARTI won a First Prize Ashden Award in 2002 for using sugar cane waste to make char briquettes.

Contact: Dr Anand Karve, President,
Appropriate Rural Technology Institute
Maninee Apartments
Survey no. 13, Dhayarigaon
Pune 411 041, Maharashtra
Tel: +91-20-24390348 / 24392284


Biogas systems take organic material (feedstock) into an air-tight tank, where bacteria break down the material and release biogas – mainly a mixture of methane with some carbon dioxide. The biogas can be burned as a fuel, for cooking or other purposes, and the solid residue can be used as organic compost.

Most biogas plants currently in operation in India and elsewhere are designed to use animal manure as their main feedstock, and are therefore used in rural areas. ARTI have developed a compact biogas plant which uses highly digestible organic materials available in urban areas, such as waste flour or kitchen waste as feedstock. The plant can be made in a small size and still produces enough gas for a family, because its feedstock has a much higher energy density than manure, and the digestion takes place more quickly (typically only one to two days, compared with 30 to 40 days for a manure-based plant); so a much smaller quantity of decomposing material needs to be held in the plant.

The compact plants are made from cut-down HDPE water tanks, which are adapted using a heat gun and standard HDPE piping. The standard plant uses two tanks, with volumes typically of 0.75 m3 and 1 m3. The smaller tank is the gas holder. It is inverted and attached to the larger one which holds the mixture of decomposing feedstock and water (slurry). An inlet is provided for adding feedstock, and an overflow for removing the digested residue. Because the feedstock is almost completely digested, the effluent contains a much smaller amount of solid matter than the residue from a manure-based plant, and ARTI recommends that the liquid is mixed with the feedstock and recycled into the plant. A pipe takes the biogas to the kitchen, where it is used with a biogas stove. Such stoves are widely available in India, which has a long tradition of using manure-based biogas plants. The gas holder gradually rises as gas is produced, and sinks down again as the gas is used for cooking. Weights can be placed on the top of the gas holder to increase the gas pressure.

ARTI has found that the gas produced in these plants has a higher methane concentration than is found using manure based plants, and therefore has a higher energy content. Experiments suggest that the reason for the high methane concentration is that the carbon dioxide dissolves in the very liquid slurry. The methane concentration is further increased when weights are used on the gas holder, because more carbon dioxide dissolves under the increased pressure.

The plant is provided as a kit that takes only two to three hours to install. It needs a space about 2 m square and 2.5 m high, although adaptations can be made if it is placed under a roof. The plant is filled with a starter mix: either cattle dung mixed with water and waste flour, or effluent from an existing biogas plant mixed with flour. The feeding of the plant is built up over a few weeks until it provides a steady supply of gas, typically 250 g of gas per day from 1 kg (dry matter) of feed. The feed can be waste flour, vegetable residues, waste food, fruit peelings and over-ripe or rotten fruit. Feedstock with large lumps (more than 20 mm) can be broken up with a food blender. Hand- and pedal-powered food blenders are being developed for when electricity is not available. Oil cake, left over from oil-pressing, is another useful feedstock. Non-edible oils (such as jatropha, castor and linseed) are being promoted in India for making biodiesel, and oil-cake that cannot be fed to animals is likely to become more abundant.

A biogas plant can become acidic and fail if it is over-fed, and this is a particular problem with a plant using highly digestible organic materials. If this happens, ARTI has found that the plant can be recovered by ceasing feeding, partially flushing out the contents with fresh water, and then building up the feed rate again slowly. This problem was more common with the early smaller systems (0.5 or 0.75 m3) than with the later, larger systems.

How users pay

At the time of writing (July 2006) 85 Rupees (Rs) = UK£1 = US$1.8

All plants are paid for in full by the owner when they are constructed. If the plant is installed by ARTI, the user purchases the plastic tanks and the relevant hardware. An ARTI technician assembles and commissions the system and the total cost to the owner is about Rs 6,500 (about £76) for a 1 m3 system, plus Rs 500 (£6) for a biogas stove. ARTI is not allowed to make a profit and so the cost of installation is lower than with private entrepreneurs, who build plants for Rs 10,000 (about £120). This includes the cost of fabrication, transport to site and installation. There are no subsidies or loan facilities. The project does not have a micro-credit facility but a supplier may accept payment by instalments.

Training, support and quality control

ARTI develops sustainable technologies and trains people to use them. Knowledge is transferred either through local entrepreneurs or through representatives of other NGOs who are trained by ARTI at their centres. About 30 people have so far been trained and ten are actually functioning as entrepreneurs. ARTI employs five people who focus on improving the biogas technology, and training entrepreneurs to install biogas plants. They also install biogas plants in places where no-one has yet been trained in this role.

ARTI's technicians each have their own biogas plant at home, and some are testing out new configurations, identifying and solving problems. Two test sites are also used to verify further improvements, such as the design of gas burners. Alternative designs, based around a fixed dome, are also being tested.

Benefits and replicability

The immediate benefit from owning a compact biogas system is the saving of the cost of kerosene or LPG for cooking. The up-front cost of a biogas system (Rs 6,000 for a 1 m3 system plus stove) is higher than for LPG, since an LPG bottle plus a two burner stove costs only Rs 5,000 (£60). However the running cost for biogas is only about Rs 2 per day if waste flour is purchased as feedstock, and can be zero if the plant uses only food waste. This is much cheaper than LPG which costs about Rs 15 per day, even with the current subsidy of 50%. Biogas can easily halve the amount of LPG used by a family. Some families who use a pressure cooker for cooking and collect food waste from their neighbours have replaced all their LPG use.

The replacement of fossil fuels reduces the emission of greenhouse gases. ARTI estimates that, for a typical urban household, biogas saves 100 kg of LPG or 250 litres of kerosene per year, which is equivalent to 300 to 600 kg CO2 per year. A rural family could save about three tonnes of wood per year, which would generate about five tonnes of CO2 if burnt.

Indoor air pollution is reduced by cooking with biogas as opposed to wood or kerosene. This is better for the health of those in the kitchen (who are mainly women). Further reductions in pollution and energy use arise from not having to transport LPG cylinders to be re-filled. The small amount of liquid residue produced by the biogas plant makes a good fertiliser.

In villages, food waste was traditionally fed to livestock or left by the side of the road for animals to devour. People in the city continue to do this, but there are fewer animals to consume it. The result is that smelly, rotting food attracts flies and rats. Some authorities collect food waste and dispose of it in landfill. The municipal authority in Pune has decided it can no longer do so because of lack of space, and local officials are therefore strongly encouraging the use of biogas plants to dispose of food waste. Local council offices have set up demonstration plants, which use local food waste and provide gas for making tea for local officials and their visitors.

Between 2003, when the project started, and May 2006, 600 biogas plants had been installed. At least 100 more plants were installed outside the project area (the project is operative only in Maharashtra), and several independent producers have copied the design for themselves. These existing plants are a useful advertisement for the advantages of biogas systems and consequently about 100 plants are now being installed per month. Several other states are interested in the technology. ARTI have produced a CD showing how the plants are made, to encourage widespread adoption of the technology.

ARTI's compact biogas plant could be replicated in any location where there is the space for a plant and the temperature is sufficiently high. Because the methane-producing bacteria normally reside in the guts of animals, their optimum operating temperature is about 38ºC, but gas production is generally satisfactory if the ambient air temperature is above 30ºC. ARTI is currently studying ways to insulate biogas plants to increase gas production in the coolest months.

Management, finance and partnerships

ARTI divided the State of Maharashtra into five regions and made a project manager responsible for each one. In each region, two NGOs work as local partners, and each NGO seconds one field assistant to this work. Several models of plant were tested in the field, and the most successful one selected for commercialisation. Artisans selected by the NGOs have been trained by ARTI to manufacture and install the systems. The United States Environmental Protection Agency was the major funder for the dissemination of the compact biogas plant.

Five people employed by ARTI and ten self-employed contractors produce and install the biogas systems. They are able to supplement their regular income (for example from plumbing or brickwork) by working with biogas systems.

ARTI focuses on developing new rural technologies and on enabling other people to spread such technology to those who will benefit. ARTI is restricted by its constitution to research, training and publicity roles, and cannot take part in commercialisation. Although ARTI does not want to restrict knowledge of their design, they are also concerned that all plants are manufactured correctly so that the technology maintains its good reputation.

How the Ashden Award will be used

The Ashden Award will enable ARTI to publicise biogas technology more widely in Maharashtra State, where there are an estimated 500,000 potential users. ARTI also wants to train entrepreneurs to produce standardised feedstock in de-centralised units.

This report is based on information from the application submitted to the Ashden Awards by ARTI; findings from a visit by one of the judges to see their work in and around Pune; discussions between Dr Karve and the Ashden judges at interview; and a presentation by Dr Karve at an Ashden Awards seminar.

Dr Anne Wheldon, Technical Director of the Ashden Awards
Jeremy Rawlings, Technical Assistant
July 2006.

The Ashden Awards has taken all reasonable care to ensure that the information contained in this report is full and accurate.  However, no warranty or representation is given by The Ashden Awards that the information contained in this report is free from errors or inaccuracies.  To the extent permitted by applicable laws, The Ashden Awards accepts no liability for any direct, indirect or consequential damages however caused resulting from reliance on the information contained in this report.