A better biogas plant for your home

The use of waste food instead of dung as feedstock makes this biogas plant distinct and more practical to use by people in both rural and urban areas

What comes to your mind first when “a biogas plant” is mentioned? Does the picture include a large smelly tank attached to a gas stove used mostly in villages?

Well, you might have been correct if not for an innovative compact biogas plant that uses waste food rather than cattle dung as the feedstock! Brainchild of Dr Anand Karve, a Pune-based biologist, this biogas plant can be used both in rural and urban households, thanks to the source of energy.

Dr Anand Karve Appropriate Rural Technology Institute (ARTI)

The use of biogas – a mixture of mainly methane and some carbon dioxide – as an alternative to conventional fuels such as coal and LPG in rural households is not new. However, the size of the plant and its reliance on large quantities of cattle dung has acted as a dampener for urban households. Also, because of the dung’s low calorific value, the energy produced per kilogram of dung is low vis-à-vis waste food. It was this that made Dr Karve decide on replacing the traditional feedstock with waste food as the input. “It is known that methane gas can be produced from sugar, starch, cellulose and fat, and one kg of food waste (dry weight) – which contains starch, sugar, protein or fat – yields about 250 gms of methane. So I decided to replace dung used in a conventional biogas plant with waste food,” says Dr Karve, who runs Appropriate Rural Technology Institute (ARTI), an NGO. The result was an efficient and less cumbersome device that can also be used in urban households.

Since 2006, nearly 3,000 such plants have been installed both in India and abroad in rural and urban households and in commercial establishments such as hostels and hotels.

So why has this biogas plant aroused a lot of interest? There are several reasons behind this.

One, the conventional biogas plant produces 250 gms of biogas from 40 kgs of excreta in 40 days. In contrast, the new plant requires just 1 kg of sugar or starch – in the form of waste food from household or hotels, spoilt grain, overripe fruit, non-edible seeds, kitchen waste, etc. – to produce the same quantity of methane in just 24 hours. According to Dr Karve, through the use of this compact system it has been demonstrated that using feedstock having higher calorific value increases the efficiency of methane generation.

Two, the choice of feedstock facilitates its use in urban households. Reliance on cattle excreta has been one of the major restricting factors limiting its usage in urban homes. Traditional plants require approximately 40 kg of input on a daily basis, and have a high retention period of 40 days. The large quantity of input and the longer period require plenty of storage space, which is a major constraint in cities.

How does it work?
The standard plant uses two tanks, which typically have a volume of 0.75 cu.m. and 1 cu.m. The smaller tank, which is the gas holder, is inverted over the larger one containing a mixture of feedstock and water. An inlet pipe is provided for adding feedstock and an overflow pipe for removing the digested residue.
A pipe takes the biogas to the kitchen, where it is used with a biogas stove.
The gas holder gradually rises as gas is produced, and sinks down again as the gas is used for cooking.
Initially the plant is fed with a starter mix, which contains either cattle dung mixed with water and waste flour or effluent from an existing biogas plant mixed with starch. The feeding of the plant is built up over a few weeks.

Three, according to Dr. Karve the higher calorific value of the input results in better quality of gas thus produced. From the point of view of conversion of feedstock into methane, this system is 400 times more efficient than conventional system. He says, the biogas thus produced has all the virtues of LPG – it produces a clean blue flame, has the same intensity, provides finger tip control of flame, produces no soot and smoke, etc. As methane has the same calorific value as LPG, it becomes as efficient and cost effective too.

Four, besides having higher efficiency compared to a conventional plant, the compact biogas plant takes care of another pressing problem faced by cities – the problem of waste disposal. Not only this, the quantity of residue produced from this plant is smaller than the traditional plant. The residue is liquid, with good nutrient content, and hence can be used as fertilizer for plants.

There are some disadvantages of the system as well. Says Dr Karve, “For a family of four, you will need about 1 cubic meter of gas, which will give you continuous flame for 2 hours. If you are a chappati consuming family, this will not be enough, because chappatis are made serially one after the other and consumes more fuel. But for a rice eating family this would be sufficient for all the meals.”

What can be used as a feedstock?
Waste flour
Vegetable residue
Waste food
Fruit peelings and rotten fruit
Oil cake, left over from oil-pressing
Even rhizomes of banana, canna, nutgrass, non-edible seeds (e.g. Leucaena, Sesbania, tamarind, mango kernels) and spoilt grain

Another disadvantage with the biogas system is that the process of fermentation requires a sufficiently higher temperature of around 35oC. Dr Karve says, “This system works best in peninsular India, because the bacteria requires high temperature to ferment – that is more than 30oC. In North India, especially in winters, the biogas production will drop considerably.”

Conventional Biogas SystemsCompact Biogas Plant
Feedstock Requirement40 kg+40 liter water1-1.5 kg+15 liter water
Nature of feedstockMostly cattle dungWaste food (high in starch)
Feedstock Reaction/Retention time40 days24 hours
Standard size for household4000 liters1000-1500 liters
Residue produced80 liter (sludge)15 liter (watery)
Capital Investment per unit
including stove
Rs. 25,000Rs. 10,000

The use of waste food as feedstock reduces the storage space (size of the tank) required by the system drastically, which has a direct impact on the price. A conventional plant may cost approximately Rs 25,000, whereas the compact biogas system costs Rs 10,000. Of that Rs 7,000 is for the hardware and the rest for installation and transport.

However, compared to LPG, the up-front cost of a biogas system is higher. “The up-front cost of a biogas system is higher than for LPG, since an LPG cylinder plus a two burner stove costs only Rs 5,000 whereas the compact biogas plan plus a biogas stove costs about Rs 10,000. However, the operational cost for biogas is only about Rs 2 per day if waste flour is used as feedstock, and can be zero if the plant uses only food wastes,” according to ARTI.

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