Listen to the text below and label the diagram using the information given.
I've been doing research into small-scale technology. That kind of technology is very useful to developing countries because it's more appropriate to their needs. I've just been reading about bio-gas plants, in fact. Here's a diagram of one. It's a fairly simple idea. It's basically a system for producing methane - that's a very useful gas for providing power.
The raw material for this kind of plant is slurry. Slurry is a mixture of all kinds of agricultural waste in a semi-liquid form. That's one of the advantages of a bio-gas plant, you see. The slurry is simply made from animal and crop wastes which are available in any village community, however small. Usually these wastes can't be put to any use, but with a bio-gas plant they can be converted to methane gas.
This part of the plant here that looks like a funnel is called a hopper. It's really a large funnel for feeding in the slurry. So the slurry is fed in at the hopper. It's then passed on down a pipe to the main tank. The part here which is suspended from what look like two poles called the gas-holder. It has a perforated flowell. The gas lifts up the gas-holder as it rises. There are counterweights attached to the end of the cables. Those make it possible for the rising gas to push up the weight of the holder.
The methane is actually produced in the main tank. It is produced by a natural process known as anaerobic fermentation. In other words, the gas is produced away from the presence of oxygen. So the methane is collected in the gas-holder and then it's ready to be used. But, first of all it has to pass through a water trap in order to remove all the water. The water trap, by the way, is connected to the drain-off tank. That tank is connected to the main tank as well. So it collects the water from the water trap and it collects the used slurry as well.
Another advantage of the bio-gas plant is that the used slurry is an especially rich fertilizer and can be spread on the land to help the crops grow. So it's a very useful by-product. It's not just thrown away.
The main product, methane, can be released from the holder by the main valve. That's on top of the gas-holder.
The bio-gas plants are very common. According to the figures in this book, there are over sixty thousand in India and four million in China. They're widely used in African villages as well. You see, they have so many advantages. They can be made from materials, such as iron sheeting, which are easily available in most countries, even in rural communities - farming villages, I mean. They are also easy to build. That's another plus. In most villages, there's a blacksmith who has the necessary skills. So that's another reason for their popularity. And, of course, above all, the methane is very useful. It provides mechanical power for jobs like pumping or transport. And, another point, you can use it for domestic purposes as well, like cooking.
The stuff that you put in the bio-gas plant, the slurry - there's no problem about obtaining that. So that must help countries to cut down on expensive imports of fuel. And chemical fertilizers as well. So bio-gas plants are just one example of how small-scale technology can help developing countries to become economically independent.
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