In our villages, dried cakes of animal dung are burnt as fuel for domestic use, It is unwise to use animal dung directly in the form of dung cakes as fuel because of the following reasons:

1. Animal dung contains important nutrients like nitrogen and phosphorous which are needs by the crops. So animal dung can serve as good manure in agriculture.

2. Dung cakes produce a lot of smoke, causing air pollution.

3. Dung cakes have low calorific value (produce less heat per unit mass.

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4. Dung cakes produce large amounts of ash as residue.

Therefore, we see that there are many disadvantages in using animal dung as a fuel directly. However, if we produce biogas from animal dung instead of burning it directly then its use becomes double. Firstly, it gives us clean fuel called biogas which does not give smoke, has high calorific value and easy to handle. Secondly, the residue, which is left after extracting biogas, is rich in nutrients and can be used as manure.