(i) Cow-dung contains important nutrients like nitrogen and phosphorus, which are very useful for crops. Now, when the cow-dung is used to prepare bio-gas, then the residue left behind is still rich in nutrients and can be used as manure.
But if the cow-dung is burnt in the form of dried cow-dung cakes, then the nutrient contained therein are destroyed and hence cannot be used as manure.
(ii) The fuel (bio-gas) obtained by the use of cow-dung in bio-gas plants does not produce any smoke on burning and does not cause air-pollution.
On the other hand, cow-dung cakes on burning produce a lot of smoke which causes air pollution.
(iii) The fuel obtained by the use of cow-dung bio-gas plants is a clean fuel and does not leave behind any ash or residue. On the other hand, cow-dung cakes do not burn completely and leave behind a lot of ash.