We have to drink only clean water. Because non-purified water contains germs,micro organisms, fungus etc. which could harm our inner body organs. 

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Clean water refers that the water is free from impurities which can be potentially harmful to our body & system.
Clean water helps our body from harmful impurities & particulate & heavy metals which are present which might be potentially harmful to our internal system as it does not get absorbed but the particles break down & reach our blood.
Clean water is free from harmful organisms which when enters our body can cause different types of diseases to us.