Fungisai, I presume that you ...

Fungisai, I presume that you are asking about the collection of the water not its treatment. This requires that you have a system of pipes which are connected to the source and carry the water where you want it to go. It could be done with open channels but that would be more problematical as regards maintenance. This is not too difficult if you have access to plastic pipes and, for grey water, they don't need to be too big as there should be no solids to pass - unlike sanitary sewers which are normally a minimum diameter of 100mm. As regards treatment - there are standards - but many rural communities use grey water for irrigation without any treatment.