Hong Kong uses grey water for flushing toilets, and I never had a problem. Each building has its own tank (I believe, to pump the water and maintain a correct w

Hong Kong uses grey water for flushing toilets, and I never had a problem. Each building has its own tank (I believe, to pump the water and maintain a correct water pressure) that needs to be washed every now and then, but that's the same with fresh water.