As you can see from my diagram, there is a HUGE opportunity to use less water in our daily lives with the right equipment to hand. Currently we use drinking quality water -cleaned to a very high standard for everything -even flushing the toilet -which is total madness when you think about it! Ladies and Gentlemen - I give you the "Water Use Hierarchy"! [click and zoom in]
The average person uses about 150 litres of water per day in the UK. A third of this water, purified to drinking water standard by your dedicated water supplier, is simply flushed down the toilet! Only about 4% of it is actually used for drinking. We don't need to use water of this quality to wash dirty laundry or flush the toilet, this is madness! It is safe to use rainwater and greywater for certain applications, provided that you understand the hierarchy of water use and which activities have to come at the front of the queue (using drinking standard water) and which activities can come at the back of the queue (like flushing the toilet). In an ideal world where rainwater and greywater are used to their full potential, we can become much sustainable in our approach to water use, perhaps even reducing our mains water usage by 50%.
I would argue that both rainwater and greywater are important sources for water use in the home, they both play a useful role, one is not more important than the other. Rainwater can be relatively clean and provided it is filtered properly it can be used for washing the car or your windows and even for washing your laundry! The greywater from bathing can be reused, for example to water the plants in your garden. If you pay for your water on a water meter, you are paying for every measured unit or cubic metre of water, so why pay more? Don't flush money down the toilet!