See this great animated tour of the water cycle by Yorkshire Water and see what happens to create fresh water. Treating water to this high standard takes a lot of energy to pump water from A to B, the waste water treatment and for the cleaning of fresh water taken from rivers and reservoirs. It has been estimated that for each 1000 litres of water arriving at your tap this requires 2.2 kWh of electricity to treat. If your water comes from a desalination plant which convert salty sea water into fresh water this requires a further 1.5 to 2.5 kWh to process, depending on how efficiency the plant is.
And shock horror! News just in .... according to the Guardian 39 million tonnes of "sewage" goes into the river Thames, in London, every year! Now that's gross! The fact remains is that many EU countries pay fines rather than fixing the problem of pumping sewage into rivers or the sea. Most rivers in the UK are over abstracted -i.e. the water companies are regularly taking too much water out of the river and this harms the river ecology.
Any greywater you can divert directly onto your garden, rather than down the drain, cuts out this long cycle of man made treatment and delivery. Water that goes into the ground will be filtered naturally and eventually end up in the sea and evapourate and return as RAIN. Likewise any water you can collect as rain also interrupts this cycle. In areas where acid rain is not a problem and air pollution is closely regulated, rain can be a relatively clean source. Obviously, you must be careful what kind of water you use like this -see my diagram of the WATER USE HIERARCHY
Next time -I'll be blogging about water butts and rain water in preparation for world water day...