Don't be in too much of a hurry to advocate the use of waste water and sewer outflow, as being polluted and therefore useless. It is easily purified, either in sewage treatment plants or reed beds. The tap water one gets in London, for example, may have been recycled several times. This may not sound very appealing but it comes through clean enough for the Coca Cola Company to have bottled and marketed it as mineral water. It always amazes me that people are evidently happy to pay more per litre for bottled water than they'd stump up for petrol.