Well-known operating wastewater reuse schemes include NEWater in Singapore (1% of drinking water consumption is covered with reclaimed wastewater), where the quality of the reclaimed water fulfils all requirements and is in most aspects better than that of the raw source water used otherwise. In the Shinjuku district of Tokyo, the water recycling centre distributes treated wastewater for toilet flushing in twenty-six high-rise office buildings. In Mexico City, untreated sewage has been used to irrigate the Mezquital Valley since 1896. As a result of this practice, the water table of the aquifer underlying the irrigation zone has been rising. The unplanned artificial recharge is about 25 cubic metres per second (m3/s), and this ‘reclaimed water’, treated only with chlorine, is being used to supply 300,000 inhabitants of the region for human consumption. Several studies have shown that the water meets potable norms and WHO human consumption guidelines, including toxicological tests (Jiménez et al., 2001).