The term water regeneration defines the treatment of previously used water for new uses and susceptible to reuse. Water reuse has different meanings from one country to another, simply because in each one there are different practices regarding its use. A standardisation of the term is difficult, because several administrative entities are usually involved. However, as a general description, it could be said that reuse is the use of treated or untreated water for a purpose other than that for which it was generated, implying a change of end user. For example, the reuse of wastewater for irrigation.
It is estimated that currently only 4% of all the water consumed in the world is reused.
To put the debate in context, we must understand that only 10% of the drinking water supply in our homes (150 to 300 l/person/day) is consumed for drinking use. 60% is for other domestic uses (cleaning, watering, etc.) (grey water) and the remaining 30%, (black water), is for use in toilets and dragging other waste. Only 45% of the domestic use would need a drinking quality.
Since the middle of the 20th century, and as a consequence of water scarcity, practices began to consider alternative sources with no need for drinkable quality. In 1942 the city of Baltimore used 4.5 m3/s of a secondary effluent for industrial use. In 1958 the Economic and Social Council of the UN promoted not to use high quality water for uses that could tolerate lower qualities.
Nowadays, the countries that occupy the first positions in the ranking of water reuse, according to three different criteria, are the following (see table 1):
Table 1. Ranking of countries reusing wastewater, using three different criteria.
According to total volume, China, Mexico and the US are the countries with the highest amount of wastewater reuse. Qatar, Israel and Kuwait are the countries with the highest level of reuse per inhabitant. When reuse is considered as the percentage of total water used, Kuwait, Israel and Singapore become the first in the ranking.
Spain and Israel are examples for the generation of new unconventional resources, since they complement the water production by desalination and reuse. Spain reuses almost 11% of its water, especially in the Mediterranean basin for primarily agricultural purposes.
Regarding the most common uses of recycled water, we can distinguish between (1) municipal reuse (urban or recreational uses, injection to aquifers, and higher quality for irrigation and potabilization) and (2) industrial reuse.
Agriculture represents 70% of global water demand, but only 2-7% of irrigated land makes use of reused water.
In spite of drinking water shortage in many developed countries, direct reuse for drinking water, which is technologically solved today, is something that still does not have the social acceptance or the necessary legal coverage.
However, several countries already consider this approach, such as the USA (Texas/California), Namibia or Singapore, where arid climatic conditions and a lack of drinking water have led people to innovate and be pioneers in this matter.