RECYCLING OF TREATED WATER AND NUTRIENTS
Usually wastewater is seen as a substance that has to put away as soon as possible. This idea has influenced the treatment techniques and has brought to conceive expensive purification solutions (big sewage net in pressure and activated sludge treatment plants), with energy waste and low reuse of nutrients and treated water. Also the recycling of depuration sludge is often not possible due to the presence of toxic substances. Moreover, the common practice of municipalities of not separating the different types of wastewater makes not easy an inversion of tendency: black water with high organic and microbial content, easily treatable grey water and rain water only initially polluted (the so called “storm water”) are collected together in a single net and can not have a differentiated treatment.
The reuse of treated wastewater, for example in toilet flushing of in irrigation, reduces the fresh water consumption, which is an important factor in situations of limited availability of water. At the same time, reducing the fresh water consumption brings an “at-the-source” reduction of the amount of wastewater to be treated. Treated wastewater to be reused has to respect different quality values from the ones for discharge in natural water bodies, and these values also change according to reuse aims. In case of reuse, therefore, it will be necessary to adapt the treatment to the chosen utilization. One of the base requirements for a safe reuse of wastewater is the hygienic-sanitary quality.
Reuse of wastewater in agriculture is a diffuse practice in many countries and it is more and more recommended by international agencies promoting sustainable development. Countries with the greatest experience are USA and Israel. In Israel since a long time used waters are considered part of the national water patrimony: already in 1970 quite the 10% of water potential was constituted of wastewater. In USA the reuse of wastewater has a great importance above all in the desert states of the south: in particular California and Texas . Just in California the first laws about regulation of quality standards for reuse of wastewater were approved.
Recently, with the raise of the environmental sensitivity in all the world, the theme of reuse of water is getting more and more diffuse: the World Water Council report on water in the 21° century (WWC 1998) cites formally the reuse of wastewater as a principal strategy to promote the utilization of “non-conventional” resources.