It is simply not necessary to use water of drinking quality for all purposes in private households and industrial plants. In the industrial sector in particular, untreated raw water, rainwater or treated greywater or wastewater would be sufficient to fulfill many purposes. What kind of pre-treatment is needed depends on the subsequent use of the treated water. For example, if recycled water is to be used for cleaning or cooling, the quality need not be as high as it must for – say – cleaning textiles.
However, water used for agricultural purposes and for bathing must be absolutely hygienic with no traces of contamination which could damage health by accumulating in human organisms.
On the other hand, if the treated water is to be used to irrigate fields, it is not necessary to remove nitrogen and phosphor entirely because their presence saves farmers costs for precisely these mineral fertilizers.
Generally speaking however, if water is to be re-used it is necessary to remove all solids and fats beforehand with appropriate technologies. Organic carbon (COD, BOD) must always be extracted from organically polluted wastewater. The best way to do this is to treat it biologically and strongly polluted wastewater is best treated anaerobically. Depending on the subsequent use of the water, it is sometimes necessary to remove the remaining COD as well e.g. in an aerobic biological process.
Innovative, adapted technologies for recovering, storing and re-using wastewater residual heat are also becoming more and more popular especially as the costs for energy are continuing to rise.