Wastewater quality and required water quality for irrigation purposes

This review summarizes the main issues related to wastewater reuse for agricultural irrigation. It discusses successively the reuse itself, the sanitary, environmental and agricultural hazards caused by human pathogens and chemicals in wastewater, the economic sustainability of wastewater reuse, and the means either legal (guidelines or regulations and standards) or economic (tariffs, subsidies, taxes) to promote wastewater reuse while minimizing the risks. Proposals are made in the conclusion to improve and promote wastewater reuse in a more rational framework. As human enteric viruses are more and more often incriminated in human outbreaks, Appendix 1 provides a review on their fate in the environment. Although wastewater reuse can address simultaneously problems in water quantity and quality, it remains low in most of the European countries. Wastewater is preferentially reused for crop irrigation in South European countries having a high Water Stress Index, high water needs for crops and large volumes of wastewater produced (Cyprus, Malta, Spain and Italy), and reuse will increase further in these countries. Wastewater reuse remains low in South European countries having a lower water stress index (Greece, France and Portugal), but it should increase because of global warming and the increase in frequency of extreme droughts. In more Northern European countries where water deficit for crops is lower or non-existent, wastewater may be reused locally for irrigation (e.g. in Germany) and/or in other sectors such as urban and industry sectors (e.g. in Belgium); several large cities and conurbations depend on recharging the surface and ground water bodies by treated wastewater. We have obtained nearly no information on wastewater reuse in Bulgaria that has one of the highest water stress index of European countries. Public acceptance is good, but some opposition exists ("psychologically repugnant", "lack of purity", "can cause disease") and justifies information to prevent project failure. Actual risks include sanitary, environmental and agricultural hazards; they result from the presence in raw sewage of human pathogens and various inorganic and organic compounds. Although it is possible to produce water of almost any quality desired from wastewater, cost-effectiveness of treatments must be ensured. In order to protect conventional water resources, reduce the risks inherent in the use of wastewaters to tolerable levels, and insure the economic sustainability of reuse projects, the management of conventional and alternative water resources requires appropriate regulations and standards, as well as economic policy. Since water requirements, properties of raw wastewaters, and human resistance to pathogens vary with regions, it would not be appropriate to use the same regulations and standards in all European countries. However, the current diversity of rules is not scientifically justified and lead to inequalities. Europe could propose guidelines with maximum tolerated risks and a methodological framework to elaborate regional or national regulations and standards that account for local specificities. A distinction should then be performed between crops for local markets or for export. New tools to support decisions such as computer programs are required. They have to account simultaneously for treatments, hazards, and cost-benefit considerations (with the monetary valuations of changes in human health, environment and crops), whereas existing tools only address part of the problem: they include models for quantitative microbial risk assessment, decision support systems for the configuration of wastewater treatment plants, and methods for the monetary valuation of the positive and negative changes in the environment. In addition there are still gaps in knowledge and not enough data or parameters estimated correctly to ensure the reliability of these models. First, the fate of pollutants (including pathogens) is not enough known and described to properly inform quantitative microbial risk assessment; second, the monetary valuation of environmental changes is recent and probably requires a step back with, where possible, a comparison of methods; third, several studies have questioned the relevance of standards in selected microbial indicators and recent methodological developments suggest that the direct monitoring of some pathogens would be more appropriate in the next years.