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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, a special emphasis was brougth on their fate in the environment.
Water reuse, an under-used means to address problems in water scarcity and quality
Wastewater reuse for crop irrigation may simultaneously address water quantity and quality problems; it is implemented in regions where conventional water resources are too limiting and/or the discharge of (treated) wastewater has too much impact on the environment, especially in coastal areas and Islands where tourism is of first concern. While wastewater reuse is already important in some countries/states including California, Israel, and Cyprus, it remains generally low. In Europe, 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, even in Cyprus where it is currently limited by the collect and treatment of wastewaters. Wastewater reuse remains low or negligible 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); and several large cities and conurbations depend on recharging surface water and groundwater bodies by treated wastewater, leading de facto to indirect potable reuse, although it is usually not acknowledged. We have obtained nearly no information on wastewater reuse in Bulgaria that has one of the highest water stress index of European countries.
Various types of reuse
Wastewater may be reused for various activities: agricultural or landscape irrigation, industrial uses, urban reuse, recreational uses, aquifer recharge, and indirect or direct potable reuse. The type of use has to be discussed with regard to local water needs, cost-benefit considerations and possible conflicts. Conflicts may result from competing uses in countries with a high water demand – e.g. between irrigation and industrial uses in Japan –, and from greatly differing quality needed for several simultaneous uses and the allocation of treatment costs – e.g. in the South of Valencia, Spain, where water from El Pindo wastewater treatment plant simultaneously irrigates submerged rice fields and supplies Albefura lake –. Public acceptance is generally good for reuse in irrigation and for other uses, with the exception of direct potable reuse although some 'success stories' exist in direct potable reuse as in Windhoek (Namibia). However, some opposition exists. They are often explained by the 'yuck factor', corresponding to a psychological aversion with the following expressions retained in surveys: "psychologically repugnant", "lack of purity", "can cause disease". Another explaination is that public opposition would result from social and cultural perceptions of risk. The 2 preceding explanations justify more information dealing with water cycle, water treatments and the actual risks in order to prevent project failure.
The prevention of sanitary, environmental and agricultural risks
Actual risks include sanitary, environmental and agricultural hazards that 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 have to be ensured. The management of conventional and alternative water resources requires appropriate regulations and standards, as well as economic policy. While the diversity of current rules between European countries seems not scientifically justified and leads to inequalities, differences between regions in water requirements, raw wastewater properties and human resistance to pathogens justify having regulations adapted to regional contexts. The European Union could propose guidelines with maximum tolerated risks and a methodological framework to elaborate regional or national regulations and standards that account for local specificities, in the same spirit that the World Health Organization encourages national governments to adapt their guidelines to their own socioeconomic and environmental realities. A distinction should then be performed between crops for local markets or for export.
Tools to assess risks, optimize treatment trains and check of project economical sustainability
Some separate tools exist to optimise wastewater treatment, assess quantitatively microbial risks or estimate the balance between benefits and costs, including a monetary valuation of environmental changes, but new tools combining risk assessment, treatment optimisation and cost-benefit considerations are required to support decisions dealing with the definition of new regulations and standards as well as economic policy (tariffs, taxes, subsidies …). In the current state of knowledge, a first generation of combined tools can be proposed, but they would have to evolve in order to (i) incorporate new data, processes and pathways of contamination, (ii) add emerging microbial or chemical contaminants, and (iii) adapt to possible changes in standards (indicators themselves, their maximum tolerable quantities and/or their minimum removal rate).
Bioindicators and chemical indicators to check for the acceptability of treated wastewater
The relevance of microbial indicators has to be discussed. Their use to characterize the efficiency of treatments is probably relevant as long as several bioindicators having different sensitivities to different treatments are used simultaneously. The major limitations of microbial indicators generally used are first weak correlations with the level of pathogens in wastewater and second their fate in the environment that may differ from the fate of pathogens. In particular, they are generally not correlated with the levels of enteric viruses that are increasingly considered as the etiologic agents of human enteric infections. An alternative could be to detect directly some pathogens from a list est
ablished using a methodology as that of the U.S. Environmental Protection Agency and the American Water Works Association. This choice is currently hampered by the low levels of enteric pathogens and the very low thresholds of tolerable concentrations; they may require the concentration of water samples prior to pathogen detection and the use of molecular methods. But great technological progresses have been performed, including those using DNA microarrays on silicone nanostructures, and molecular methods would allow to obtain results quickly. Another alternative would be to partly replace microbial controls with more advanced treatments and/or more controls on some treatments (e.g. by measuring the residual content in disinfectant), all the more that treatments will probably have to become more stringent, partly to avoid environmental discharge of water too much contaminated by chemicals. In the case of separate sewage networks for domestic wastewaters and rain runoff without industrial wastewaters, indicators of emerging organic pollutants have probably to be chosen among chemicals from personal care products, pharmaceutical products, bisphenol A and phthalates; otherwise they must cover a broad spectrum of toxicological and ecological risks as well as possible technical disorders.
Scientific challenges to improve wastewater management
Additional knowledge is needed to optimize wastewater reuse. First, it is important to better understand and describe the processes implied in microbial contaminations in order to improve quantitative microbial risk assessment: via air (aerosolization, atmospheric speciation of pathogens, inactivation/mortality and transport in the atmosphere, redeposition, transfer from human respiratory tract to gastro-intestinal track), via food (internalisation through the roots, the leaf stomata and/or injuries in the aerial parts of plants), and via water to some extent to better assess the reactive transport of pathogens from soil surface to underlying aquifer. It is also important to assess the combined effects of organic pollutants on human health and environmental biodiversity, as well as their cumulative consequences over long-term periods, as concentrations of most of them in urban wastewaters are generally below the toxic levels for humans. Quantitative microbial risk assessment and quantitative chemical risk assessment need also informations on population habits (food, displacements, protection of farmers during field labour …) that are probably not available in each European country. Second, new probe methodologies are required to monitor in situ microbial and chemical contaminations in real time and at low costs, whereas current cost greatly vary between microorganisms, as well as between organic micro-pollutants. Third, the monetary valuation of the changes resulting from wastewater reuse (human health, environment, recreational activities, industry, jobs …) must be refined, especially the monetary valuation of environmental changes, which is recent and may result from different calculation procedures.