Wastewater is becoming more widely recognized as an asset to exploit rather than a liability to be treated and disposed of to the environment. It is virtually 100 percent recyclable and is a source of nitrogen, phosphorus and other agricultural nutrients; soil amendments and conditioners; biofuel oils; methane gas for heat and electric power production; and treated water for cooling, irrigation and potable human consumption. Wastewater is an ideal source for recycling because it is collected and delivered to water treatment facilities as sewage, thus taking advantage of the existing infrastructure and obviating the high cost of collection and transport that limits other types of product recycling. Finally, beneficial reuse eliminates the disposal costs and the environmental concerns of discharges to ambient waters and leachates from landfills.
Agriculture accounts for about 75 percent of all world water uses. Agriculture water reuse has been rediscovered as an important sustainability, conservation and cost-reduction opportunity, and many national and international organizations have produced duplicative reviews, recommendations and guidelines. Those reviews are aimed at encouraging and expanding applications by describing the benefits and providing operating and quality criteria to assure health protection for consumers of agricultural crops and fieldworkers who would have greater opportunity for contact with the recycled waste irrigation water and the solids that could accumulate in soil as well as in and on crops.
The principal health concerns primarily involve the necessity of having sufficient treatment processes for elimination of the pathogenic microorganisms in the water and biosolids, accumulation of pesticides, nonbiodegradable commercial and industrial chemicals, and the human hormones and drug residues that could be in the waste and then become accessible for uptake by food crops.
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Agricultural water reuse guidelines
Guidelines have been produced in recent years by several national and international groups, and they are conceptually similar while coming from somewhat different perspectives and aimed at different audiences, namely, developed and developing countries. Examples include: U.S. Food and Drug Administration (USFDA), U.S. Environmental Protection Agency (EPA), World Health Organization (WHO), the European Union (EU) and the United Nations (UN). States can also have agricultural reuse guidelines, and some are in development.
An example of USFDA nonbinding recommendations is the Guidance for Industry: Guide to Minimize Microbial Food Safety Hazards for Fresh Fruits and Vegetables. Agricultural water includes water for irrigation, cooling, frost protection, processing and as a carrier for fertilizers and pesticides. Sources include surface waters from rivers, irrigation ditches, canals and impoundments, wells and municipal supplies. The guide describes microbial hazards from wastewater and includes information on control of potential hazards, recommended but not obligatory microbial testing, manure management and treatments of manure to reduce microbial hazards, worker health and sanitation in the field, packing and transportation. Good management practices are recommended to reduce microbial food safety hazards. Recycled process water is acceptable within a process, such as for chiller water used in meat production.
EPA has produced iterations of its Guidelines for Water Reuse. Guidelines are a function of the end use. There are no federal requirements specific to recycled wastewater; states have that authority. They include reuse for agricultural, industrial, environmental, recreational and potable applications.
WHO has produced guidelines specific to agricultural reuse applications and, more recently, for potable reuse. The agricultural reuse guidelines provide a preventive management framework that covers from the point of wastewater generation to consumption of products that have been exposed to recycled water and excreta. Treatment technologies are dependent upon the end uses of the water and especially, the potential for human contact during production and consumption of the crop. The WHO guidelines are described as reasonable minimum requirements of good practice but they are not mandatory limits; WHO provides guidance and then defers to the national governments to determine if and how they will apply the guidance.
The WHO guidelines include principles and procedures to protect the health of agricultural workers as well as consumers. However, WHO guidelines of this type may be more sensitive to conditions in developing countries, so it would be expected that developed countries have more stringent specifications or guidelines. The WHO guidelines summarize a range of microbial health risks from helminth, bacterial, virus and protozoan infections that could be derived from contact with inadequately treated wastewater. It also calculates quantitative microbial risk assessment (QMRA) results for several microbes potentially associated with unrestricted irrigation risks to consumers from several crops and for farmers and other heavily exposed populations. It also describes different irrigation and food processing scenarios and estimates log pathogen reductions that could be achieved. The guidelines also include values for maximum tolerable soil concentrations for 40 typical water, industrial and pesticide chemicals.
The EU has produced an “Implementation Strategy for the Water Framework Directive (WFD) and Guidelines for Integrating Water Reuse into Water Planning and Management in the Context of the WFD.” It includes:
- Applications of wastewater reuse to contribute to environmental objectives and making water available for future uses
- Agricultural/horticultural uses for food and nonfood crop irrigation, orchards, pastures and aquaculture
Industrial uses, e.g., cooling water, process water, soil compaction and dust control
- Municipal landscape uses
The EU guidelines do not recommend any chemical, microbial or other water quality or technology elements, but defer to member states for developed standards.
The UN World Water Development Report discusses: prevention and reduction of pollution at the source; wastewater collection and treatment; using wastewater as an alternative source of water; and recovery of useful byproducts from wastewater. With regard to wastewater treatment facilities, it encourages consideration of decentralized treatment facilities and lower cost sewage systems as opposed to investing in large-scale centralized wastewater treatment facilities. Decentralized facilities provide greater opportunities for commercial and industrial water recovery developments and for locally sustainable applications of the water and the byproducts for agricultural use.
Next steps for agriculture water reuse
Multiple use of water and wastewater, rather than single-pass use, has many opportunities to produce sustainable and more stable access to water, as well as for generating valuable products such as agricultural nutrients, soil amendments and fuel. Both farmers and localities are benefiting from wastewater recycling, and industries also can achieve cost reductions and even improved community relations by more water recycling and less de novo consumption.
Wastewater recycling applications will continue to increase – driven by economics and necessity. Public education can play a role leading to acceptance of the concept for applications on food crops and drinking water.
Greywater recycling for some agricultural and nonhuman contact uses is also in the ascendancy. The entry of refractory contaminants into waste streams should be managed by pretreatment and diversion so that the desired end-use quality can be more readily achieved and at a lower cost. Rational quality specifications for end uses developed by qualified bodies are essential.
Numerous versions of agricultural reuse guidelines exist; however, their contents should be similar because most developed counties import substantial portions of their food supplies from other countries, often developing countries. Therefore, the quality and production specifications should be universal. Food supplies and food worker conditions in developing countries should reflect high standards for the safety of agricultural workers and for consumers in both environments. If for no other reason, well-managed, high-quality and safe agricultural reuse of wastewater in developing countries should be strongly encouraged because it allows opportunities for commercial developments in those countries to provide high-quality food for profitable export and raise their health status and standards of living