**7. Standards and guidelines**

Standards and guidelines for reuse of wastewater in agriculture are very different in different countries of the world. One of the most important common guidelines regarding the physical and chemical parameters of irrigation water is Ayers and Westcott's guideline, which is summarized in **Table 4**. Because Ayres and Westcott's guidelines are based on many studies and researches and taking into account factors such as leaching percentage, changes in soil permeability due to EC and SAR, plants' tolerable capacity against salinity, sodium, and bar toxicity and other trace elements have been developed, it can be a suitable basis in evaluating the quality of wastewater for use in agriculture [28].

Regarding the microbiological parameters of sewage and effluent due to their importance in public health, guidelines have been provided by various organizations such as WHO and American EPA for use in agriculture, which are shown in **Tables 4**–**6**, respectively. The purpose of WHO in developing such guidelines is to determine the guiding indicators for design engineers to choose the appropriate technologies for wastewater treatment and for planners to choose the best management options. Comparison of the mentioned tables shows that the American EPA guidelines are more strict than the WHO guidelines [13, 14, 30]. According to the EPA, for the


#### **Table 4.**

*Water quality guidelines for irrigation [28].*

application of wastewater in the irrigation of plants that are not processed (for example, plants that are eaten raw), no fecal coliforms should be detectable in 100 ml of wastewater samples. Meanwhile, for watering such plants, WHO has considered the total number of fecal forms in 100 ml of wastewater to be less than or equal to 1000.

Investigations show that heavy metals are the most important harmful pollutants in urban wastewater and irrigation with these wastewaters increases its amount in plants, especially vegetables and soil. Of course, the wastewaters of small and nonindustrial cities are less contaminated with heavy metals and chemical compounds. The maximum permissible nitrate concentration that can be present in drinking water is set by the US Environmental Protection Agency as 45 mg/liter. Also, according to the standard of Iran's Environmental Protection Organization, the permissible limit of nitrate for discharging into surface water is 50 mg/liter and for discharging into water


*a In special cases, epidemiological, social, cultural conditions, and environmental factors should be considered and the guidelines should be modified based on them.<sup>b</sup> Scaris and Nericoris species and hookworms.c During the irrigation period.<sup>d</sup> For public green spaces such as hotels, i.e., where direct public contact is possible, a stricter guideline (less than or equal to 200 coliforms per 100 ml) should be considered.<sup>e</sup> In the case of fruit trees, irrigation should be stopped 2 weeks before fruit picking and no fruit should be collected from the ground. In addition, rain irrigation should not be used.*

#### **Table 5.**

*WHO (1989) Guidelines for the Application of Effluent (Treated Sewage) in Agriculture<sup>a</sup> [14].*


#### **Table 6.**

*USEPA guidelines for wastewater reuse in irrigation [28].*

*Investigating Issues and Problems of Using Sewage Effluent in Agriculture DOI: http://dx.doi.org/10.5772/intechopen.108636*


**Table 7.**

*Allowable concentration standard of some heavy metals for irrigation with wastewater.*

absorbent wells, it is 10 mg/liter. **Table 7**. It shows the permissible concentration standard of some heavy metals (mg/liter) for irrigation with wastewater.
