**4. Results and discussion**

**Tables 1** and **2** show the comparison of the effluent quality of Arak sewage treatment plant with the standard of Iran Environmental Organization regarding reuse in agriculture. According to **Tables 1** and **2**, the average total and fecal coliforms were 878.9 and 379.6 per 100 ml, respectively, with a standard deviation of 17.2 and 11.4. The average parameters of COD and BOD5 were 49.6 and 23.3 mg/liter, respectively, and the standard deviation was 6.12 and 2.19. Therefore, it is below the recommended limits of 1000 total coliforms and 400 fecal coliforms per 100 ml. Parasite eggs in the samples taken from the sewage are less than the standard with an average of 0.5 and a standard deviation of 0.1134. The average of heavy metals cadmium, copper, and lead were 0.0564, 0.08, and 0.513 mg/liter, respectively, and the standard deviation was 0.0147, 0.0361, and 0.0921. It complies with the standard of the environmental organization in this field. The average values of pH, turbidity, and dissolved oxygen parameters in wastewater were 7.55, 21.31 NTU and 5.14 mg/L with standard deviation of 0.304, 10.3, and 1.2, respectively, which are all in


#### **Table 1.**

*Comparison of the quality of Arak wastewater treatment plant effluent with the standard of Iran Environmental Organization.*

accordance with the standard of the Environment Organization. Life has been consistent in these areas. According to **Table 2**, the values of bar, chromium, and oil in the laboratory samples are lower than the standards, and in terms of these parameters, there is no problem in different uses. Also, the average values of the elements in the treated wastewater have been compared with the existing standards (**Figure 3**). The results show that there is no problem in terms of use for agriculture. In **Figure 4**, the comparison between the average values of heavy elements in treated wastewater in two hot and cold seasons has been done, the results show that cold and heat do not have a great effect on the elements. **Figure 5** compares the average values of the elements in the treated wastewater in different seasons. The results show that the average values of the above eight parameters have the least amount of change in the whole year. The same results can be seen in the two parameters of total coliform and fecal coliform. **Figure 6** shows the comparison of the average values of elements in treated wastewater in different months of the year. Except for cadmium, DO, and pH elements, the results show the highest value in July and the lowest value in February. Of course, in the case of the COD parameter, the highest value happened in September.

#### **5. Discussion**

According to **Tables 1, 2** and **Figure 3**, the factors in the effluent are within the appropriate range compared to the existing standards. Therefore, from the chemical point of view, the use of wastewater in agriculture has no special limitations. From the microbial point of view, considering that the products that will be irrigated with wastewater are fodder products or products that cannot be consumed raw by humans,


 **2.** *Comparisonofobservedvalues ofqualityparameters*

 *of wastewater*

 *with existing standards.*

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

#### **Figure 3.**

*Comparison of the average values of the elements in the treated wastewater with the existing standards.*

it can be said that the transmission of pathogenic bacteria is very weak and there is a limit for the use of wastewater in agriculture, green space, and artificial feeding of underground aquifers does not. Considering that the daily flow rate of the effluent from the treatment plant is 1000 liters per second, and the average hydromodulus of *Investigating Issues and Problems of Using Sewage Effluent in Agriculture DOI: http://dx.doi.org/10.5772/intechopen.108636*

**Figure 4.**

*Comparison of the average values of the elements in the treated wastewater in two hot and cold seasons.*

crops in the region is 1.4 liters per second per hectare in the month of August, it is possible to use this water in the cropping season for about 714 hectares of land. He allocated the lands that are barren due to lack of water in the villages around the treatment plant to agriculture and green spaces, and in the non-agricultural season, he used this water for the artificial feeding of the underground aquifers or to the Miqan desert. By artificially feeding the underground aquifers around the desert, the advance of salty water to the underground aquifers of the surrounding villages is prevented. If the effluent is allocated to the irrigation of only one crop, with about 86,400 cubic meters of treated wastewater per day, according to **Table 3**, approximately 4198 hectares of wheat or 1998 hectares of alfalfa or 3679 hectares of clover or He irrigated 16,898 hectares of fodder corn or 18,814 hectares of sunflowers or 30,115 hectares of spruce or 30,586 hectares of walnut trees.

In the application of wastewater for agriculture, the selection of plants should be in accordance with the principles that do not cause contamination of the irrigated crops with pathogenic agents and transfer to the consumer. Therefore, according to the information obtained from the quality of wastewater, compared to the standards of the World Health Organization, the said wastewater is suitable for drip irrigation and tree irrigation, but not suitable for watering sports fields and green spaces of hotels. Also, watering vegetables and products that are consumed raw are not recommended at all. Irrigation of root crops such as potatoes and sugar beets is also not recommended due to direct soil contact. Due to the fact that wheat, barley, and legumes are not consumed directly, they can be cultivated with the effluent of this refinery. It is important that in all cases of application, irrigation with wastewater should be stopped at least 2 weeks before harvest (WHO publication, 2006) [13]. Irrigation of fruit gardens is recommended, but irrigation of fruits that are consumed fresh is not recommended due to the possibility of soil contamination. Trees whose products are consumed as dry fruits such as walnuts and almonds can be suggested. Timber trees such as cypress, pine, elm, and fir can be irrigated with wastewater without restrictions. Also, irrigation of industrial crops such as cotton has no restrictions. Although in this study, the concentration of heavy metals was lower than the recommended standards of the Environmental Protection Organization, but due to the cumulative effect of these elements, the first priority is to irrigate non-edible industrial plants such as cotton and wood trees (WHO publication, 1989) [14].

*Comparison of the average values of the elements in the treated wastewater in different seasons.*

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

**Figure 6.** *Comparison of the average values of the elements in the treated wastewater in different months.*

#### **5.1 Mixing the wastewater of this treatment plant with primary water**

Suitable quality water or primary water can be used directly for product production. On the other hand, raw water mixed with wastewater can be reused. This


#### **Table 3.**

*Water required for the production of some agricultural products.*

combination is possible in two ways: periodic use and mixing. In intermittent use, two water sources are used alternately during the growing season (intra-seasonal intermittent use) or both water sources are used separately during the seasons for different plants (inter-seasonal intermittent use). Choosing a safe option for reuse depends on several important factors, which include: the quality of wastewater, plant resistance to the amount of materials in the wastewater, and the amount of access to freshwater sources. For example, in a place where wastewater is supposed to be used as irrigation water in a cropping season, the important issue is whether the wastewater is used directly or intermittently. The direct use of wastewater is usually done at the farm level without mixing with suitable primary water. The results of the research conducted in India, Pakistan, Central Asia, and Egypt show that surface irrigation with the direct use of wastewater is possible without reducing the yield if the salinity of the wastewater does not exceed the tolerance threshold for the desired plants and the conditions. Drainage should be in good condition. Since plants are more sensitive to salinity in the early stages of growth, according to the research conducted in India, pre-irrigation with water of appropriate quality is of particular importance. In order to obtain a higher yield, it is necessary to pre-irrigate with suitable water and to use wastewater in the subsequent periods of irrigation. Under such conditions, it is possible to use wastewater with a salinity level higher than the plant's tolerance threshold for salinity while preserving the product. Mixing wastewater with water of a suitable quality in such a ratio that the substances in the resulting irrigation water are less than the tolerance threshold of the plant is an acceptable practical method and has been used by many. The option of mixing primary water with wastewater is another management and practical method. It is easy to use because in this method a tank is not needed to mix water from two sources. In addition, many scientists have used good-quality water during the critical stage of plant growth and low-quality water in other growth stages. When the effluent quality is higher than the threshold tolerance value for optimal product production, it can be mixed with other available water sources so that it has an acceptable quality for growing the desired plants. When the mixing operation is carried out at the field level, the water quality can be modified as much as the tolerance limit for the quality of each of the plants. Intermittent use, which is also known as intermittent, is a method that provides the possibility of combined use of suitable wastewater. In this method, wastewater replaces primary water in a predetermined cycle. Intermittent use is used in cases where the effluent

quality has exceeded the tolerance of the plant threshold. The intermittent method of using wastewater can be used intra-seasonally and inter-seasonally.

### **6. Challenges of wastewater application**

#### **6.1 Adverse effects on soil, plant, and public health**

In the reuse of wastewater in agriculture, due to its inherent characteristics and also due to the occurrence of processes such as decomposition of organic substances, ion exchange, oxidation of minerals, sedimentation, filtration, etc., in the soil system, soil properties can be affected and change especially in the long term. Many researchers in different parts of the world have studied and analyzed the effects of wastewater on the physical, chemical, and biological properties of soil. The increase of soil salinity and sodium as a result of irrigation with wastewater, which respectively causes conditions of reduced water availability for plants and destruction of soil structure, has been reported by various researchers [15–17]. Saber during his research on irrigation with wastewater in Cairo showed that with the increase in the years of using wastewater, the amount of dissolved salts in the depth of 0–20 cm of the soil, significantly up to about three times, compared to non-irrigated soils. It has increased [18]. The results of Ebrahimizadeh et al.'s research (1) also indicate that as a result of irrigation with wastewater compared to conventional water, soil salinity in the layers of 20–40 and 40–60 cm and SAR and sodium in the soil at depths of 0–20, 20–40, 40 and 60 cm has had a significant increase [19]. Smart compared the properties of soils in the northern Adelaide area in Australia that were irrigated with water or wastewater, reported that irrigation with wastewater increased the salinity, sodium, and boron content of the soils in the region, although the observed increase was still to a certain extent. It has not affected the performance of agricultural products. But the observed increase in sodium and SAR of the soil is alarming in terms of the destruction of the soil structure and the reduction of its drainage capacity [20]. The study and research conducted on the soils of the Moose Jaw region in the Canadian state of Saskatchewan, which has been irrigated by wastewater in an area of about 1200 hectares since 1982, have shown that the soil salinity has increased significantly so that the average EC Soil has reached from 0.75 to 1.6 in 1997. More salt accumulation is reported in the 1 meter surface layer of the soil. The results of investigations on shallow underground waters in the mentioned area also indicated an increase in sodium, chloride, sulfate, and bicarbonate concentrations. Also, the studies conducted in connection with another big project that started in Swift Current in the same state in 1978 in an area of about 338 hectares indicate that there has been a significant increase in soil salinity in some places. In the recent region, the amount of chlorine, hardness, sodium, sulfate, and manganese in shallow underground water has increased [21]. In his research in Australia, Patterson concluded that the high SAR in the effluent from domestic sewage treatment plants leads to a decrease in the saturated hydraulic conductivity of the soil, so that with the increase of SAR from zero to 3, the hydraulic conductivity The saturation is 50%, and if it increases to 15%, the hydraulic conductivity is reduced by 75% [22]. In Parvan's research report, it has been reported that saturated hydraulic conductivity decreased by 30% in the surface layer of the soil due to long-term irrigation with wastewater [23]. Alizadeh et al. showed in their research that corn irrigation with the treated wastewater of Mashhad city for 2 years resulted in a 156% decrease in the permeability of the soil compared to the time before the beginning of

the research [24]. Shadkam and others also observed a significant decrease in the hydraulic conductivity of the studied soils as a result of irrigation with wastewater [25]. Gholamhossein and Al-Saati in the study of the characteristics of production wastes in Saudi Arabia reported that the use of such wastes due to the inappropriate amount of salinity and sodium can increase the salinity of the soil and change the exchangeable sodium percentage of the soil. Therefore, they recommended the use of wastewater only as auxiliary water [26]. Moadad and Hanifeh Lu in the investigation of the quality of the effluent from the wastewater treatment plant west of Ahvaz city for use in agriculture came to the conclusion that in terms of some parameters such as sulfate, chloride, and salinity, the said effluent exceeded the standards of the Environmental Protection Organization of Iran [27]. And especially in terms of salinity, according to Ayers and Westcott irrigation water quality guidelines, it is evaluated as having a very bad outcome. In the reuse of wastewater, the pH of the soil can also be changed. Since the availability of nutrients required by the plant as well as the solubility of many elements and toxic compounds depends on the pH of the soil, changing this parameter can reduce the absorption of the nutrients required by the plant and in this way or by affecting the availability Toxic elements and compounds affect plant growth and performance [28]. The studies of Saber show that the irrigation of the lands of Cairo city with sewage has led to a decrease in the pH of the soil, but in the investigations carried out by Mahida, an increase in the pH in dry and semiarid soils of India due to irrigation with wastewater was reported. Irrigation with wastewater, especially due to the chlorination process in treatment plants, can increase the concentration of this element in the soil and reach the level of toxicity for plants [29]. Agricultural plants and fruit trees are sensitive to chlorine ion, and if the amount of this element in the saturated soil extract reaches about 10, it causes poisoning for many plants [28]. In his research work on the long-term effects of wastewater, Parvan has reported an increase in the amount of chlorine in different depths of the soil. Among the other impurities that are found in the wastewater of treatment plants, especially in industrial areas, stone metals.
