# Using Wastewater in Irrigation: The Effects on Infiltration Process in a Clayey Soil

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## Abstract

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## 1. Introduction

## 2. Materials: Field Experiments

#### 2.1. Experimental Site

_{3}content. The three plots were subjected to three different TWW irrigation patterns. The first plot was not-irrigated (rain-fed) and is used here as the no-TWW benchmark. The second plot was irrigated with TWW for two years, while the third plot was irrigated with TWW for five years. The TWW used in the experiments was supplied from a wastewater treatment plant located in the JUST campus, which uses rotating biological contactors. More details about water characteristics and irrigation strategies can be found in Gharaibeh et al. [9]. In the following sections, these treatments are referred to as 0 YR, 2 YR, and 5 YR, respectively. The main chemical soil characteristics of the three plots are presented in Table 1.

#### 2.2. Measurements of Hydraulic Properties

_{s}($\mathsf{\Psi}$) is the steady infiltration rate at the fixed tension $\mathsf{\Psi}$, K($\mathsf{\Psi}$) is the unsaturated hydraulic conductivity, r is the infiltrometer radius and α is the sorptive number. Substituting the exponential model of Gardner [12]:

_{s}is the saturated hydraulic conductivity. Equation (3) highlights a linear relationship between ln[q

_{s}($\mathsf{\Psi}$)] and $\mathsf{\Psi}$ with α representing the slope. Linear regression can be therefore used on experimental pairs of ln[q

_{s}($\mathsf{\Psi}$)] and $\mathsf{\Psi}$ for estimating α. The estimate of K

_{s}can be then obtained by Equation (1) for $\mathsf{\Psi}\text{}$= 0:

_{s}is not significantly different among the three considered treatments even though a slightly decreasing trend can be detected with increasing the number of TWW irrigation years. This result suggests that the usage of TWW in irrigation does not alter morphology and connectivity of the largest pores which mainly influences the saturated hydraulic conductivity. However, the estimate of the sorptive number shows a relevant difference in the three plots with values significantly increased for TWW irrigated sites where, as a consequence, the unsaturated hydraulic conductivity expressed through the Gardner [12] model had lower values (see Figure 2). Considering that the sorptive number parameter indicates the relative magnitudes of gravity and capillarity forces during unsaturated flow [14], this outcome suggests a significant reduction of fine pores, that drain water at suction levels < 0 cm, with respect to the total porosity. This evidence was justified by Gharaibeh et al. [9] also with the presence in TWW of both high loads of organic material and suspended solids that tend to settle in the finer soil pore spaces where the flow velocity is lower. Furthermore, the application for long periods of wastewater determined a reduction and disconnection of soil micro- and mesopores leading to a significant drop in hydraulic conductivity of unsaturated soils.

## 3. Methodology

## 4. Results and Discussion

_{s}(Equation (8)). In this context small differences of antecedent soil conditions in terms of ${\mathsf{\theta}}_{\mathrm{s}}-{\mathsf{\theta}}_{\mathrm{i}}$—slightly decreasing from 0YR to 5YR treatments—were observed among the three plots (see Table 3).

## 5. Conclusions

_{s}, and sorptivity S. From infiltration simulations performed by the model under the hypothesis of ponded conditions, applicable during irrigation, a quantitative estimate of TWW usage effects has been carried out.

- The continuous usage of TWW for irrigation determines a lower capacity of water drainage in unsaturated conditions mainly due to the clogging process of the smaller pores by the accumulation of suspended sediments. This leads to a significant decrease of the S parameter in 2YR and 5YR plots, while the saturated hydraulic conductivity linked with the connectivity of larger pores is only weakly affected.
- The simulations by the Philip model with the decreased values of sorptivity have highlighted reductions of cumulative infiltration in a plot with TWW treatment. For an irrigation pattern with a duration of 1.5 h, the reduction of absorbable water amount with respect to 0YR plot has been found equal to 40% and 47% in 2YR and 5YR plots, respectively, with irrigation duration equal to 3 h the percentages reduce to 38% and 44%, respectively. Equivalent increases of runoff have to be expected if the irrigation water amount remains the same. Hence, the percentage reductions of cumulative infiltration have been here interpreted as water amounts that can be saved for that planned irrigation pattern and have been considered a measure of the gained irrigation efficiency.
- The above-defined advantages of TWW usage (in terms of irrigation efficiency) slightly decrease with increasing irrigation duration (30 min up to 180 min) ranging from 50% to 44% and from 48% to 38% for the 5YR and 2YR plots, respectively. Anyway, the irrigation efficiency is significant and can be relevant in arid and semi-arid areas.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**(

**a**) Instantaneous infiltration rates associated with the first replicate; (

**b**) mean cumulative infiltration curves obtained on the three plots with different earlier irrigation treatments (0YR, 2YR, and 5YR) [9].

**Figure 2.**Unsaturated hydraulic conductivity, K, as a function of tension, $\mathsf{\Psi},\text{}$ according to the Gardner (1958) model estimated for the three experimental plots with different irrigation treatments (0YR, 2YR, and 5YR).

**Figure 3.**Cumulative infiltration curves obtained by Philip’s model for the three experimental plots subjected to different irrigation treatments (0YR, 2YR, and 5YR).

**Figure 4.**Reduction of water amount absorbable for periods of irrigation up to 3 h in 2YR and 5YR plots if compared with the benchmark plot (0YR).

**Figure 5.**Irrigation efficiency obtained in 2YR and 5YR plots for periods of irrigation up to 3 h in terms of cumulative infiltration reduction if compared with the benchmark plot (0YR).

**Table 1.**Selected chemical properties for the soil of the three plots: pH, electrical conductivity (EC), organic matter (OM), and cation-exchange capacity (CEC).

Treatment/Plot | pH | EC (dS m ^{−1}) | OM (%) | CEC (cmole _{(+)} kg^{−1}) |
---|---|---|---|---|

0YR | 6.9 | 0.7 | 2.77 | 32.49 |

2YR | 7.7 | 1.68 | 4.37 | 31.16 |

5YR | 7.4 | 2.09 | 7.19 | 33.44 |

**Table 2.**Saturated hydraulic conductivity, K

_{s}, and sorptive number, α, estimated through the procedure described in Section 2.2 and averaged on five replicates of the infiltration tests performed on each plot [9].

Treatment/Plot | K_{s}(cm/h) | α (1/cm) |
---|---|---|

0YR | 2.94 | 0.056 |

2YR | 2.75 | 0.161 |

5YR | 2.69 | 0.212 |

**Table 3.**Philip’s model parameters estimated for the three experimental plots of this study. The difference between saturated soil water content, ${\mathsf{\theta}}_{\mathrm{s}}$, and initial soil water content, ${\mathsf{\theta}}_{\mathrm{i}}$, is also given.

Treatment/Plot | ${\mathsf{\theta}}_{\mathbf{s}}-{\mathsf{\theta}}_{\mathbf{i}}$ | S | A |
---|---|---|---|

(cm/h^{0.5}) | (cm/h) | ||

0 YR | 0.49 | 6.84 | 1.18 |

2 YR | 0.42 | 3.61 | 1.10 |

5 YR | 0.41 | 3.08 | 1.08 |

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**MDPI and ACS Style**

Albalasmeh, A.A.; Gharaibeh, M.A.; Alghzawi, M.Z.; Morbidelli, R.; Saltalippi, C.; Ghezzehei, T.A.; Flammini, A.
Using Wastewater in Irrigation: The Effects on Infiltration Process in a Clayey Soil. *Water* **2020**, *12*, 968.
https://doi.org/10.3390/w12040968

**AMA Style**

Albalasmeh AA, Gharaibeh MA, Alghzawi MZ, Morbidelli R, Saltalippi C, Ghezzehei TA, Flammini A.
Using Wastewater in Irrigation: The Effects on Infiltration Process in a Clayey Soil. *Water*. 2020; 12(4):968.
https://doi.org/10.3390/w12040968

**Chicago/Turabian Style**

Albalasmeh, Ammar A., Mamoun A. Gharaibeh, Ma’in Z. Alghzawi, Renato Morbidelli, Carla Saltalippi, Teamrat A. Ghezzehei, and Alessia Flammini.
2020. "Using Wastewater in Irrigation: The Effects on Infiltration Process in a Clayey Soil" *Water* 12, no. 4: 968.
https://doi.org/10.3390/w12040968