Treated wastewater quality improvement through the unsaturated and saturated zones in an aquifer storage and recovery system (laboratory scale)

Document Type : Research Paper

Authors

1 PhD Candidate, Department of Irrigation and Reclamation. Eng.Dept. -Faculty of Agriculture- Agricultural and Natural Resources Faculties-University of Tehran-Karaj-Iran

2 Irrigation and Reclamation Eng. Dept. -Faculty of Agriculture- Agricultural and Natural Resources Faculties-University of Tehran-Karaj-Iran

3 Irrigation and Reclamation Engineering Dept.-Faculty of Agriculture- Agricultural and Natural Resources Faculties-University of Tehran-Karaj-Iran

4 Irrigation and Reclamation Eng. Dept.-Faculty of Agriculture- Agricultural and Natural Resources Faculties-University of Tehran-Karaj-Iran

Abstract

The groundwater level decline has caused an environmental issue in most of the Iranian aquifers. In this regard, aquifer storage and recovery (ASR) system can be used as a solution. On the other hand, the huge volume of treated wastewater in big cities can be used as a water source for aquifer storage purpose. In this study, the treated wastewater, soil and groundwater quality have been investigated through the unsaturated-saturated porous media. An experimental model, as L-shaped, has been designed using PVC pipe material with a diameter of 200 mm and a length of 15 m (2.5 m vertical and 12.5 m horizontal). The wastewater used in this research was obtained from the outlet of the sewage treatment plant in the south of Tehran. In order to avoid changes in the quality of wastewater during storage in the tank, it was tried to a regular change the treated wastewater in short intervals. After the initial investigations, a zone situated in center of in the Feshafuye plain, has been selected for treated wastewater injection. Then the required soil for laboratory setup has been collected for tis area. In this regard, firstly, the surface layer soil was removed, then the subsurface soil was transferred to the laboratory for the experimental setup. The required inflow water for this setup was taken from the aquifer of the same area and was changed regularly in order to establish the maximum real conditions. During this study, treated wastewater have been sampled and analyzed at 12 points along the flow path for 60 days. The results showed that the nitrate concentration decreased to less than 0.5 ppm after 7 m and microbial pollutants, total coliform and E. coli had been removed in a 100-cc sample after 6 m. In terms of salinity, the treated wastewater and soil quality have been improved from an EC of 4 to 3.3 dS/m. The BOD and COD have decreased by 81% and 87%, respectively. Due to improvement of treated wastewater quality through the unsaturated- saturated porous media, the ASR system can be implemented as a solution to prevent the groundwater level decline.

Keywords

Treated wastewater quality improvement through the unsaturated and saturated zones in an aquifer storage and recovery system (laboratory scale)

EXTENDED ABSTRACT

 

Introduction

A large amount of treated wastewater is annually produced in greater cities of the world. On the other hand, groundwater overexploitation in most countries, especially in arid and semi-arid zones, causes a huge rate of groundwater drawdown and its quality. In such conditions, the aquifer recovery and storage (ASR) approach can be proposed. As it is done in most wastewater treatment plants, stage 2 is implemented, in this research, the ability of porous media to increase the quality of effluent in the ASR system is investigated. An experimental model in laboratory scale is designed and set up to evaluate the variation in the quality of the effluent in both its unsaturated and saturated zones, which according to our knowledge has not been addressed in previous studies.

Methods and Materials

The treated wastewater, soil and aquifer water of the region, which are used for the experimental model, have been collected and analyzed in the chemical laboratory. The designed experimental model has a vertical height of 2.5 meters and a horizontal length of 12.5 meters, respectively, to simulate the unsaturated and saturated zones, which was made with a PVC pipe with a diameter of 200 mm. In the mentioned model, a hole with a diameter of 6 cm was created at a distance of one meter from the vertical part and on the horizontal part, and a pipe of the same diameter and a height of 2.5 meters was placed inside it; in which the water level can be adjusted. For the treated wastewater and aquifer water injection site, a platform was considered at the top of the vertical part, and a 200-liter tank for wastewater and a 60-liter tank for water were installed on it. A float was used to create a fixed head at the top of the vertical part of the model, where the effluent enters.

Results and Discussion

The results show that after 60 days, the unsaturated-saturated zones showed a good effect to improve the treated wastewater quality. The BOD and COD of the treated wastewater shows a significant decrease, the BOD value has reached less than 2 mg/L in all periods, which is natural in ground water. The unsaturated zone performed well in removing these parameters. Microbial parameters such as Escherichia coli and Escherichia coli, after passing through the unsaturated zone, their value decreased drastically. The amount of salinity of the injected treated wastewater improves the quality of the soil in the unsaturated zone and improves the quality of the aquifer water in the saturated zone. After 60 days of injection, this process continued, and the more time causes more unsaturated quality improvement. The amount of salinity in the saturated zone has increased over time. But its amount is still lower than the salinity of the aquifer water along the way. After passing through the unsaturated zone, the nitrate concentration of the treated wastewater decreased to a great extent and also improved the water quality of the aquifer, so that after a distance of 7 meters from the beginning of the setup, its value was close to zero.

Conclusion

The groundwater level decline and a large volume of treated wastewater in the metropolises are two important issues in different countries of the world, especially in arid and semi-arid regions. In this regard, the aquifer-storage and recovery approach can be a good solution to manage these two problems. In this research, the ability of unsaturated and saturated porous media to increase the quality of wastewater injected into the aquifer has been investigated in a laboratory experimental model. According to the results, a significant increase in the quality of treated wastewater passing through unsaturated and saturated porous media, shows the high efficiency of this system. So, we propose to use our successful method in the ASR project.

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Iranian Journal of Soil and Water Research
Volume 55, Issue 1
April 2024
Pages 131-143

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