Investigation of the effect of treated wastewater injection on the permeability of unsaturated and saturated porous media in the aquifer storage and recovery system

Extended Abstract: The permeability of porous media due to clogging of pores is one of the problems of aquifer storage and recovery (ASR) systems. The more pore clogging occurs when treated wastewater is used as water resources for ASR In addition to physical clogging, the biological clogging also plays an important role in reducing the permeability and hydraulic conductivity of the porous media. In most of previous studies, the infiltration and clogging of the unsaturated zone have been evaluated by measuring the input-output flow from the soil columns. In this study, the permeability and hydraulic conductivity variations due to the passage of treated wastewater through the unsaturated and saturated zone have been evaluated simultaneously. The main goal of this study is the investigation of permeability and clogging variations in unsaturated-saturated zones in the aquifer storage and recovery system using the treated wastewater. For this study, an experimental model was designed with 2.5 m vertical height (unsaturated layer) and 12.5 m horizontal length (saturated layer). It was made with a PVC pipe with a diameter of 200 mm. The input-output flow rates had been measured for a period of 70 days. The reduction of inlet and outlet flow is due to physical and biological clogging of soil pores. The physical clogging usually occurs earlier and in the early parts of the model and then there is a gradual decrease of infiltration velocity and hydraulic conductivity. The rate of increase of biological clogging is slower than physical and with the growth of bacteria, its amount increases to a constant rate. Then, as the bacterial population decreases, the flow rate in the porous media increases and results in a temporary increase in permeability and outlet flow rate. The bacterial growth cycle in a closed environment consists of four stages. This growth pattern corresponds to the fluctuations of the discharge output from the end of the setup. In the first stage (lag phase) when the bacterial population is the smallest, the output discharge is maximum. Then, entering the second stage (log phase), the bacterial population increases up to the maximum. With this increase in growth, the output discharge is reduced to a minimum. After that, the bacterial population enters the third stage (stationary phase) and their population remains constant, and the output discharge in this stage is also almost constant. Then the growth of bacteria enters the fourth stage (death phase) and some of the bacteria die to regain balance and the output flow increases to an almost constant value. This bacterial growth cycle and discharge output continues. In fact, what causes biological clogging is the activity of bacteria. The gases produced by their activity clogged some of the pores of the porous medium. The nitrate concentration decreases to some extent as the treated wastewater passes through the unsaturated soil. Then, as it continues to move in the saturation zone, its concentration decreases much more and at a distance of 7 meters from the beginning of the setup, its value reaches less than 0.5 mg / liter and this concentration is almost constant at the end of the path. The main reason for the large decrease in nitrate concentration is due to denitrification phenomenon. This is also hydraulically justified by the height of the water inside the piezometers along the flow path. The hydraulic head had many fluctuations in piezometers, which are largely proportional to the output flow fluctuations. The quantitative (inlet and outlet flow and pressure) and qualitative (nitrate concentration) measurements on the model indicate the types of clogging in the porous media. The output flow of the experimental model after two days from the start of injection reached its maximum value of about 6.7 liters per day and after 6 days began to decrease to about 2.6-2.1 liters per day and had a variation of the same range for about 30 days. Then, its amount has been increased to 4.1 liters per day for 6 days and decreased to 2.1 liters per day in 70 days after the injection. The hydraulic conductivity of the soil also changes in proportion to the changes in the output flow. Before injecting the treated wastewater into the soil column, the amount was 1.32 meters per day and gradually decreased to 0.47 meters per day. The maximum and minimum soil permeability is 14.8 and 4.33 cm per day, respectively. After the injection of treated wastewater, over time, part of the pores of the porous medium is clogged for physical, chemical and biological reasons and reduces the permeability. This permeability reduction can initially be up to 70%, which is the simultaneous effect of three factors of physical, chemical and biological clogging, but with the entry of bacteria into the fourth phase, the effect of biological clogging decreases and the permeability increases so that the penetration rate is 35% less than its original value. If the clogging of the pores is physical, the reduction of permeability and hydraulic conductivity becomes almost permanent, but if the clogging is biological, the reduction of permeability and hydraulic conductivity is temporary. Therefore, a cycle of biological clogging changes in the treated wastewater injection system and using the dry and wet interval periods in accordance with this cycle, the performance of injection ponds can be significantly increased in terms of quantity and quality.