Design and manufacture of the experimental apparatus
In order to consider only the characteristics of the flushing device, the same characteristics of the limestone layer were ensured by using glass beads of the same shape and size; further, this aspect with regard to the sediment was ensured by using sediment having the same density, amount, and the origin of its production, in the experiment. In addition, to establish the same flow characteristics as the SAPS, the conditions of natural drainage under a constant head without an inflow rate were used in the experiments.
A schematic diagram of the experimental apparatus is shown in Fig. 2. The dimensions of the apparatus are as follows: 1 m length, 0.5 m width, and 1.2 m height. Several pipe connections and valves were installed in the tank. The pipe connection is a device that facilitates the changing of pipes, and three of these were installed in the tank at a spacing of 0.33 m; on/off valves were also installed. On the right side of the tank, a valve for drainage was installed, and a transparent plate and a scale were installed to check the water level and observe the flow situation in the tank.
To simulate the limestone layer of a SAPS with glass beads, a glass bead layer with an average thickness of 19 cm was laid above the pipe. The glass beads had a spherical shape and were 20 mm in diameter, as shown in Fig. 3. The porosity of the glass bead layer was found to be an average of 39.0%, which is similar to the porosity of 39.70%±1.24% in orthorhombic packing (Alberts 2005).
In this study, three cases were set for each factor to understand the effect of orifice size and spacing and pipe diameter on the flushing effect. The pipe used in the experiment was a commercial polyvinyl chloride (PVC) PVC pipe with a diameter of 16 mm, 50 mm, and 50 mm. The diameter of the orifice was 3 mm, 6 mm, and 9 mm, and the orifice spacing (OS) was 100 mm, 200 mm, and 300 mm.
The sediment used in the experiment was the mine drainage sediment produced at the Hwangji-Yoochang passive treatment facility located in Samcheok city, South Korea. The main component of this sediment is iron oxide, and the dry density of the sediment is 3.85 g/cm3. The photograph of the sediment and the particle size distribution of the sediment used in the experiment are shown in Fig. 3 (a) and Fig. 3 (b). The density of the sediment was measured using pycnometers with capacities of 25 ml and 50 ml, and the particle size distribution of the sediment was measured with Malvern Mastersizer 3000. For each experiment, the same dry weight of sediment was mixed with water and put into the water-filled tank, and the experiment was carried out after the sediment had completely settled. By this process, the sediment was uniformly distributed within the layer of glass beads (Fig. 3 (c)).
Experimental method
Flushing involves the installation of a network of pipes with orifices in the limestone layer of the SAPS, which is filled with water at a constant height, and the removal of the sediment that accumulates in the limestone layer using water by rapidly opening the valve connected to the pipe. The experimental procedure was as follows.
- Install a pipe with the orifice facing upward at the bottom of the tank, and lay glass beads in a layer 19 cm thick.
- Close all valves in the tank and fill it with water to 1.03 m.
- Mix 740 g dry weight of sediment with water and evenly sprinkle it on the surface of the water.
- Wait for the sediment in the tank to settle completely to the bottom.
- Place the bucket in the discharge area. Open the valve rapidly and close the valve when the water level in the tank reaches 0.95 m.
- Measure the volume of the mixture recovered in the bucket, and then weigh only the sediment.
- Repeat the above procedure by varying the spacing of the orifices and the pipe diameter.
The amount of the recovered sediment was evaluated by measuring its weight after drying it in a dryer at 80 °C.
During this experiment, the discharge flow rate was first checked for consistency with the existing theoretical models, such as Bernoulli formula and Blake–Kozeny formula, by draining only the water in the tank, and then the experiment from step 3 was performed after putting the sediment in the tank.
Evaluation of water flow rate in limestone layer
The limestone layer in a SAPS resists the flow of mine drainage. The magnitude of the flow resistance in the limestone layer can be assessed indirectly using the discharge flow rate when flushing is performed. Therefore, it is important to check the discharge flow rate by flushing under a certain water head condition. In this study, it was relatively easy to predict the discharge flow rate because glass beads of spherical shape and uniform size were used instead of limestone (which has particles with irregular shapes and of various sizes).
The flow velocity in a pipe installed at the bottom of the tank filled with water to a constant height is generally expressed by the Bernoulli equation, as shown in Equation 1.
See equations 1 and 2 in the supplementary files.
Conceptual suggestion for the influence radius of orifice
The velocity distribution around the orifice is important in removing sediment by flushing. This is because if the magnitude of the flow velocity around the orifice is large enough to move the sediment, the sediment can escape out of the pipe with the water, and if the velocity is small, the sediment remains stationary.
Singh et al. (2020) studied the flow phenomenon that occurs when a pipe sucks in the surrounding fluid, which is similar to the phenomenon of an orifice sucking water in a limestone bed. They presented the velocity distribution when a small pipe sucks in the surrounding fluid, as shown in Fig. 4. If the Reynolds number (Re) is small (Fig. 4 (b)), the volume representing the same flow velocity shows a long ellipsoid in the direction of flow; when the Reynolds number is large (Fig. 4 (a)), the ellipsoid is distorted perpendicular to the direction of flow.
In the initial stage of flushing, the flow velocity is high due to the high water head above the orifice; it decreases over time as the head is lowered. Thus, the velocity distribution around the orifice varies over time but can be assumed to be close to a sphere on average. This velocity distribution shape has the same shape if the porous medium is homogeneous. Based on this, this study assumes that the volume where sediment is removed by the orifice during flushing is in the shape of a sphere.
If sediment is uniformly distributed in the glass bead layer, the volume of sediment removed by an orifice can be calculated from the amount of sediment removed by flushing. In this study, the influence radius of orifice Rl refers to the radius of the sphere when the volume of sediment removed from the porous medium by one orifice is assumed to take a spherical shape, which allows an evaluation of the effect of flushing. It is described by the formula as follows.
See equation 4 in the supplementary files.
However, depending on the flow conditions, such as the hydrostatic head, inflow rate, and orifice diameter (OD), the shape of the velocity distribution around an orifice during flushing may not take the shape of a sphere.