1. Introduction
Sedimentation, the process by which particles suspended in water settle under the influence of gravity, plays a critical role in numerous industrial processes, including solid-liquid separation in the chemical, mining, pulp and paper, wastewater, food, pharmaceutical, and ceramic industries [1–6]. The behavior of settling particles is influenced by factors such as concentration and flocculation, and can be classified into four primary regimes: discrete non-flocculent settling (Class I), discrete flocculent settling (Class II), zone or hindered settling (Class III), and compressive settling (Class IV) [7, 8].
In mine drainage treatment, the active process typically involves neutralizing acidic drainage with alkaline agents, which precipitates dissolved contaminants into solid form. Over time, the resulting sludge sequentially undergoes flocculent, hindered, and compressive settling behaviors. Mine drainage is characterized by high concentrations of dissolved metals (e.g., Fe, Mn, Al) and sulfates, which significantly influence sedimentation dynamics. The presence of metal hydroxides and sulfate precipitates can alter the flocculation process and result in heterogeneous settling behaviors, leading to variations in particle size distribution, settling rates, and sludge consolidation efficiency [9, 10]. Understanding the specific settling type occurring during this process is crucial for the optimal design of sedimentation equipment, such as continuous clarifiers and thickeners [11].
For high-concentration suspensions like those found in Class III and IV settling, the traditional method for characterizing sediment behavior relies on column experiments, which measure the settling velocity at the interface between sediment and water—a point referred to as the sediment interface in Fig. 1 [12]. However, this approach primarily characterizes the behavior of the entire sediment mass and does not account for spatial variations within the column. Moreover, there are inherent challenges in accurately defining the interface between the clear water and settling particles, particularly at the early stages of sedimentation, which can lead to operator-induced errors [13].
Despite extensive research into sediment characteristics using these classification frameworks [11, 13–21], there remains a critical gap in understanding the localized transition of settling types within the column. The traditional column test setup, which relies on batch sampling, lacks the capability to continuously track changes in sediment characteristics over time and space. Column tests have continued to be widely used in recent studies to evaluate sediment behavior and validate numerical models [2, 22–24].
Recent advancements in continuous sediment behavior monitoring have leveraged advanced sensing technologies to enhance data resolution and accuracy. Some studies have installed cameras in column experimental setups to analyze settling curves and initial settling rates of flocculated sludge through image analysis and software-based evaluations [25, 26]. Other approaches include the development of an on-line settlometer using a scanner to measure sludge settling curves [27]. However, these studies primarily focus on measuring the sediment interface height and do not capture variations within the settling layer. In contrast, some researchers have implemented ultrasonic transceivers to continuously monitor the sediment interface in column settling experiments, applying the results to physical models to predict velocity and sediment concentration changes within the settling layer [28–30]. While this method enables continuous sediment interface measurements, the internal variations within the settling layer are still inferred through physical models rather than directly measured in experiments. To date, no study has experimentally determined sedimentation characteristics within the settling layer itself.
In this study, we developed a novel experimental apparatus by integrating turbidity sensors into the traditional column test setup to continuously monitor sediment behavior. This new approach enabled continuous measurement of sediment characteristics, allowing for the identification of transition points between hindered, transitional, and compressive settling regimes at various locations within the column. By continuously capturing sediment behavior throughout the experiment, we constructed a comprehensive spatial and temporal distribution model of settling types within the sediment layer. This model provides critical insights into sedimentation dynamics, offering a more detailed understanding of sludge treatment processes such as sedimentation tank design and operation and serving as a valuable tool for optimizing engineering systems involved in solid-liquid separation, especially in water and wastewater treatment plants and industrial applications such as settling tank design and operations.
2. Experimental Equipment and Method
2.1. Selection of Measurement Factor
Turbidity and total suspended solids (TSS) are widely used parameters for assessing the sedimentation characteristics of suspended materials in water. Turbidity provides a qualitative evaluation by measuring the optical transparency of water, while TSS offers a quantitative measure, defined as the mass of suspended particles greater than 2 μm per unit volume of water [31, 32]. However, turbidity readings can be prone to overestimation in the presence of dissolved organic matter or sediment deposition, and they are sensitive to variations in ambient lighting conditions [33, 34]. Due to these limitations, TSS was selected as the primary parameter in this study for characterizing sediment behavior. TSS measurement methods include the gravimetric technique, based on EPA standard methods 2540C and 2540D [35, 36], and the photometric method as described by Krawczyk and Gonglewski [37]. The gravimetric approach involves filtering a sample through a 2 μm filter, followed by drying and weighing the retained solids. While this method is highly accurate, it requires extensive sample preparation and drying time, making it unsuitable for continuous monitoring applications. Conversely, the photometric method, as implemented by devices such as HACH’s DR 900, offers rapid and straightforward sample processing, making it more appropriate for this study’s objective of frequent TSS measurement.
Despite the advantages of the photometric method, it remains inherently batch-based, limiting its ability to capture continuous changes in sediment concentration. To address this limitation, we integrated a turbidity sensor into the experimental setup to enable continuous monitoring of sediment concentration in conjunction with batch-type TSS measurements. The continuous TSS estimation was achieved by correlating the TSS values obtained from the photometric method with the turbidity readings at 10-minute intervals. Between sampling points, TSS values were interpolated using the continuous data from the turbidity sensor, providing a continuous profile of sediment concentration throughout the experiment.
2.2. Configuration of Turbidity Measuring Device
The experimental setup included turbidity and temperature sensors, as illustrated in Fig. 2. The turbidity sensor (model SENO189, developed by DFROBOT) and the temperature sensor (model DS18B20, developed by Dallas Semiconductor) were connected to an Arduino Mega microcontroller to facilitate data transmission to an external computer via serial communication. To enable accurate time-stamped data collection and storage, a real-time clock (RTC) module and a micro secure digital (SD) card storage module were integrated into the system, as depicted in Fig. 3. A total of 10 turbidity sensors and 1 temperature sensor were employed in the experimental apparatus.
The microcontroller unit (MCU) was programmed using the Arduino Integrated Development Environment (IDE) software, which enabled the sensors to record and store measurements at predetermined time intervals. This configuration provided reliable, continuous monitoring of turbidity and temperature throughout the experiment.
To validate the performance of the turbidity sensors, a preliminary experiment was conducted to assess the relationship between sensor voltage output and TSS. For this test, varying concentrations of TSS were simulated using solid coffee dissolved in water. The same mixture used for turbidity measurement was also analyzed for TSS. As shown in Fig. 4, the voltage outputs of all 10 turbidity sensors demonstrated a linear decrease with increasing TSS concentration. Despite minor variations in sensor voltage at the same TSS levels, the overall linear trend confirmed that the turbidity sensors used in this study are capable of reliably detecting changes in TSS, making them suitable for continuous monitoring in sedimentation experiments.
Fig. 4 is presented to demonstrate that the turbidity sensor readings exhibit a linear correlation with TSS variations for sediments with a color similar to that of the mine drainage sludge. Instead of applying a predetermined calibration equation, we adopted a dynamic calibration approach. Every 10 minutes, water samples were collected from the column at the sampling tabs, and the corresponding TSS concentrations were measured. New calibration equations were then established based on the paired turbidity reading and TSS value for that interval. This dynamic calibration method was employed to minimize potential errors caused by sensor repeatability and reproducibility issues.
2.3. Configuration of Column Settling Test Equipment
The experimental apparatus, as shown in Fig. 5, consists of a vertical column designed to perform sedimentation experiments on mine drainage sludge using the developed turbidity measuring system. The column is constructed from transparent acrylic, with dimensions of 0.3 m in diameter and 2.0 m in height. A graduated scale is affixed vertically along the column to allow for precise measurement of the settling sediment height.
Turbidity sensors and sampling ports are installed symmetrically on opposite sides of the column. The sampling ports, aligned with the turbidity sensors, facilitate the collection of specimens for TSS measurements. Both the sensors and sampling ports are positioned at predetermined intervals, beginning at 0.05 m from the bottom of the column. The sensors are spaced at intervals of either 0.2 m or 0.4 m, with a total of 10 turbidity sensors and 6 sampling ports. The highest sampling port is situated at 1.8 m from the base of the column.
Additionally, a temperature sensor is fixed 0.2 m from the top of the column to monitor temperature variations in the mine drainage throughout the experiment. The temperature and turbidity sensors are connected to the previously described turbidity-measuring system, which outputs real-time data to a computer display and stores the measurements on a microSD card. The sensors are configured to record data at 1-second intervals, ensuring high temporal resolution for both temperature and turbidity measurements.
The experiments were conducted indoors within the mine drainage treatment facility, ensuring controlled environmental conditions. Temperature was monitored using a thermometer installed in the equipment and remained relatively constant throughout the experimental period, thus its variation was deemed negligible and not included in the results. Humidity was not actively controlled, but the indoor environment of the facility provided relatively stable humidity levels during the experiments.
2.4. Mine Drainage Sediment used in the Experiment
The sediment utilized in this study was sourced from the mine drainage treatment facility at the Ilkwang Mine, located in Busan, South Korea. The geochemical characteristics of the mine drainage are presented in Table 1, with particularly high concentrations of iron (201 ppm) and sulfate (SO44−) (1,050 ppm). The facility temporarily stores incoming mine drainage in a collection tank before transferring it to a pH adjustment tank, where lime is added as a neutralizing agent and mixed via an agitator. The neutralization process within the pH adjustment tank leads to the formation of mine drainage sediment.
Following its generation, the sediment is directed to a settling pond, where it undergoes sedimentation. The clarified water from the settling pond is subsequently discharged through a constructed wetland system. In this study, mine drainage sediment collected directly from the pH adjustment tank was pumped into the experimental column using a submersible pump, allowing for controlled simulation of sedimentation processes.
3. Results and Discussions
3.1. Changes in Sediment Layer and TSS in Vertical Column over Time
Fig. 6 illustrates the progressive changes in the sediment layer within the vertical column over time. As the sediment particles settle under the influence of gravity, a clear separation between the sediment and the overlying water is observed. The settling velocity of the sediment layer gradually decreases over time as the sediment continues to accumulate at the bottom of the column.
The results of TSS measurements, taken every 10 minutes from the sampling ports located at various heights within the column, are presented in Fig. S1. The legend indicates the detention time elapsed after filling the column with mine drainage. When a neutralizing agent is introduced into the low-pH mine drainage, dissolved metal components precipitate as flocculent particles [38]. As these flocs (Class II) settle, the sedimentation process transitions from flocculent settling (Class II) to hindered settling (Class III), and ultimately to compressive settling (Class IV) [39]. The transition point from Class II to Class III varies depending on TSS concentration and the degree of flocculation, with typical TSS values ranging between 600 and 5,500 ppm [8, 40]. In this study, the initial average TSS of the mine drainage sediment was 697 ppm, placing it near the boundary between Class II and Class III settling.
Upon injection of the mine drainage into the vertical column, TSS values were initially uniform throughout the column. Over time, however, the TSS at the top of the column decreased, while TSS at the bottom increased as the sediment progressively settled downward. This behavior reflects the reduction in sediment concentration at the top and the accumulation of sediment at the bottom as the settling process proceeds.
Interestingly, TSS levels in the middle section of the column remained relatively constant for a period before eventually declining. This indicates that the sediment from the middle of the column flowed downward at a rate that matched the inflow of sediment from above, maintaining a stable concentration. This behavior suggests that the middle section of the column exhibited characteristics of hindered settling, where the settling velocity was impeded by the collective movement of sediment particles.
The transition from hindered settling to transition settling in mine drainage sludge occurs when the collective motion of particles slows due to increased interparticle interactions and the formation of a denser sediment network. As settling progresses, compressive forces dominate in the lower layers, leading to the transition from transition settling to compressive settling. These transitions are influenced by factors such as particle size distribution, ionic strength, and the presence of organic matter, which affect flocculation and sediment consolidation [41].
3.2. Continuous Change in TSS by Location of Turbidity Sensors in Vertical Column
Fig. S2 presents the results of continuous TSS measurements at various turbidity sensor locations within the vertical column over time. The legend indicates the height of each sensor from the bottom of the column. Temperature data were excluded from the analysis as temperature variations in the mine drainage were negligible during the experiment and had minimal impact on sediment behavior.
3.3. Changes in Settling Type by Location of Turbidity Sensors
The TSS trend observed at the 0.2 m measurement point (Fig. S2) remains unchanged for the first 8 minutes, after which it begins to increase at a steady rate. After 32 minutes, the rate of TSS increase begins to decline. A similar trend is observed at the 0.4 m measurement point. These trends can be correlated with the three primary sediment settling types: hindered settling (HS), transition settling (TS), and compression settling (CS). Specifically, at the 0.2 m measurement point, the data initially show hindered settling (no change in TSS) during the first 8 minutes, followed by transition settling (a steady increase in TSS) from 8 to 32 minutes, and finally compression settling (a slower rate of TSS increase) beyond 32 minutes.
The behavior at the 0.4 m measurement point follows a similar pattern, with settling type transitions occurring at approximately 27 minutes (HS to TS) and 53 minutes (TS to CS). The sharp decline in TSS observed after 103 minutes corresponds to the sediment interface passing the 0.4 m measurement point.
At the 0.6 m measurement point, the transition from HS to TS occurs at 50 minutes, but the TSS decreases rapidly before transitioning to CS due to the sediment interface moving past this point. Therefore, only HS and TS were observed at this location.
For sensors positioned at heights above 0.8 m, the TSS remained constant before decreasing, as these sections only exhibited hindered settling, and the sediment interface passed these points before transitioning to TS.
Notably, the slope of TSS increase at the 0.2 m measurement point during CS is steeper than at the 0.4 m point. This is attributed to the greater compression of sediment at the base of the column, where the load from the overlying sediment increases the TSS.
In summary, HS was observed at the top of the column, HS and TS in the middle, and all three settling types (HS, TS, and CS) at the bottom. Below 0.2 m, it is anticipated that a section may exist where only TS and CS occur, without HS. This indicates that the sediment settling type varies with column height.
In Fig. S2, the period during which the sediment displays hindered settling is marked by a constant TSS across all measurement points. As noted earlier, the classification of sediment into Class II or Class III depends on its flocculation state and TSS. For the sediment used in this study, which exhibited hindered settling, the TSS remained constant at an average of 723 ppm. The period of constant TSS serves as an indicator of hindered settling and may be utilized as a reference for classifying sediment types.
3.4. TSS Distribution of Sediment over Time in the Column
Fig. S3 illustrates the TSS distribution map, derived through interpolation of the continuous TSS measurements obtained at each sensor location using the minimum curvature method. This experimental approach allows for the visualization of TSS distribution over time, a feature that has not been effectively demonstrated in previous studies. Initially, the TSS is uniform throughout the column; however, as sediment accumulates at the bottom and the thickness of the sediment layer increases, the region exhibiting constant TSS progressively diminishes.
3.5. Change of Sediment from HS to TS in the Column
This study demonstrates that continuous TSS measurements within the column can identify the timing of sediment settling type transitions. The transition from hindered settling (HS) to transition settling (TS) is referred to as the “switch to TS,” while the transition from TS to compressive settling (CS) is referred to as the “switch to CS.”
Fig. S4 presents the time and height from the column base at which the switch to TS occurs for three measurement points, as shown in Fig. S2. Eq. (1) was derived using linear interpolation (with a coefficient of determination of 0.997) to represent these points. Here, hht is the column height at the switch to TS (m), and t is the detention time (min) following the introduction of mine drainage into the column.
As the sediment continues to settle in the column, it forms three distinct layers from bottom to top: a compressive settling layer, a transition settling layer, and a hindered settling layer. In the early stages of sedimentation, the compressive settling layer is relatively thin, and the effects of compression on the sediment can be neglected. Therefore, sediment accumulates at a constant rate at the bottom, causing the interface between HS and TS to rise steadily. This linear increase in the switch to TS is evident in the initial stages of settling. Because the TSS of the sediment is initially uniform throughout the column, the line representing the switch to TS will not intersect the column height axis but will converge toward the origin, as depicted in the figure.
Fig. S5 overlays the TSS distribution map, the switch to TS line, and the sediment settling curve. Based on earlier work by DallaValle et al. [42], which demonstrated that the TSS of sediment in the hindered settling zone remains constant, the regions of constant TSS on the map can be considered hindered settling zones. The extension of the switch to TS line aligns closely with the end of the straight section of the settling curve, accurately delineating the boundary of the constant TSS region. This confirms that the switch to TS line is a reliable indicator of the transition between HS and TS, validating the method proposed in this study as an effective tool for predicting the transition points between different sedimentation regimes.
3.6. Change of Sediment from TS to CS in the Column
The experimental results indicate two measurement points where the transition from transition settling (TS) to compressive settling (CS) occurs: 0.2 m and 0.4 m (Fig. S2). To better understand the trends in the switch to CS, additional data points are required. At the 0.6 m measurement point, although a switch to TS is observed, the sediment interface passes this point before the transition to CS can occur. Thus, an additional measurement point between 0.4 m and 0.6 m would be necessary to capture the switch to CS more comprehensively.
Eq. (2) represents the linear interpolation of the two existing points where the switch to CS occurs. Here, htc is the height of the column where the switch to CS occurs (m), and t is the detention time (min).
In the early stages of sedimentation, the compressive settling layer at the bottom of the column is thin, causing the upward movement of the TS and HS interface to remain constant. As a result, the switch to CS increases linearly from the bottom of the column during the initial stages of settling. While this linear relationship can be predicted using Eq. (2), as the compressive settling layer thickens over time, the load from the upper sediment compresses the lower layers, slowing the upward movement of the CS-TS interface and causing the switch to CS to deviate from the initial linearity. This nonlinear behavior could not be captured in this experiment due to limited data. However, the potential nonlinear range can be estimated using the settling curve and the interpolation line of the switch to CS.
The intersection of the interpolation line and the settling curve indicates the minimum height (0.56 m) and detention time (70 minutes) for the switch to CS. Fig. S6 (a) illustrates the qualitative classification of settling types by time, and Fig. S6 (b) presents the settling curve and the switch to CS line. Based on these figures, the transition settling range is estimated to occur between 58 and 80 minutes. The detention time of the switch to CS is therefore estimated to lie between 70 and 80 minutes, with the switch to CS trend shown as both a straight line and a curve in Fig. S6 (b).
3.7. Schematic Model for Settling Type Distribution of Sediment in Column
Fig. S7 displays the trend lines for both the switch to TS and the switch to CS on the settling curve, along with the distribution of TSS in each zone. Since the boundaries of the settling types are derived from interpolating a limited number of measurements, they may not fully capture the precise boundaries of all settling types. A more accurate representation would require additional measurement points, particularly in sections of the column where transitions between settling types occur.
The simultaneous presentation of the TSS distribution and settling type boundaries, as shown in Fig. S7, provides valuable insight into how TSS and settling behavior evolve over time. This figure allows for the determination of both the settling type and the time at which the transition between settling types occurs at any point within the vertical column. Additionally, the TSS at all positions in the column can be estimated based on detention time. Along the settling curve, sediment to the left of the switch to TS line represents hindered settling, while sediment between the switch to TS and switch to CS lines undergoes transition settling. To the right of the switch to CS line, sediment is compressively settled.
Examining the changes in settling types within the column, Fig. S7 shows that at a height of 0.75 m, the sediment exhibits only hindered settling, maintaining a constant TSS of 723 ppm. After 65.2 minutes, the sediment interface passes this point, and the TSS begins to decrease rapidly. The sediment at a height of 0.68 m undergoes the longest period of hindered settling (58 minutes), representing the maximum height where only HS is present. Between 0.68 m and 0.5 m, the sediment undergoes both hindered and transition settling. At a height of 0.5 m, the sediment experiences 38 minutes of hindered settling followed by 42 minutes of transition settling. Sediment between 0.5 m and 0.13 m undergoes all three settling types: hindered, transition, and compressive settling.
By examining the sediment behavior in the column, Fig. S7 shows that only hindered and transition settling layers are present before 11 minutes of detention time. After this time, a compressive settling layer forms at the bottom, resulting in three distinct layers: hindered, transition, and compressive settling. A consistent height difference of 0.236 m (the sum of the intercepts from Eqs. (1), (2)) is observed between the switch to TS and switch to CS in the linear section of Fig. S7. This indicates that as the compression layer at the bottom increases linearly due to sedimentation, the transition settling layer rises upward while maintaining a constant thickness. As sediment accumulates, the boundaries between settling types shift upward. Notably, the slopes of the switch to TS and switch to CS lines in Eqs. (1), (2) are identical at 0.0095 m/min, signifying that the upward movement speeds of these boundaries remain constant. This suggests that the sediment settling types in the column progress upward at a constant speed of 0.0095 m/min, with the transition settling layer rising while maintaining a stable thickness of 0.236 m.
The schematic model derived in this study encapsulates the various behaviors of sediment in the column and can be applied to understand sediment dynamics and optimize sludge treatment in various engineering fields.
3.8. Changes in Sediment Interface according to Settling Type
When mine drainage containing sediment is introduced into the vertical column, the sediment particles settle downward, gradually forming a distinct interface between the water and sediment layers. Fig. S8 presents a series of photographs of the sediment interface over time, alongside the corresponding settling curve. Initially, the sediment interface appears diffuse, with water and sediment intermingling and no clear separation. Over time, however, the interface becomes progressively thinner and more defined.
This behavior can also be observed in the TSS trends presented in Fig. S2. At the 1.8 m measurement point, located near the top of the column, TSS gradually decreases during the first 10 minutes, which corresponds to the period when the sediment interface passes this location. The gradual reduction in TSS is attributed to the initial thickness of the sediment interface, causing changes in TSS to occur incrementally as the interface passes the measurement point.
As the sediment continues to settle and the interface moves downward, its thickness decreases, and the boundary between water and sediment becomes sharper, as shown in the photographs from 1.6 m to 0.4 m in Fig. S2. This thinning of the sediment interface reflects the transition in settling type over time.
By the 60-minute mark, the sediment interface becomes thin and remains so, and after 70 minutes, it effectively disappears, with a clear boundary forming between the water and sediment. Fig. S8 shows that the disappearance of the sediment interface thickness occurs within the transition settling zone. In this experiment, this point was observed near the intersection of the settling curve and the straight line representing the switch to CS. These findings suggest that changes in settling type can be inferred indirectly and qualitatively by observing the variations in sediment interface thickness over time.
3.9. Considerations for Evaluating TSS using a Turbidity Sensor in Column Test
For TSS evaluation using turbidity sensors in column tests, selecting an appropriate turbidity sensor is crucial. While I utilized a general-purpose, lower-precision turbidity meter, it proved adequate for our study due to the significant TSS concentration variations characteristic of mine drainage sludge. However, turbidity measurements can be influenced by particle size and color. To mitigate potential errors from these factors and ensure reliable TSS monitoring, we employed a dynamic calibration approach, as detailed previously. For experiments with smaller TSS concentration ranges or differing sediment optical properties, more sophisticated turbidity sensors less sensitive to particle characteristics, or alternative direct TSS measurement methods, may be necessary to enhance accuracy.
To achieve more accurate and precise measurements of TSS distribution within a settling column, it is necessary to install a greater number of sampling tabs at closer intervals along the column height and to sample the mixture at shorter time intervals. However, this approach increases the volume of mixture removed during sampling, leading to a progressive reduction in the total volume of mixture remaining in the column. If the sampled volume is significant relative to the column volume, it can potentially influence the experimental results. To mitigate this issue, increasing the column diameter is advisable. A larger diameter minimizes the proportional reduction of the mixture within the column due to sampling, thereby ensuring that the experimental results remain representative and are less affected by the sampling process itself.
4. Conclusions
This study developed an experimental apparatus and methodology capable of continuously measuring the sedimentation characteristics of mine drainage sediments within a vertical column. Through continuous measurement of total suspended solids (TSS) at multiple points within the column, the temporal and spatial distribution of TSS was determined. Furthermore, the transition points for hindered, transition, and compressive settling types were identified, providing a comprehensive understanding of the sedimentation behavior over time. Based on these findings, a schematic model of the sediment settling type distribution was proposed, which allows for the prediction of settling behavior and transitions in sediment types throughout the column.
The proposed model encapsulates the various sedimentation behaviors observed in the column, offering valuable insights for applications in wastewater treatment, mining, and industries that deal with sludge management. The identification of transition points, particularly in hindered settling zones, provides a direct method for optimizing settling tank design. The proposed methodology enables determination of the critical settling depth where the highest settling efficiency occurs, which is crucial for improving sludge removal in full-scale treatment facilities. Additionally, the approach allows for predicting the thickness of sediment layers accumulating over time, providing key data for operational adjustments in settling tanks. To further demonstrate the practical applicability of the proposed model in real-world engineering scenarios, it can be utilized to determine the optimal height of settling tanks based on the identified transition points between settling regimes. Moreover, the model can assist in establishing sludge discharge intervals by predicting sediment accumulation rates and sludge layer thickness. These applications can contribute to enhanced operational efficiency in industrial and environmental sedimentation processes.
By analyzing the sludge concentration distribution within the settling layer, this approach also enhances the understanding of complex sedimentation phenomena occurring inside the sediment interface. For instance, it can be used to validate existing physical models of sludge settling behavior and assess their accuracy under different conditions. Furthermore, the model provides a basis for verifying two-phase flow simulations of water and sediments within the settling layer, aiding in the control and optimization of sedimentation dynamics in large-scale treatment systems.
Although this study provided a robust methodology for determining sediment settling types and transitions, limitations were noted due to the relatively wide intervals between measurement points. This constrained the precision of the data, particularly in identifying the nonlinear transition zones between the switch to transition settling (TS) and the switch to compressive settling (CS). Future research should focus on refining the experimental setup by increasing the spatial resolution of measurements, which would allow for more precise identification of transition points and enhance model accuracy. Additionally, the applicability of the model to different sludge types and environmental conditions should be investigated to ensure broader usability.
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