1. Introduction
The growing global demand for wastewater treatment is driven by the urgent need to mitigate the adverse effects of pollutants such as organic matter (referred to as biochemical oxygen demand or BOD), nutrients (especially nitrogen and phosphorus), and suspended solids (SS) on aquatic ecosystems [1]. Discharge of untreated or inadequately treated wastewater into natural water bodies can accelerate the eutrophication of rivers and lakes, which in turn reduces oxygen levels, harming aquatic life [2]. To address these issues wastewater is subjected to a series of treatment processes to ensure it meets environmental standards and does not harm ecosystems after discharging. Among these treatment process, SBR stands out for its operational versatility and effectiveness in treating wastewater. SBR technology is cost-effective, generally resulting in lower overall expenses compared to traditional methods. It is also space-efficient, requiring less land area. The system handles wide fluctuations in flow and load effectively. Additionally, SBR technology offers better control over filamentous growth and settling issues, simplifying maintenance with fewer components. It demands less frequent monitoring from operators, providing greater operational flexibility. Biomass is retained within the system, preventing washout. Settling problems can be identified and corrected quickly. The technology can also incorporate Powdered Activated Carbon (PAC) to enhance treatment performance [3]. SBR faces various challenges in wastewater treatment, during the settling and decanting phases. Settling problems such as sludge bulking and insufficient settling time can lead to the deterioration of wastewater quality for especially SS. Hydraulic overload especially during heavy rainfall, further complicates the settling process [4,5,6,7]. Decanting problems including floating sediment, resuspended solids, and suboptimal decanter design, directly affect the quality of purified water [8,9]. Applying or adapting industrial methods from other fields can enhance the design of existing decanters, leading to improved suspended solids (SS) removal efficiency during decanting process [10].
Building on the momentum for modernizing SBR systems, integrating cyclone separators as a novel approach in the decanting phase presents a promising technology for enhancing wastewater treatment processes. During the sedimentation phase allows suspended solids to settle at the bottom of the reactor, while lighter solids may float or remain suspended. Floating solids can be managed through skimming and proper tank design. Hydro cyclones offer an additional method for enhancing solids removal by using centrifugal force to settle and seperate remaining suspended solids during water is discharge thru the hydro-cyclone from the SBR. This combination of discharging process thru the hydro-cyclone treatment ensures high-quality effluent and effective management of suspended solids.
This study focuses on application a hydro-cyclone decanter for SBR to enhance the separation of SS from liquids, leveraging the centrifugal forces of cyclonic systems. The design aims to optimize solid-liquid separation without the need for extensive modifications to existing treatment processes. By improving the water inflow method at the top of the SBR, potentially through the use of floating devices, the sedimentation process can be significantly enhanced. This method is intended to complement existing wastewater treatment operations, optimize water quality, and SS separation during top flow discharge. The results of this experiment will be able to provide applicable data for optimizing SBR operations in the pilot scales.
2. Materials and methods
2.1. Hydro-cyclone design
Figure 1(a), the reactor is designed with a height of 1.75 meters and has a diameter of 0.50 m and a volume of roughly 0.34. Figure 1(b), elucidates the hydro-cyclone separator, a key part of this system, has an inlet and outlets (both top and bottom) with a width of 2.5 cm each and stands at a height of 18.5 cm. This decanter is placed at the top of the reactor. It uses a cyclone shape to use centrifugal force to clean the water by effectively separating and settling particles. The design includes a bottom mechanism for collecting settled particles and an effluent pipe that can be automatically positioned with external power, supported by vertical structures for steady movement, aiming for efficient removal of SS and better water cleanliness. The design includes a bottom mechanism for collecting particles and an effluent pipe that can be automatically positioned with external power, supported by vertical structures for steady movement, aiming for efficient removal of SS and better water cleanliness. The hydro-cyclone was designed based on a thorough review of published researches on cyclonic separators. The optimal shape and dimensions for hydro-cyclone were determined from deep literature analysis and the separation efficiency of the designed hydro-cyclone analyzed by using a Computational Fluid Dynamics (CFD) simulations [11,12,13].
2.2. SBR operation and waste water preparation.
The experiments were conducted to identify the operation conditions of lab-scale SBR, the main goal was to understand does the lab-scale SBR imitate the actual SBR and to evaluate the effectiveness of the SBR in controlling nitrification and denitrification processes. The total volume processed was 130 L, with 30 L drawn off and replaced with 30 L of feeding volume (23%). The total experiment time was about 260 min. The influent concentrations were COD: 200 mg/L, T-N: 25 mg/L, and T-P: 5 mg/L. Suspended solid (SS) concentrations were adjusted to different concentrations depending on the purpose of each experiment.
To imitate suspended solids in waste water 20 g of grinded clay (silver/milky color) used with 1L distilled water. Silver clay particle size up to 350 µm, while milky clay is composed of particles with a diameter of 160 µm distributed in smaller sizes compared to silver clay. After stirring for about 20 min, the resulting mixture closely resembled common substances found in wastewater, making it an ideal test sample. Actual wastewater (1 L) was stirred thoroughly and then utilized for the experiment. The actual wastewater was collected at a wastewater treatment plant of Uijeongbu Water Reclamation Center.
2.3. CFD analysis
The design of the decanter based on our design analyzed by the computational fluid dynamics software COMSOL Multiphysics 6.1 (USA). The simulation utilized the Laminar Flow (Navier-Stokes model) and Particle Tracing physics modules to model the behavior of particles within the reactor. The flow rate was varied from 5 L/min to 40 L/min, while particle sizes ranged from 100 to 800 μm with a particle density of 1800 kg/m³. drag force calculations based on Stokes' law show how the particles interact with the fluid flow and hydro-cyclone included to analyze how it moves in its environment. The primary objective of CFD analysis was to identify an optimal pump location to achieve suitable outflow patterns and bottom outlet condition to escape the settled sludge wake up .
In practical operation, the SBR incorporates a pump and a transformer at the decanter's effluent outlet. This setup allows precise control over the flow rate passing through the decanter, ensuring that the system can handle varying operational conditions and maintain high suspended solids (SS) removal efficiency.
2.4. Statistical analysis and COD/TN/TP experiments
Regression analysis using SigmaPlot version 10 (Systat Software Inc., Germany) was conducted to examine the correlation between flow rate and removal efficiency of hydro-cyclone decanter, employing a hyperbola model, single rectangular hyperbola model I, and a three-parameter model. The results suggest that modifying the flow rate can improve particle treatment in the SBR. Additional experiments were carried out according to the Standard Method [14] to measure T-N, T-P, and COD levels in wastewater samples. These measurements were aimed at assessing particle treatment effectiveness and determining the extent of pollutant removal achieved in the SBR. The particle size distribution of each sample was measured by a particle size analyzer (nanoSAQLA, Japan).
2.5. Operation conditions
To simulate colloidal SS in the reactor, 1L of the synthetic wastewater was introduced and allowed to settle in the reactor for 2 hrs before operating the cyclone decanter. Influent water was collected through the upper valve just before activating the decanter, and effluent water was then collected at 30 sec and 45 sec intervals after activation. The effluent flow rate from the decanter was adjusted to 0.60, 1.20, and 2.40 m3/hr (equivalent to 10, 20, and 40 L/min), and the removal efficiency was analyzed based on the measurement of SS. For the experiments with actual wastewater, samples were collected from the aeration tank in the activated sludge process, and the removal rates were measured at different concentrations, including low concentration (25 mg/L ~ 86 mg/L) and high concentration (370 mg/L ~ 790 mg/L). The effluent flow rate from the decanter was adjusted in the same manner as in the synthetic wastewater experiments (0.60, 1.20, 2.40 m3/hr), and the particle concentrations in the influent and effluent were measured.
3. Results and Discussions
3.1 Operation of laboratory scale SBR
Figure 2 represents the concentrations of T-N, T-P, and COD concentration throughout the lab-scale SBR operation. T-N initially increases with the addition of ammonia and organic nitrogen in the feeding solution, then decreases significantly under aerobic or anoxic conditions, reflecting efficient nitrification and denitrification. COD peaks early due to the addition of organic matter and gradually decreases, demonstrating effective removal of organic pollutants. The removal efficiencies of COD, T-N, and T-P were calculated as 96.3%, 61.3%, and 62.7% respectively. Thus, the lab-scale SBR effectively regulates nitrification and denitrification processes, demonstrating favorable results in the removal of organic matter and nitrogen-phosphorus compounds. The design was not aimed at maximizing the overall effectiveness of the SBR. Instead, it was designed to be flexible and maximize the performance of the hydro-cyclone.
3.2. Simulation experiment for inlet/outlet model design
The performance of a hydro-cyclone decanter for SBR was evaluated using CFD software four main designs were tested based on where the outflow pump was placed and whether the top and bottom of the decanter were open or closed. The influent flow rate from the decanter was adjusted to 0.60, 1.20, and 2.40 m3/hr. The particle size distribution and density were set from 0.1~100um and 1800kg/m3 respectively for CFD simulation.
The results, shown in Figure 3, revealed that when the supernatant entered from the decanter's right side with both the top and bottom inlets open Figure 3(a), there was a 62.6% removal efficiency at a 2.40 m³/hr inflow. This setup was not chosen for further use because it could cause previously settled sludge to mix back into the water through bottom outflow. In addition, this method was also seen as not cost-effective since it required a much higher rate of supernatant inflow to get the needed outflow when both outlets were open. Closing the bottom outlet Figure 3(b), achieved a 10% removal rate at a 2.40 m³/hr inflow. Pumping out supernatant from the top with both the left and bottom sides of the decanter open Figure 3(c), resulted in virtually no particle removal. However, a setup with the bottom closed and the top open for outflow Figure 3(d), showed that about 40% of particles could be removed.
Therefore, the final design chosen for the cyclone decanter in the lab-scale SBR reactor had the bottom closed and used the top for discharging supernatant. This design was selected because it showed the best results for removing particles from the water with bottom closed outlet.
3.3. Decanter operation for the optimal flow rate setting
3.3.1. Estimation of particle removal efficiency using the synthetic wastewater
In laboratory-scale reactor experiments, the effluent flow rate of the decanter was adjusted to 0.60, 1.20, and 2.40 m3/hr to measure the concentration of floating particles in the influent and effluent, calculate the removal efficiency, and analyze the correlation between the removal efficiency of SS and flow rate (or velocity) through regression analysis, as shown in Figure 4(a,b).
The synthetic wastewater was named milky or silver clay, representing the characteristics of the suspended particles in the water. These particles consist mainly of kaolinite clay and were named based on the color of the supernatant when composed as synthetic wastewater. Synthetic wastewater was prepared using clay particles, settled in the reactor for 2 hours, and then the supernatant containing colloidal suspended particles was introduced into the cyclone decanter for discharge. The experiment was repeated four times at the same flow rate, and the optimal flow rate (optimal velocity) of the hydro-cyclone decanter was selected using the average values obtained from each repetition.
As shown in Figure 4, under the experimental conditions, it was observed that as the outlet flow rate approached 1.2 m3/hr, the sedimentation of suspended particles did not increase significantly with increasing flow rate. At the optimal flow rate obtained in this experiment, the removal efficiencies of milky and silver clay particles were approximately 21.3% and 26.9%, respectively. Using an exponential growth curve, the correlation between the calculated removal efficiency and the outlet flow rate was analyzed, yielding R2 values ranging from R2=0.933 to 1.000. The average upper limit removal efficiency of the synthetic wastewater with milky clay was calculated to be approximately 32.3%. The particle removal trend indicates that the decanter is optimized for particles larger than 100µm, while smaller particles (less than 20µm) were less effectively segregated. This suggests that the hydro-cyclone's efficiency is limited by particle size, which is expected given the principle of operation relying on centrifugal forces.
To ascertain the pattern of particle diameter removal, the particle size distributions of the inlet and outlet streams of the decanter were analyzed. The optimal control point in this experiment was identified as the point where the removal efficiency began to converge to its maximum value, following the outlined methodology. It was observed that adjusting the outlet flow rate to approximately 1.20m3/hr resulted in convergence to the maximum value. This observation points toward the need for optimizing flow rates in practical applications, especially when dealing with variable particle sizes.
To analyze particle removal efficiency through the hydro-cyclonic decanter, the particle size distribution was measured in the influent and effluent of the decanter while operating at a flow rate of 2.4m3/hr (see Figure 5). In both types of clay-containing supernatant, the average and median particle sizes decreased in the effluent compared to the influent, indicating that larger particles were removed by the decanter. Since the cyclone operates on the principle of using centrifugal force to settle particles, the removal of larger particles was prominently observed. However, considering that particles with a diameter of less than 100 μm may remain in the supernatant (especially in the case of silver clay), it may be worthwhile to consider discharging at a higher flow rate to induce the sedimentation of particles smaller than 100μm.
In terms of particle size distribution Figure 5, it was determined that the removal rate by the decanter was higher for silver clay (Figure 5(a)) compared to milky clay (Figure 5(b)), as the particle distribution of silver clay extends up to 300 μm, relatively larger than that of milky clay. Particularly noteworthy in this experiment is the almost negligible sedimentation of particles smaller than 20μm for each type of clay. In conclusion, the optimal discharge flow rate for the cyclonic decanter was determined to be 1.20 m3/hr, and the estimated removal limit of particle size was around 20μm.
Using the cyclonic decanter, the concentrations of some key water quality parameters in the influent and effluent were measured, and the removal efficiency was calculated accordingly. For T-N, removal ranged from 1%, while for T-P, approximately 1.6% removal on average was observed. COD was reduced by an average of around 11.4%. Considering the characteristics of clay, which typically has low levels of particulate nitrogen and phosphorus, and the poor settling of soluble nitrogen and phosphorus, effective removal by the cyclone decanter was limited.
3.3.2. Estimation of particle removal efficiency using actual wastewater
Experiments with a hydro-cyclone decanter using actual wastewater were simulated by collecting samples from the sedimentation tank at the U Water Reclamation Center and injecting them into a laboratory-scale SBR reactor. The SS concentrations were set at low concentrations (25~86 mg/L) and high concentrations (280~790 mg/L). The decanter's discharge flow rate was adjusted to 0.60, 1.20, and 2.40 m³/hr to measure the concentration of SS in the influent and effluent and calculate the removal efficiency. Regression analysis was conducted to analyze the correlation between flow rate and removal efficiency, as presented in Figure 6(a, b).
In the reactor with prepared wastewater, colloidal suspended particles were introduced through the hydro-cyclone decanter after settling particles for 2 hours. The experiments for low concentration (25 mg/L ~ 86 mg/L) were repeated seven times at the same flow rate, and the optimal flow rate of the cyclone decanter was selected using the average values (Figure 6(a)). An average particle removal rate of 67.9% to 73.2% was observed for the seven experiments, with the maximum achieved at a flow rate of 2.4 m³/hr. The results of the experiments with high concentrations (280~790 mg/L) of wastewater are presented in Figure 6(b). The experiments were repeated seven times at the same flow rate, and the optimal flow rate of the cyclone decanter was determined using the average values. An average particle removal rate of 63.2% to 70.9% was observed for the seven experiments, with the maximum achieved at a flow rate of 2.4 m³/hr. The removal efficiency pattern was similar to that of the low concentration, showing consistently high removal rates exceeding 70% for both low and high concentrations. The efficiency remained consistently high even as the particle size and concentration increased, further emphasizing the hydro-cyclone's capability to handle diverse wastewater conditions. The study also observed that the decanter performed best for particles larger than 650 µm, with a full removal efficiency beyond this size. For smaller particles, particularly those between 5 µm and 20 µm, the decanter was less effective. This trend suggests that the hydro-cyclone decanter may need to be paired with other technologies or processes to effectively manage finer particles in wastewater treatment.
As shown in Figures 6(a, b) it was observed that as the discharge flow rate approached approximately 1.2 m³/hr, there was not a significant increase in the sedimentation of suspended particles even with an increase in flow rate. The particle removal efficiency at the optimal flow rates obtained in this experiment was approximately 73.2% for low concentration and 70.9% for high concentration. Utilizing an exponential growth curve, the average calculated removal efficiency for each flow rate and its correlation with the discharge flow rate were analyzed, resulting in an R-value of over 0.998.
Considering that suspended particles in actual wastewater are distributed over a wide range of particle sizes, and given the considerable correlation between the removal efficiency of suspended particles by the hydro-cyclone decanter and the particle size distribution, the particle size distribution of the influent and effluent of the decanter was analyzed to confirm the relationship between particle size and removal pattern. The optimal control point in this experiment was identified as the point where the removal efficiency begins to converge toward its maximum value. It was found that this convergence occurred when the discharge flow rate was approximately 2.40m³/hr.
Figure 7 presents a clear view of how particle sizes in wastewater adjust with the concentration levels. It reveals that the particle size predominantly measures around 600 µm in the low concentration (Figure 7(a)). As the concentration of particles in the water increases to the high concentration (Figure 7(b)), the average size of the particles also rises, hitting about 1,200 µm. Despite the escalating sizes of particles in more concentrated wastewater, the hydro-cyclone decanter demonstrates high efficiency, managing to remove a significant amount of these particles. This efficiency suggests that the decanter is capable of performing well under various conditions, including those with higher levels of particulate matter. The main takeaway is that the decanter is better at removing larger particles, specifically those over 650 µm in size, where it reaches full removal efficiency. For smaller particles, particularly those between 5 and 20 µm, the decanter's effectiveness is limited. The most efficient removal occurs at particle sizes beyond 500 µm for the low concentration and 650µm for the high concentration conditions.
In addition to calculating the effective removal diameter using the cyclone decanter for actual wastewater, the concentrations of key water quality parameters in the influent and effluent were measured to calculate the removal efficiency. Three samples were collected for each case, and the removal rates were calculated based on the average values. Approximately 4.5% of T-N was removed, while T-P showed an average removal of about 1.0%. COD exhibited an average removal of approximately 36.5%. For T-N and T-P, given the characteristics of clay, where the content of particulate nitrogen and phosphorus is not high, and the fact that dissolved nitrogen and phosphorus have little sedimentation, effective removal through the application of a cyclone decanter was deemed unlikely. The fact that a certain degree of COD removal was observed indicates that the hydro-cyclone decanter effectively removes particles that contribute to COD from the activated sludge, suggesting that some COD removal can be expected from the use of the cyclone decanter.
4. Conclusion
The study shows how utilizing a hydro-cyclone decanter in SBR improves particle segregation in the decanting phase. It was found that the hydro-cyclone decanter particularly worked well at cleaning synthetic wastewater with different sizes of particles. The highest removal efficiency for silver clay stood at 32.3% while for milky clay 21.3%. Hydro-cyclone decanter showed the highest efficacy with larger particles, those over 100 um, but was less effective for smaller ones, under 20 um. This presents its potential to make solid-liquid separation better for certain sizes of particles. The actual wastewater at various concentrations, the hydro-cyclone decanter showed high efficiency even in different flow rates, the highest efficiency for low and high concentrations were 73.2% and 70.9% respectively of particles. These results highlight the decanter has high performance in segregating particles from wastewater. However, it didn't significantly reduce T-N and T-P levels because they are dissolved in the water, while it moderately improved COD removal, attributed to particulate COD. The study represented the capability of a hydro-cyclone decanter to enhance the separation of particles in the decanting phase of SBR. It suggests that this technology can be integrated effectively into SBR to better segregate particles during wastewater treatment. Further studies are recommended to conduct full-scale experiments assessing the influence of different hydro-cyclone dimensions on particle distribution to expand the use of hydro-cyclone decanters in full-scale SBR systems. Additionally, research should investigate the impact of varied wastewater compositions on flow dynamics in larger reactors.