Secondary Clarifier Research Articles

Planning, Design and Estimation of Waste Water Treatment Plant Sector 1 of Virat Khand

Abstract: This study's primary goal is to carry out the design of a sewage treatment facility for Lucknow's Virat Khand Sector 1. due to constant domestic and municipal sewage will be produced at a faster rate as the population grows. All species become sick from the foul stench that sewage produces. To prevent this however, appropriate treatment is required prior to disposal on land in order to utilise the treated water and avoid dumping sewage directly into natural resources, which ultimately lowers the demand for fresh water as a whole. Its goal is to generate solid waste and fluid waste that are safe for the environment and can be recycled or disposed of. In a single day, how much sewage was produced overall, based on the anticipated population of Virat Khand Sector 1 over the following 20 years? As a result, the sewage generation in the Virat Khand area is the main topic of this study, and a sewage treatment plant is constructed in accordance with it. It is suggested that the different parts of the sewage treatment plant be designed taking into account the various standards and allowable limits of treated sewage water. Screening, a grit chamber, a main sedimentation tank, a biological reactor, a secondary clarifier, an activated sludge tank, and drying beds are some of the parts of a sewage treatment plant.

The fate of SARS-CoV-2 viral RNA in coastal New England wastewater treatment plants

Municipal sewage carries SARS-CoV-2 viruses shed in the human stool by infected individuals to wastewater treatment plants (WWTPs). It is well-established that increasing prevalence of COVID-19 in a community increases the viral load in its WWTPs. Despite the fact that wastewater treatment facilities serve a critical role in protecting downstream human and environmental health through removal or inactivation of the virus, little is known about the fate of the virus along the treatment train. To assess the efficacy of differing WWTP size and treatment processes in viral RNA removal we quantified two SARS-CoV-2 nucleocapsid (N) biomarkers (N1 and N2) in both liquid and solids phases for multiple treatment train locations from seven coastal New England WWTPs. SARS-CoV-2 biomarkers were commonly detected in the influent, primary treated, and sludge samples (returned activated sludge, waste activated sludge, and digested sludge), and not detected after secondary clarification processes or disinfection. Solid fractions had 470 to 3,700-fold higher concentrations of viral biomarkers than liquid fractions, suggesting considerably higher affinity of the virus for the solid phase. Our findings indicate that a variety of wastewater treatment designs are efficient at achieving high removal of SARS CoV-2 from effluent; however, quantifiable viral RNA was commonly detected in wastewater solids at various points in the facility. This study supports the important role municipal wastewater treatment facilities serve in reducing the discharge of SARS-CoV-2 viral fragments to the environment and highlights the need to better understand the fate of this virus in wastewater solids.

Screening and characterization of bioflocculant isolated from thermotolerant Bacillus sp. ISTVK1 and its application in wastewater treatment

Bioflocculants are extracellular polymeric substances (EPS) that are produced by several microorganisms including bacteria. They find application in wastewater treatment wherein they are employed as flocculating agents for removal of suspended particles during the secondary clarification process. In this study, EPS produced by a previously studied thermotolerant strain Bacillus sp. ISTVK1 was analyzed for its flocculation property. The bacteria was cultured in basal and mineral media and the EPS extracted from the two types of cultures were analyzed for their structure, composition, and flocculation efficiency (%), which was tested on kaolin in terms of parameters like duration, pH, temperature and salts. Both types of EPS showed highest flocculating activity on 6th day, at pH 6, and temperature 32.7 °C and with salt AlCl3. When tested on real wastewater, physico-chemical analysis showed reduction in organics in samples treated with EPS and EPS+AlCl3. Aromatic contaminants like Diisobutyl benzene-1,2-dicarboxylate (Retention Time (R.T.) 22.55) and 2-Phenyl-n,n-bis(1-phenylethyl)-1,1-cyclobutanedicarboxamid (R.T. 44.21) were not detected in the bioflocculant treated samples. The study highlighted the applicability of bacterial EPS as a cost-effective bioflocculant for real wastewater.

Single‐use centrifugal separator enables intensification of the clarification process in biomanufacturing of recombinant proteins

AbstractBACKGROUNDThe need to produce more biologics at lower costs and hence intensify bioprocessing has significantly improved upstream productivity (cell culture titers ≥3 g L−1 and cell densities ≥20 × 106 cells mL−1), shifting the economic and production bottleneck to downstream manufacturing. The first downstream unit operation is the clarification of the cell culture broth to remove various insoluble (such as cells and cell debris) and soluble (such as host cell DNA and proteins) contaminants, producing a feed stream suitable for further downstream purification. The high cell‐density cultures obtained in modern intensified bioprocessing have made legacy single‐use clarification trains (based primarily on depth filtration of the cell culture broth) ineffective and impractical, significantly increasing the economic and material burden of the clarification process.RESULTSWe have developed and characterized approaches that enable intensification of the clarification operation, by increasing process capacities while reducing material requirements and footprint, for single‐use manufacturing facilities. These approaches are based on primary clarification using the Alfa Laval CultureOne™ centrifuge (a continuous solid discharge disc‐stack centrifuge with a single‐use flow path) followed by secondary clarification using single‐use disposable depth filters. We show that these intensified clarification trains are suitable for processing high cell‐density cultures (6–12% packed cell volume) with significantly high yields (≥90%) and impurity clearance, along with reduced operating costs (30–60% savings).CONCLUSIONIncorporating a single‐use continuous discharge centrifuge into the clarification train enables clarification of high cell‐density cultures in single‐use manufacturing facilities and enhances process economic and environmental sustainability. © 2023 Society of Chemical Industry (SCI).

Integration of Pre-Treatment with UF/RO Membrane Process for Waste Water Recovery and Reuse in Agro-Based Pulp and Paper Industry

This recent study aims to evaluate the efficacy of membrane filtration on recovery of water resource from agro-waste such as bagasse, crop-based pulp and paper mill waste. A mini pilot scale membrane system having a combination of pre-treatment filter unit (pre-filter, sediment filter and pre-carbon filter), ultra-filtration and reverse osmosis with spiral wound configuration were employed to evaluate the water reuse efficacy of effluent coming from the secondary clarifier of the conventional treatment plant of the mill. The operational conditions were optimized using Taguchi method at pH 8, temperature 32 °C, and pressure 2 bar and a flow rate of 60 l/hr. The qualities of the effluent from the secondary clarifier, and the permeate from both the combination, viz. Combination 1 (pre-treatment + ultra-filtration) and Combination 2 (pre-treatment + ultra-filtration+ reverse osmosis) were analyzed and the percentage reduction in pH, TDS, TSS, BOD, COD, Color, Lignin, Potassium and Sodium were calculated. The elimination of TDS, COD and BOD with Combination 1 was not promising (<22%). However, the installation of a RO membrane greatly reduced (>88%) the contaminants in both paper mill effluents. The obtained qualities of water from all the combinations were compared with the tolerance standard for reuse as process water. The quality of effluent from the secondary clarifier did not agree with any class of water quality. The permeate from the combination of pre-treatment and UF sufficiently reduced the TSS to reach the requirement. However, the combination of (pre-treatment + UF + RO) adequately complied with the quality standard required for reuse in the making of all grades of paper.

Helmet-Mounted Real-Time Toxic Gas Monitoring and Prevention System for Workers in Confined Places.

Occupational health and safety hazards associated with confined places are mainly caused by exposure to toxic gases and oxygen deficiency. Lack of awareness, inappropriate monitoring, and improper evacuation methods can lead to worker fatalities. Although previous studies have attempted to develop systems to solve this issue, limited research is available on their application in confined places. In this study, a real-time helmet-mounted system was developed to monitor major toxic gases (methane (CH4), hydrogen sulfide (H2S), ammonia (NH3), and carbon monoxide (CO)), oxygen, temperature, and humidity. Workers outside and inside confined spaces receive alerts every second to immediately initiate the rescue operation in the event of a hazard. The test results of a confined environment (wastewater treatment unit) highlighted that concentrations of CH4 and H2S were predominant (13 ppm). Compared to normal atmosphere, CH4 concentration was 122- and 130-fold higher in the landfill and digestion tanks, respectively, while H2S was 36- and 19-fold higher in the primary and secondary clarifiers, respectively. The oxygen content (18.2%) and humidity (33%) were below the minimum required limits. This study will benefit future research to target appropriate toxic gas monitoring and alert workers by studying the existing issues and associated factors in confined places.

Comparison of ultrasound and UV technologies to control bulking and foaming in a wastewater treatment facility. A case study in an industrial park in Morelos, Mexico

Most wastewater treatment plants use activated sludge-based biological systems for this purpose. The latter must effectively remove organic matter and, at the same time, show good sedimentability. However, sometimes there is an excessive proliferation of certain bacteria, giving rise to filamentary swelling, compromising the excellent sedimentability of the sludge. In this sense, the study’s objective was to evaluate the effect of applying two different technologies, the application of low-frequency ultrasound and UV radiation. Some bench-scale experiments were performed using the bulked sludge from the secondary clarifier of a wastewater treatment facility in an industrial park (CIVAC) in Morelos, Mexico, affected by filamentous organisms. Results showed that for the UV application for two, four, and 6 min, the settleability of the mixed liquor suspended solids was not improved; on the other hand, the cavitation effect caused by the ultrasound application demonstrated effective action against the destruction of filamentous organisms. The 10 min condition showed a significant decrease in the filament integrity of the microorganisms and a significant improvement of sedimentation velocity and sludge volume index (SVI) values and settleability of the sludge, but not enough to satisfy national discharge regulations related to total suspended solids in the treated effluent. Molecular identification indicates the presence of the genera Thauera and Brevundimonas in the sludge.

Continuous depth filtration in perfusion cell culture

As the biopharmaceutical industry embraces continuous manufacturing there has been significant progress made in converting standard unit operations from batch to continuous. Cellulose-based depth filters are composed of a complex mix of cellulose fibers, inorganic filter aids, and resins that create wide pore size distributions and multiple modes of separation (e.g. size exclusion, adsorption). They are heavily used in batch processing due to their scalability, footprint, and cost-effectiveness, but have not yet been widely adopted in continuous processing and questions remain on their ability to provide consistent long-term performance. Here, we use a continuous secondary clarification application to investigate depth filter performance over long continuous operation. After initial offline screening, select depth filters were sterilized and connected inline with a CHO cell perfusion bioreactor and run for between one and nine days. We evaluate how fouling changes in continuous operation by fitting fouling models to the differential pressure data. Instantaneous filtrate quality is evaluated over run time and filter loading. Finally, we assess how flux and run duration impact filter capacities and process economics.

Occurrence and removal of psychiatric pharmaceuticals in the Tehran South Municipal Wastewater Treatment Plant.

Psychiatric drugs released by humans in wastewater have received more attention because of their potential risks for aquatic organisms. In this study, the occurrence of the two most common groups of psychiatric drugs (sedatives-hypnotics-anxiolytics and antidepressants) were evaluated in the Tehran South Municipal Wastewater Treatment Plant. All the target sedatives-hypnotics-anxiolytics (alprazolam, phenobarbital, and thioridazine) and antidepressants (fluoxetine, citalopram, sertraline, and venlafaxine) were observed in influent and secondary clarification (SC) effluent. Thioridazine (164.25 ± 218.74ng/L) and citalopram (672.53 ± 938.56ng/L) had the highest mean concentrations in the influent, while alprazolam (5.09 ± 2.33ng/L) and citalopram (776.97 ± 1088.01ng/L) had the highest concentrations in the SC effluent. The higher concentrations of the psychiatric drugs, except thioridazine, were detected in the SC effluent compared to the concentrations in the influent. The increased drugs concentrations, with negative removal efficiencies, were more distinctive in the cold season samples. Psychiatric drugs processed in the chlorination unit followed a completely different pattern compared to the drugs in the biological treatment unit. All the drugs' concentrations, except thioridazine, decreased in the chlorination unit, ranging between 27 ± 14% for alprazolam and 75 ± 10% for citalopram. However, the mean concentrations of the detected drugs were as follows: sertraline (11.96 ± 11.62ng/L) and venlafaxine (184.94 ± 219.74ng/L) which could cause environmental and ecological concerns.

Spatiotemporal characteristics of microplastics in a university wastewater treatment plant: Influence of sudden on-campus population swings

We studied the occurrence and characteristics (sizes, shapes, and polymer compositions) of microplastics (MPs) in a secondary wastewater treatment plant (WWTP) at the University of Mississippi both spatially and temporally. Putative MPs were isolated by sieving, matrix digestion, and density separation, and quantified using stereomicroscopy, with a subset of samples analyzed by Focal Plane Array (FPA) µ-FTIR imaging. In the influent, the highest MP concentration (particles/L ± 1 SD, n = 3) occurred after a football game on campus (62.3 ± 7.6) and the lowest (19.7 ± 2.1) during the summer with little activity on campus. Over 90% of the MPs were removed in the primary treatment. Downstream, MPs were most abundant in the closed loop reactor with concentrations as high as 1962 particles/L. Concentrations in secondary clarifier and final treated effluent were both < 4 particles/L during both normal flow (∼2000 m3/d) and high-flow (>2500 m3/d) periods, and between 16 and 39 particles/L during a low-flow (<1500 m3/d) period. This difference likely stems from changes in plant operations during the low-flow period to support the activated sludge, including longer process times. MPs were mainly composed of polyester, polyethylene, acrylates, polypropylene, polyurethane, polyvinyl chloride, and polystyrene. Fibers were most abundant throughout the system, averaging 61%, followed by fragments (21%), films (13%), and beads and foams (∼5%). Overall, we show that flow rates and treatment times can profoundly influence MPs concentrations in the treated effluent, and that the optimal wastewater treatment conditions also yield the best MP removal efficiency.

Performance of Micropollutant Removal during Wet-Weather Conditions in Advanced Treatment Stages on a Full-Scale WWTP

The reduction of organic micropollutants (OMP) in rivers, lakes and groundwater is an essential legal obligation of the European Water Framework Directive. Since OMP treatment in full-scale wastewater treatment plants (WWTPs) is not state of the art yet, there is little knowledge regarding removal performances, in particular during wet weather. We aimed to contribute to filling this knowledge gap by providing insights from a German case study. On-site measurements were conducted to investigate the impact of rain events on OMP removal with activated carbon processes using powdered activated carbon (PAC) and granular activated carbon (GAC). The study focused on 26 OMPs with different entry paths in the combined sewer system (CSS) and various physico-chemical properties. The monitored OMPs showed higher mass loads during wet weather at all sampling points: effluent of the secondary clarifier, effluent of the PAC treatment stage, and effluent of the GAC filter. As a result of shortened hydraulic retention time (HRT) due to rain events, the overall OMP removal was significantly lower (42% PAC and 46% GAC) than during dry weather (68% PAC and 62% GAC). In order to achieve constant removal rates during all weather conditions, the critical process control parameters are currently being investigated in ongoing studies.

A mathematical model to predict the performance of the secondary clarifier of a municipal wastewater treatment plants

The activated sludge process for urban wastewater treatment has a high purification performance and reliability, especially for organic pollution. However, it presents several operational insufficiencies, such as sludge control, pollution degradation monitoring, and liquid/solid separation problems in the biological reactor/secondary clarifier couple. This paper presents an analysis of a secondary clarifier of wastewater treatment plant (WWTP) via an activated sludge, which could be used for the plant optimization. The models of the settling velocity and the secondary clarifier based on the sludge volume index (SVI) are addressed. We have proposed these models to explain the settling velocity activities and illustrate the relationship between the settleability characteristics. This research aims to develop and predict a one-dimensional model (1-D model) based on an exhaustive experimental study performed on the WWTP and online information, which are the total suspended solids (TSS), the SVI, and the influent characteristics. The results of the novel model for the secondary clarifiers integrating the empirical settling velocity equation presented a good agreement and a better fit with our experimental results, with an error of approximately zero. This simulation is a perfect complement to the management solution and the hydraulic overload monitoring of WWTPs.

XAI(eXplainable AI) 기반 하수처리 활성오니공정 용존산소 제어를 위한 신경회로망 모델링

In this paper, we proposed Dissolved Oxygen(DO) neural network model of activated sludge process using XAI(eXplainable AI) in wastewater treatment system. To improve the model performance, input water qualities are to be reliable and have a much influences in DO biological operation. In regulations, COD, T-N, T-P, pH, SS of effluent are hourly to transmitted in Korea Environment Corporation. If these values are exceed the legal standards, the penalty is given. Therefore these data are very reliable and is monitored by operators critically. So these data is to be inputs of DO neural network model. And XAI(eXplainable AI) is utilized to decide which input water qualities have much influences in the process. LRP(Layer-wise Relevance Propagation) is used among various XAI(eXplainable AI) methods. NH4, MLSS, pH in aeration tank and COD, TN, TP, SS in secondary clarifier are input candidates of model for Do neural network modeling. Using LRP, COD, NH4, MLSS, SS are decided to be inputs of Do neural network model.BRThe validity of the proposed method was proved by applying to the DO neural network model of activated sludge process which was developed in previous research. 3 years hourly data was used for modeling and estimation. The result show that the performance of the proposed model was improved in comparison of conventional neural network models.BRIn the future, absolute values of weight in LRP will be more considered because we considered only the inputs orders of influencing on DO biological operation.

Preliminary Design Analysis of Membrane Bioreactors Application in Treatment Sequences for Modernization of Wastewater Treatment Plants.

By using modeling with the Capdetworks software package, the study examines the definition of the essential elements of operational expenses at wastewater treatment facilities with a capacity of 1 to 100 thousand cubic meters per day. Four different treatment sequences were examined in the study; the first three revealed a standard setup with an activated sludge reactor and secondary clarifier (operating under various operating conditions), and the fourth scheme combined an activated sludge reactor with a submerged membrane bioreactor for sludge separation. The values of concentrations of key pollutants common for urban wastewater before treatment as well as technological parameters of operation were utilized as initial data for calculations because it was crucial to obtain conclusions that could be applied at real facilities. For each of the four treatment sequences, values for pollutants concentrations in effluent wastewater and hydraulic retention time were obtained and analyzed. The expenses of operating biological treatment facilities and treatment facilities in general, as well as the specific cost of power for treating 1 m3 of wastewater, were taken into account. Additionally, the price of purchasing membrane modules, which can be categorized as operational due to their replacement frequency of around every 7 to 10 years, was determined. The study’s findings demonstrated that the use of membrane technologies at the secondary treatment stage might significantly affect the rebuilding of wastewater treatment plants under conditions of increased capacity (flow rate) and constrained area for growth.

Cytology and High-Risk Human Papillomavirus Test for Cervical Cancer Screening Assessment.

Background: A new nationwide screening strategy was implemented in Germany in January 2020. No data are available for women referred to certified dysplasia units for secondary clarification after primary diagnosis by a local physician. We therefore investigated combined testing with Papanicolaou smears and high-risk human papillomavirus (hrHPV) and compared the data with the final histological findings. Methods: Between January 2015 and October 2020, all referred women who underwent colposcopy of the uterine cervix in our certified dysplasia unit were included. Cytology findings were classified using the Munich III nomenclature. Results: A total of 3588 colposcopies were performed in 3118 women, along with Pap smear and hrHPV co-testing, followed by histology. Women with Pap II-p (ASC-US) and a positive hrHPV co-test had a 22.4% risk for cervical intraepithelial neoplasia (CIN) 3/high-grade squamous intraepithelial lesion (HSIL). The risk of CIN 3/HSIL was 83.8% in women with Pap IVa-p (HSIL) and a positive hrHPV co-test. A positive hrHPV co-test increased the risk for HSIL+ (OR 5.942; 95% CI, 4.617 to 7.649; p < 0.001) as compared to a negative hrHPV co-test. Conclusions: The accuracy of Pap smears is comparable with the screening results. A positive hrHPV test increases the risk for HSIL+ fivefold. Colposcopy is necessary to diagnose HSIL+ correctly.

Long-term effects of hydrocyclone operation on activated sludge morphology and full-scale secondary settling tank wet-weather operation in long sludge age WWTP

This paper presents the study concerning long-term effects of a full scale hydrocyclone unit implemented in a continuous flow long sludge age system, on sedimentation, treatment efficiency and sludge morphology. The research concentrates on identifying the mechanisms of sludge behaviour within the system. The gravimetric selection of activated sludge via a hydrocyclone is a recent development for enhancing sludge separation, where heavier flocs are retained in the system, and lighter ones are discarded as waste sludge. The effects of implementing hydroclyclones were analysed with the use of SEM imagining and fractal dimensioning through the frequent assessment of sludge settling capabilities, effluent quality, and floc properties. Over the course of 60 weeks of hydrocyclone operation, sedimentation efficiency varied significantly. Sludge volume index values of 40 mL/g, achieved during the warm season, were not sustained when the temperature decreased and an overgrowth of filamentous bacteria occurred. Good settling efficiency was also observed in batch tests, where settling velocity of experimental sludge was app. 1 m/h higher than for the reference train at the same concentrations. This was confirmed during wet weather, as the experimental train sustained safe sludge blanket height in secondary clarifiers. SEM imaging and fractal dimension analysis revealed that the underflow that returned to the system had a more compact and spherical shape, which led to an increased content of granule-like particles in the reactor. The presence of flocs with a diameter exceeding 900 μm in the underflow, which is not observed in the feed, indicated agglomeration within the hydrocyclone. This is contradictory to most of the literature data coming from laboratory experiments. This phenomenon was attributed to differences in the size and geometry of the used hydrocyclones, and the potential process mechanism was presented.

Features of operation of ultra-small sewage water treatment plants of northern design with membrane technology

The conditions of operation of ultrasmall sewage treatment plants SWTP - 30 with membrane technology located at a warehouse production facility in the Arctic of Russia with coordinates 67.4973, 86.3261 are considered. The parameters of the SWTP operation are explained after the modernization to supplement the cleaning scheme with a secondary clarifier.

Direct recycling of discarded reverse osmosis membranes for domestic wastewater treatment with a focus on water reuse

The recycling of discarded membranes (end-of-life) represents a relevant alternative for sustainability of reverse osmosis (RO) desalination plants in the context of circular economy. This work evaluated the feasibility of using discarded commercial RO membranes in the treatment of domestic secondary wastewater to obtain water with a certain standard quality. Crossflow filtration tests were conducted to evaluate desalination and wastewater filtration performance at different operating pressures on RO membranes discarded from desalination plans at different working positions (primary M1; secondary M2). The standard manufacturer desalination tests showed a superior performance on M1 membranes, in terms of rejection (∼25 LMH, 97%), compared to M2 (∼33 LMH, 50%); both having a lower performance than a standard membrane (38 LMH± 15%; 99.6%). The failure is sufficient for discarding due to loss of lifespan. Moreover, in wastewater filtration tests using the secondary clarifier outlet effluent from a WWTP at different working pressures, both types of membranes were shown to be effective, with degrees of performance highly dependent on the working pressure. Thus, the operating values of permeate flux/salt rejection were between 56 and 59 LMH/ 96–97% for 600-psi: 33–34 LMH/ 94–96% for 300-psi and in the range of 10–11 LMH/ 90–94% for 80-psi test. Surface characterization of the membrane showed a pressure-related increase in fouling and bacterial adhesion post-filtration. Finally, the operating performance was verified in M1 wastewater filtration at 300 psi over long times (14 h), yielding stable and promising values (∼27 LMH; 96%). The permeate obtained has a low concentration of fecal coliforms (<2 MPN/100 mL, 99.99% removal) and meets local standards for irrigation and drinking water in terms of conductivity, phosphorus and nitrogen concentration in treated water.

Assessment of Possibilities of Spreading of Bioaerosol from Different Technological Objects in Small Sewage Treatment Plants

Abstract The Covid-19 pandemia increased the attention of the world community to air biocontamination. Sewage treatment plants (STPs) generate a bioaerosol during different technological operations. Research aimed to estimate the range of bioaerosol emission from different technological objects of 5 small STPs. Such knowledge is very important for risk assessment, monitoring programs and pollution limitation. The sedimentation method was used for the detection of mesophiles, psychrophiles, Escherichia coli, pigmented bacteria, Streptococcus faecalis, Pseudomonas fluorescens, and mold fungi. The highest level of psychrophiles and mold fungi (&gt; 1000 cfu/m3) was detected in points located near activated sludge chambers, sludge thickening tanks, and secondary clarifiers. The mesophiles (&gt;500 cfu/m3) and E. coli aren’t a normal component of air microflora, but were detected in all measurement points, especially near a pomp station (inflow), grit, activated sludge, sludge thickening chambers. At the points located at the leeward, the number of microorganisms was higher than in the windward. The research results indicate the necessity of constant monitoring of the STP impact on the air quality.

Comparison of Effluent Suspended Solid Concentrations from Two Types of Rectangular Secondary Clarifiers

Secondary clarifiers play a significant role in the successful operation of activated sludge systems. Because of the restriction of available land, South Korean domestic wastewater treatment plants tend to employ rectangular clarifiers to settle mixed liquor suspended solids (MLSS) for activated sludge systems. A high MLSS concentration must be maintained in the bioreactor to ensure nitrification during winters, and achieve stringent effluent quality. The effluent suspended solid (SS) concentrations of two clarifier types currently being used in South Korea, primary rectangular clarifier-type and Gould Type I secondary clarifiers, were compared using computerized fluid dynamic simulations and hourly secondary effluent suspended solid concentrations. In addition, operational data such as hourly influent flow, daily MLSS concentrations, and sludge volume index were obtained and reviewed. This comparison reveals that the Gould Type I secondary clarifier is resilient to loading variation and produces effluent with a consistently lower suspended solid concentration than the primary rectangular clarifier-type under similar loading conditions and higher loading variations. The results suggest that the existing primary rectangular clarifier-type secondary clarifiers must be converted to Gould Type I.