Circular Clarifier for Secondary Sedimentation June 1, 2025 sazenviro Post in Uncategorized In the activated sludge process of sewage and industrial wastewater treatment, secondary sedimentation is a crucial step that ensures the separation of biological solids from treated water. Among the available clarifier geometries, the circular clarifier remains the most preferred design for this purpose due to its hydraulic symmetry, efficient sludge handling, and long-term performance. This article explains why circular clarifier for STP (Sewage Treatment Plant) applications is ideal for secondary treatment and how secondary clarifier design considerations can impact overall plant efficiency and effluent quality. What Is a Secondary Clarifier?Contents1 What Is a Secondary Clarifier?2 Why Circular Clarifier for STP Applications?3 Design Features of a Circular Secondary Clarifier3.1 Core Elements of Secondary Clarifier Design in Circular Geometry4 Design Parameters for Secondary Clarifier4.1 Critical Secondary Clarifier Design Considerations5 Advantages of Circular Clarifier for STP5.1 Hydraulic Advantages of Circular Clarifier for STP5.2 Maintenance Advantages in Secondary Clarifier Design5.3 Operational Benefits6 Comparison: Circular vs. Rectangular Secondary Clarifiers7 Use Case: Circular Clarifier in a 20 MLD STP (Bangalore)8 Automation Options for Modern Secondary Clarifiers9 Circular Clarifier Material of Construction10 Top Manufacturers of Circular Secondary Clarifiers11 Future Trends in Secondary Clarifier Design12 Conclusion A secondary clarifier, also known as a final clarifier, is a gravity-based sedimentation unit that separates microbial biomass (biological sludge) from the treated effluent in biological wastewater treatment systems. Secondary clarifiers are commonly used in: Activated Sludge Process (ASP) Sequencing Batch Reactors (SBR) Membrane Bioreactors (MBR) Moving Bed Biofilm Reactors (MBBR) Why Circular Clarifier for STP Applications? A circular clarifier for STP is ideal due to: Uniform hydraulic flow and radial symmetry Central feed well for controlled flow entry Efficient solids settling without short-circuiting Central sludge hopper for easy removal Continuous operation and low maintenance These features result in stable sludge blanket formation and minimal sludge carryover, ensuring high-quality effluent even with biological loading variations. Design Features of a Circular Secondary Clarifier Core Elements of Secondary Clarifier Design in Circular Geometry Component Description Feed Well Central inlet to dissipate influent energy and distribute flow radially Sludge Hopper Central cone-shaped structure where sludge settles and is collected Rotating Bridge/Arm Carries the scraper mechanism for continuous sludge scraping Peripheral Effluent Trough Collects clarified water evenly across the circumference Skimming Mechanism Removes floating scum (grease, foam, debris) from the top Design Parameters for Secondary Clarifier To optimize secondary clarifier design, engineers must consider: Parameter Typical Range for STPs Surface Loading Rate (SLR) 20 – 30 m³/m²/day (0.8–1.2 m³/m²/hr) Hydraulic Retention Time (HRT) 2.0 – 3.5 hours Solids Loading Rate 60 – 120 kg/m²/day Side Water Depth (SWD) 3 – 4.5 meters Diameter (for circular tank) 8 – 30 meters (based on flow rate) Return Sludge Pumping Capacity 25–50% of influent flow Critical Secondary Clarifier Design Considerations Flow EqualizationUneven inlet flows can disturb sludge blanket formation. A feed well prevents this by dissipating energy. Scum RemovalFloating matter (foam, grease) must be regularly skimmed using surface skimmers. Sludge WithdrawalControlled sludge removal ensures a stable sludge blanket and prevents sludge rising or washout. Compact Effluent Treatment Plants Advantages of Circular Clarifier for STP Hydraulic Advantages of Circular Clarifier for STP Radial symmetry ensures even flow distribution Prevents short-circuiting and dead zones Helps maintain a consistent sludge blanket across the tank Maintenance Advantages in Secondary Clarifier Design Only one rotating mechanism (scraper arm) Easy access from walkway bridges Simple sludge pump-out arrangements Operational Benefits Handles peak loads with minimal performance drop Minimal operator intervention after commissioning Adaptable for SCADA, PLC-based automation, and sludge blanket monitoring Comparison: Circular vs. Rectangular Secondary Clarifiers Feature Circular Clarifier Rectangular Clarifier Footprint Shape Round Long rectangular Hydraulic Efficiency High Medium Sludge Removal Mechanism Center hopper, rotating arm Chain-and-flight scraper Installation Time Higher (due to ring foundation) Shorter Space Use Better for large open plots Better for narrow plots Preferred For Municipal STPs, large ASP units Compact ETPs, linear plants Use Case: Circular Clarifier in a 20 MLD STP (Bangalore) Design flow: 20,000 m³/day Secondary clarifier diameter: 20 meters Side water depth: 4.5 meters Sludge return rate: 40% of influent Result: Suspended solids reduced to <20 mg/L Stable operation even during monsoon inflow variations Minimal manual sludge intervention due to automated sludge pump and turbidity sensor Automation Options for Modern Secondary Clarifiers Sludge Blanket Level SensorsAlert if sludge blanket rises above set point Scum DetectorsAutomatically activate skimmer drives Return Activated Sludge (RAS) ControlAdjust sludge return flow based on MLSS or turbidity sensors SCADA/PLC IntegrationEnables centralized control and alarms for clarifier performance Circular Clarifier Material of Construction Depending on site and budget: Material Usage RCC Large STPs with civil works MS with epoxy Medium-sized clarifiers SS 304 Compact STPs, portable units FRP Pre-engineered package plants Top Manufacturers of Circular Secondary Clarifiers WesTech Engineering (USA) – Specializes in peripheral-driven clarifiers Ovivo (Canada) – Advanced energy-saving designs HUBER SE (Germany) – Clarifier packages for MBBR/MBR systems SAZ ENVIRO (India) – Global supplier of circular clarifier for STP up to 30 MLD, with automation options Future Trends in Secondary Clarifier Design Inclined Tube or Plate Settler Add-ons to boost surface area Floating sludge detectors and AI-based scum prediction Remote cloud-based monitoring of sludge blanket and scum overflow Self-cleaning clarifier mechanisms for sludge bridge avoidance Conclusion A well-engineered circular clarifier for STP ensures stable and efficient secondary sedimentation. Its radial flow pattern, central sludge removal, and high hydraulic efficiency make it a superior choice for medium to large sewage and effluent treatment plants. By focusing on critical aspects of secondary clarifier design—such as surface loading rate, sludge withdrawal rate, and automation—plant operators can maintain consistent performance, meet discharge norms, and minimize operating costs. For any STP upgrade or new design, selecting a circular secondary clarifier with proper sizing and instrumentation is a decision that pays off in both performance and reliability.