Comparison of Circular Clarifier and Rectangular Clarifier June 1, 2025 sazenviro Post in Uncategorized In any sedimentation-based wastewater treatment system, the choice between circular and rectangular clarifiers is critical. The clarifier’s geometry affects not only footprint and civil cost but also hydraulic flow, sludge handling, and long-term efficiency. Both configurations have proven their worth in diverse applications, but modern treatment plants often face trade-offs in space, cost, and performance. This article compares the pros and cons of circular clarifier design vs. rectangular clarifiers—particularly in the context of clarifiers for municipal sewage and industrial ETPs. What Are Clarifiers in Wastewater Treatment?Contents1 What Are Clarifiers in Wastewater Treatment?2 Overview of Circular Clarifier Design2.1 Circular Clarifier Design – Core Features3 Overview of Rectangular Clarifiers4 Key Differences Between Circular and Rectangular Clarifiers5 Advantages of Circular Clarifiers6 Advantages of Rectangular Clarifiers7 Clarifier Design Considerations8 Clarifier for Municipal Sewage – Which Is Better?8.1 📌 H2: Clarifier for Municipal Sewage with High BOD Load9 Cost Comparison10 Global Examples11 What Do Clarifier Manufacturers Recommend?12 Conclusion Clarifiers are primary or secondary settling tanks that remove suspended solids from wastewater by gravity. The influent enters at a controlled velocity, allowing heavier particles to settle at the bottom, forming sludge. Clarifiers are designed for: Consistent hydraulic flow distribution Maximum solids removal Minimal short-circuiting and turbulence Overview of Circular Clarifier Design Circular clarifiers consist of a round tank with a central feed well, radial flow pattern, and peripheral effluent weirs. Circular Clarifier Design – Core Features Center-feed mechanism: Ensures symmetrical flow and better hydraulic balance Radial flow pattern: Sludge settles and moves toward the center for removal Rotating scraper arm: Collects sludge and directs it to a central hopper Overview of Rectangular Clarifiers Rectangular clarifiers are long, straight tanks with flow entering at one end and exiting at the other. Simpler construction, ideal for linear plant layouts Sludge moves longitudinally toward a collection trough Mechanized chain-and-flight scrapers for sludge conveyance Key Differences Between Circular and Rectangular Clarifiers Parameter Circular Clarifier Design Rectangular Clarifier Flow Pattern Radial Plug Flow (Linear) Sludge Removal Central hopper via scraper arms End trough using chain scrapers Hydraulic Performance Better distribution, low short-circuiting More prone to velocity gradients Maintenance Complexity Moderate (rotating mechanism) Lower (simpler chains) Footprint More compact, but wider Space-efficient in narrow layouts Civil Construction More complex due to circular concrete ring Easier with standard rectangular tanks Typical Use Municipal STPs, large industrial ETPs Smaller ETPs, limited footprint facilities Advantages of Circular Clarifiers Hydraulic Symmetry Uniform radial flow reduces dead zones Enhanced settling efficiency Central Sludge Collection Shorter sludge path Reduces sludge aging Aesthetics & Accessibility Easier to place in landscaped STP facilities Structural Stability Better resistance to external soil pressure Advantages of Rectangular Clarifiers Ease of Construction Built with standard concrete shuttering Faster construction, lower cost Modular Design Can be installed in series or parallel Easier to scale Maintenance Access Simpler scraper system Easy access along straight channels Footprint Efficiency Ideal for long narrow plots, especially in industrial estates Clarifier Design Considerations When choosing between circular and rectangular clarifiers, engineers must consider: Design Factor Circular Clarifier Rectangular Clarifier Flow rate (m³/hr) >500 preferred Up to 500 common Sludge type High solids – better circular Thin sludge – rectangular acceptable Plant layout Radial flexibility Linear pipelines preferred Retention Time 2–3 hours typical 2–3 hours Surface Loading Rate (SLR) 1.2–1.6 m³/m²/hr 0.8–1.2 m³/m²/hr Clarifier for Municipal Sewage – Which Is Better? Most municipal sewage treatment plants opt for circular clarifiers due to: Large influent variability Higher sludge load Need for better hydraulic balancing In contrast, small towns or decentralized STPs often use rectangular clarifiers for cost savings and layout flexibility. 📌 H2: Clarifier for Municipal Sewage with High BOD Load In plants where influent BOD is >300 mg/L and solids are non-uniform, circular clarifiers outperform rectangular ones due to better sedimentation uniformity. Cost Comparison Item Circular Clarifier (25 MLD) Rectangular Clarifier (25 MLD) Civil Cost High (excavation + ring slab) Lower (rectangular base + walls) Mechanical Cost Moderate (rotating arms, scrapers) Moderate (chain & flight system) Installation Time 4–6 months 2–4 months O&M Cost Slightly higher Lower Global Examples Singapore NEWater Plant – Uses high-capacity circular clarifiers for primary and secondary sedimentation Los Angeles Hyperion STP – Mix of circular and rectangular clarifiers for redundancy and flow flexibility Riyadh STP – Modular rectangular clarifiers to fit within limited desert land What Do Clarifier Manufacturers Recommend? Leading manufacturers such as: WesTech Engineering Ovivo Inc. Xylem Water Technologies …offer both designs, but recommend circular clarifiers for centralized STPs and rectangular clarifiers for decentralized and retrofit applications. Conclusion There is no one-size-fits-all when it comes to clarifier geometry. The right choice between circular clarifier design and rectangular systems depends on flow rates, space availability, influent type, sludge characteristics, and budget. For large municipal sewage treatment plants, circular clarifiers are generally preferred for better sedimentation performance. In contrast, rectangular clarifiers are suitable for smaller or modular installations where civil simplicity and linear layouts are priorities. Before finalizing your design, always consult an experienced clarifier manufacturer who can analyze your influent characteristics and plant layout.