Primary Clarifier Sizing Calculator
Calculate surface area, tank diameter, sidewater depth, surface overflow rate (SOR), weir loading, BOD and SS removal, and primary sludge production — based on Metcalf & Eddy (5th ed.) Chapter 6 design criteria for primary sedimentation tanks.
Primary Clarifier Design Parameters
Enter your wastewater flow and quality parameters to size the primary sedimentation tank per Metcalf & Eddy (5th ed.) Chapter 6.
Design average daily flow
Peak hour / average ratio
Influent suspended solids
5-day biochemical oxygen demand
Typical: 1.5–2.5 hr
Typical primary: 32–48 m³/m²·d
Max 186 m³/m·d (M&E)
How to Use This Calculator
- 1Enter the average daily flow (m³/day) and peak flow factor. The peak factor accounts for morning peak flows and storm events; typical values are 2.0–3.0 for municipal sewage and 1.5–2.5 for industrial effluents. The calculator uses this to check the peak SOR against the 120 m³/m²·d limit.
- 2Enter influent SS (mg/L) and influent BOD₅ (mg/L). Typical domestic sewage values are 150–350 mg/L TSS and 150–400 mg/L BOD. These are used to compute removal efficiency, effluent quality, and primary sludge production.
- 3Set the detention time (1.5–2.5 hr is the M&E recommended range) and the design SOR (32–48 m³/m²·d for primary clarifiers). The design surface area is the larger of the SOR-based area and the area required to keep peak SOR below 120 m³/m²·d.
- 4Enter the weir loading rate (max 186 m³/m·d per M&E) and select the number of parallel tanks. A minimum of 2 tanks is recommended for continuous operation so one tank can be taken offline for maintenance. The calculator distributes the total design area equally across tanks.
- 5Click Calculate and review the results. The design check table shows whether each key parameter falls within the M&E recommended range — green tick for within range, red cross for outside. Adjust inputs and recalculate until all checks pass or the deviations are justified for your project.
Primary Sedimentation: The First Stage of Wastewater Treatment
Primary sedimentation is the first treatment stage after preliminary processes (screening, comminution, grit removal) in a conventional wastewater treatment plant. The primary clarifier removes settleable suspended solids and associated BOD from raw wastewater by gravity settling, reducing the load on the downstream biological treatment system. Correctly sized primary clarifiers protect the biological stage from organic and solids overloads, reduce aeration energy demand, and lower biological sludge production.
At the design surface overflow rate of 32–48 m³/m²·d and a detention time of 1.5–2.5 hours, primary clarifiers achieve 50–65% TSS removal and 25–40% BOD removal from typical municipal sewage. The removed solids are collected as primary sludge from the tank floor by a rotating scraper mechanism and withdrawn to the sludge treatment train for thickening, digestion, and dewatering. Primary sludge is characterised by high moisture content (93–95%), high volatile content (60–80% VS), and good digestibility.
For some treatment configurations — high-rate activated sludge, integrated fixed-film activated sludge (IFAS), or MBBR — primary clarifiers may be omitted or replaced with fine screens to reduce capital cost and footprint. However, including primary clarification is generally recommended for larger municipal STPs and industrial ETPs where sludge management is integrated. For conventional clarifier design and supply, see the conventional clarifier page for Spans Envirotech's clarifier solutions.
Surface Overflow Rate and Clarifier Design (M&E Guidelines)
The surface overflow rate is the fundamental sizing parameter for primary clarifiers. It represents the upward liquid velocity: SOR = Q / A [m³/m²·d]. When this velocity exceeds the settling velocity of a suspended particle, the particle is carried upward and out of the tank rather than settling. Primary settling tanks must be sized to ensure the SOR is low enough that the majority of settleable solids can reach the tank floor before the liquid exits over the weir.
Metcalf & Eddy (Wastewater Engineering, 5th ed., Table 6-13) specifies the following design criteria for primary clarifiers serving municipal wastewater:
| Parameter | Typical Range | Notes |
|---|---|---|
| SOR at average flow | 32–48 m³/m²·d | Primary clarifier, municipal |
| SOR at peak flow | max 120 m³/m²·d | Absolute maximum; M&E Table 6-13 |
| Hydraulic detention time | 1.5–2.5 hr | At average flow |
| Sidewater depth | 2.5–4.5 m | Typical 3.0–3.6 m for primary |
| Weir loading rate | max 186 m³/m·d | Peripheral weir, circular tank |
| TSS removal | 50–65% | At 1.5–2.5 hr detention time |
| BOD removal | 25–40% | Primarily particulate BOD removal |
The design surface area is selected as the larger of the area required to meet the average SOR criterion and the area required to stay below the peak SOR limit of 120 m³/m²·d. This ensures the clarifier performs adequately at both average and peak flow conditions. For industrial wastewater with high suspended solids concentrations or difficult-to-settle particles (colloidal solids, light fibres), the design SOR should be reduced to 24–36 m³/m²·d or chemically enhanced primary treatment (CEPT) considered.
Primary Clarifier Design for Indian STPs and ETPs
In India, primary sedimentation tanks for sewage treatment plants (STPs) are designed in accordance with the CPHEEO Manual on Sewerage and Sewage Treatment Systems (2013) and IS 4764 (Specifications for Septic Tanks and Related Structures). The CPHEEO Manual recommends surface loading rates of 24–36 m³/m²·d for primary settling tanks and a minimum detention time of 1.5–2.0 hours at average daily flow — broadly consistent with Metcalf & Eddy guidelines, though at slightly lower SOR values reflecting typical Indian influent characteristics (lower velocities in gravity sewers, more grit and inorganic solids in combined sewers).
For combined sewer systems — common in older Indian cities where stormwater and sewage share the same network — the peak flow factor can reach 3.0–5.0 during monsoon events. This makes it critical to check the peak SOR constraint and size primary clarifiers with adequate hydraulic headroom. Flow equalisation upstream of the primary clarifier is often the most cost-effective solution for plants receiving highly variable flows.
For industrial ETPs, CPCB and State Pollution Control Board consent conditions typically specify effluent quality limits (BOD, COD, TSS, pH) at the discharge point rather than specifying primary clarifier design parameters directly. Process engineers must size the primary clarifier to achieve sufficient pre-treatment for the downstream biological process to meet consent limits. Textile effluents, food processing wastewater, and paper mill effluents often require primary clarification as a critical first step before biological treatment due to high SS loads.
Spans Envirotech designs and supplies primary sedimentation systems — including circular clarifiers with rotating scraper mechanisms, sludge hoppers, scum removal, and effluent launders — for STPs and ETPs across India. Contact us at bd@spans.co.in or +91-98100 00233 for project-specific design support.
Primary vs Secondary Clarifiers: Key Design Differences
Primary and secondary clarifiers share the same fundamental operating principle — gravity sedimentation — but differ significantly in their design constraints, the nature of the solids being settled, and their role in the treatment process.
A primary clarifier receives raw or screened/degritted wastewater and settles discrete, chemically inert suspended solids (sand, grit, organic fibres, food particles). These solids have relatively high settling velocities, allowing high SOR values (32–48 m³/m²·d) and moderate detention times (1.5–2.5 hr). The primary clarifier does not need to thicken the settled sludge to a high concentration — primary sludge at 2–5% TS is acceptable for downstream gravity thickening and digestion.
A secondary clarifier (secondary settling tank or SST) receives the biological mixed liquor from an activated sludge aeration tank or MBBR and must perform two simultaneous functions: (1) clarification — separating the biologically treated water from the activated sludge floc; and (2) thickening — concentrating the return sludge (RAS) to maintain the MLSS in the aeration tank. The biological floc has a much lower settling velocity than primary solids, requiring lower SOR values (16–28 m³/m²·d) and additional design checks against the solids loading rate (SLR). The secondary clarifier is also more sensitive to peak flow events, which can cause sludge blanket rise and solids washout. For secondary clarifier sizing, use our secondary clarifier sizing calculator.
The primary sludge from a primary clarifier has higher volatile solids content (60–80% VS) and better digestibility than secondary (biological) sludge, making it more suitable for anaerobic digestion and biogas production. In many Indian STPs, combined primary and secondary sludge is co-digested in anaerobic digesters to produce biogas for electricity generation or heat recovery.
Frequently Asked Questions
What is a primary clarifier and what does it remove?
A primary clarifier is the first solid-liquid separation unit in a wastewater treatment plant, positioned before biological treatment. It removes 50–65% of suspended solids (TSS) and 25–40% of BOD from raw wastewater by gravity settling, reducing the load on the downstream biological system. Removed solids are collected as primary sludge.
What is the surface overflow rate (SOR) for a primary clarifier?
Metcalf & Eddy (5th ed.) recommends 32–48 m³/m²·d at average flow for primary clarifiers treating municipal wastewater, with a maximum of 120 m³/m²·d at peak flow. Exceeding the peak SOR limit causes the upward hydraulic velocity to carry settleable solids out of the tank, increasing effluent TSS. Industrial wastewater with fine or light particles may require a lower design SOR of 24–36 m³/m²·d.
What is the typical detention time for a primary sedimentation tank?
The hydraulic detention time for a primary clarifier is typically 1.5–2.5 hours at average design flow per M&E Chapter 6. A detention time of 2.0 hours is the standard design value for municipal sewage. Shorter times (1.5 hr) are used for high-rate primary treatment; longer times (2.5 hr) improve TSS and BOD removal. The detention time alone is not sufficient for sizing — the SOR constraint must also be satisfied.
How is primary clarifier diameter calculated?
The diameter is derived from the design surface area. The design area is the larger of: (a) Q_avg / SOR_design; and (b) Q_peak / 120 (to satisfy the peak SOR limit). Dividing by the number of tanks gives the area per tank, and the diameter follows from D = sqrt(4 × A_per_tank / π). In practice, diameters are rounded up to standard fabrication sizes in 0.5 m increments.
What is the weir loading rate and why does it matter?
The weir loading rate (WLR) is the volumetric flow per unit length of the effluent weir: WLR = Q / L_weir [m³/m·d]. Metcalf & Eddy specifies a maximum WLR of 186 m³/m·d for primary clarifiers. Excessive WLR creates a hydraulic draw-down effect near the weir that lifts settled sludge back into the effluent. For large-diameter tanks, additional internal weirs or finger weirs are used to increase total weir length.
How much BOD and SS does primary treatment remove?
Primary sedimentation removes 50–65% of TSS and 25–40% of BOD₅ from typical municipal sewage at a detention time of 1.5–2.5 hours and SOR of 32–48 m³/m²·d. This calculator uses the Ekama simplified equation: SS removal (%) = [1 − 1/(1 + 0.35 × t)] × 100, with BOD removal estimated at 60% of SS removal efficiency. Industrial wastewater with a higher fraction of settleable solids can achieve removal at the upper end of this range.
What is the volume of primary sludge produced per day?
Primary sludge volume depends on the mass of SS removed and the moisture content. The dry solids mass = (influent SS − effluent SS) × flow / 1000 [kg/day]. At 5% total solids (95% moisture), sludge volume = dry mass / (0.05 × 1000) × 1000 [L/day], or divided by 1000 for m³/day. For example, removing 100 kg/day of solids at 5% TS produces 2.0 m³/day of primary sludge, which is thickened and dewatered downstream.
Related Tools
Secondary Clarifier Sizing
Size secondary settling tanks using SOR and solids flux methods for activated sludge systems.
Equalization Tank Sizing
Size flow equalization tanks to dampen peak flows before primary treatment.
Sludge Production Estimator
Estimate total sludge production from primary and secondary treatment stages.
Design Your Primary Treatment System
Spans Envirotech designs and commissions primary sedimentation systems — clarifiers, sludge scrapers, scum removal, and integrated primary treatment units — for municipal STPs and industrial ETPs across India. Contact our engineering team for a project-specific sizing review and detailed design.
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