Primary Clarifier Design: Overflow Rate, Weir Loading and Depth

What does a primary clarifier do, and where does it sit?

A primary clarifier (primary sedimentation tank) is a gravity settling basin placed after screening and grit removal but before secondary biological treatment. Its job is to remove readily settleable organic and inorganic solids so that the downstream aeration or biofilm process receives a lower, more stable solids and BOD load. Removing settleable solids here is cheap: gravity does the work, whereas removing the same mass biologically consumes oxygen and produces more secondary sludge.

Typical duties are 50–70% TSS removal and 25–40% BOD₅ removal at design flow. The collected primary sludge (typically 4–6% dry solids for a well-operated tank) is drawn from a central or end hopper and sent to thickening and digestion. Floating scum and FOG are skimmed from the surface. Because the tank is quiescent, it also equalises short hydraulic transients before the biology.

Primary sedimentation is one of the workhorse unit operations in municipal and industrial clarifier equipment, and its sizing logic underpins secondary and tertiary settlers too.

Which of the four settling regimes governs a primary tank?

Classical sedimentation theory recognises four regimes, distinguished by particle concentration and interaction:

• Type I — discrete (free) settling. Dilute suspensions of non-flocculating particles (e.g. grit). Each particle settles independently at its terminal velocity, described by Stokes' law.
• Type II — flocculent settling. Particles coalesce as they fall, so settling velocity increases with depth. This governs the bulk of solids removal in a primary clarifier.
• Type III — hindered (zone) settling. At high concentration particles settle as a mass with a distinct interface; relevant to secondary clarifiers on activated sludge, not primaries.
• Type IV — compression settling. In the sludge blanket, particles are supported by those beneath and consolidate under their own weight; this occurs only in the sludge hopper zone.

A raw-sewage primary tank is dominated by Type II flocculent settling in the clarification zone, with Type IV compression confined to the sludge hopper. Because floc velocity rises with depth, deeper tanks capture more solids than pure discrete-settling theory predicts — which is why side-water depth is a genuine design variable, not just freeboard.

How does overflow rate control the design? (Hazen theory)

The master criterion is the surface overflow rate (SOR), also called the surface loading rate. Hazen and Camp's ideal-settling model shows that, for a horizontal-flow ideal basin, whether a particle is captured depends only on its settling velocity relative to the overflow rate — not on tank depth or detention time.

The elegant result is that capture is set by surface area, not volume: doubling the area halves the critical velocity and captures finer particles. Depth still matters in the real world because flocculation continues with depth, because a minimum side-water depth suppresses scour and density currents, and because the hopper needs volume to thicken sludge — but the primary area is fixed by SOR.

For an idealised discrete particle the critical velocity follows Stokes' law:

What about weir loading, depth and detention time?

SOR sets the area, but three secondary checks confirm the geometry:

• Weir loading rate (m³/m·d) — flow per unit length of effluent weir. Excessive weir loading draws up local velocities near the outlet, lifting rising solids over the weir. Design to <125–250 m³/m·d at average flow (WEF/Metcalf & Eddy). Peripheral or finger weirs are used to add length on circular tanks.
• Side-water depth (SWD) — commonly 3.0–4.5 m. Deeper tanks give flocculation room and buffer density currents, improving effluent quality at a given SOR.
• Hydraulic detention time — t = V / Q, typically 1.5–2.5 h at average flow. Below ~1 h septicity and short-circuiting risk rises; well above 2.5 h the sewage can turn septic and release odour and dissolved sulphide.

Detention time and SOR are geometrically linked through depth: t = V/Q = (A·H)/Q = H/SOR. So once you fix SOR and SWD, detention time is determined — a useful cross-check that the three criteria are mutually consistent.

Worked example: sizing a primary clarifier

Size a primary clarifier for an average design flow of Q = 8,000 m³/d (0.093 m³/s), with a peak factor of 2.5, using an average SOR of 40 m³/m²·d.

• Surface area: A = Q / SOR = 8,000 / 40 = 200 m².
• Peak-flow check: at peak Q = 20,000 m³/d, SOR = 20,000 / 200 = 100 m³/m²·d — within the ~120 m³/m²·d peak limit, so the area holds.
• Tank diameter (circular): D = √(4A/π) = √(4 × 200 / 3.1416) = √254.6 = ~16 m (round to a standard 16 m mechanism).
• Volume & detention time: at a side-water depth of 3.5 m, V = A·H = 200 × 3.5 = 700 m³, so t = V/Q = 700 / 8,000 × 24 = 2.1 h — inside the 1.5–2.5 h band.
• Weir loading: a single peripheral weir has length L = π·D = 3.1416 × 16 = 50.3 m. Weir loading = Q / L = 8,000 / 50.3 = 159 m³/m·d — above the 125 target, so add an inboard (finger) weir launder to roughly double the length and bring it to ~80 m³/m·d.

Result: one 16 m diameter tank at 3.5 m SWD (or, for redundancy, two 11.3 m tanks each of 100 m²) meets SOR, detention and depth, with a double weir launder to satisfy weir loading. Confirm scour and sludge withdrawal before finalising.

Circular vs rectangular tanks — and the scour limit

Both geometries are proven; the choice is driven by site footprint, number of units and sludge-collection preference.

Whatever the geometry, the horizontal velocity must stay below the scour velocity that re-suspends settled sludge:

How does chemically enhanced primary treatment (CEPT) change the design?

Dosing a coagulant (ferric chloride, ferric sulphate or alum, often 20–80 mg/L as metal) with a small polymer dose destabilises colloids and builds larger, denser, faster-settling flocs. This chemically enhanced primary treatment (CEPT) raises removals to roughly 70–90% TSS and 50–70% BOD, and lets the tank run at a higher SOR (60–80+ m³/m²·d) for the same effluent quality — a retrofit route to uprate an existing basin without civils.

The trade-offs are more chemical sludge (higher volume, though denser), added phosphorus removal (often a co-benefit for consent), and running cost. CEPT is attractive where land is constrained, where wet-weather flows spike, or where P removal is required. For finer or lighter solids that settle slowly even after coagulation, inclined-plate settling can add effective area within the same footprint — see our note on lamella and high-rate settler design and the sibling article on lamella clarifier design. Where flotation suits the solids better than settling — for example FOG-laden or low-density solids — a dissolved air flotation (DAF) system is the alternative primary separation route.

Primary clarifier sizing sequence

1. Establish design flows. Set average and peak flows including recycles. Size the surface area on average flow and check capture at peak.
2. Select the surface overflow rate. Choose SOR of 30–50 m³/m²·d (average) from Hazen theory and comparable duty; lower for weak or cold sewage.
3. Compute surface area and dimensions. A = Q / SOR. Convert to a diameter (circular) or length×width (rectangular) using a standard mechanism size.
4. Set side-water depth and check detention. Adopt 3.0–4.5 m SWD; confirm detention time t = H/SOR lands within 1.5–2.5 h to avoid septicity.
5. Check weir loading. Keep weir loading below ~125–250 m³/m·d; add finger/inboard weirs to gain length if the single perimeter weir is overloaded.
6. Verify scour and sludge withdrawal. Confirm horizontal velocity stays well below the scour velocity, and size the hopper and draw-off for the expected primary sludge rate.