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Rectangular Secondary Clarifier

The rectangular tank trades the circular scraper for a chain-and-flight mechanism and buys a much better footprint on tight sites. It brings a check the circular design does not: horizontal velocity. Push the water through too fast and you scour the blanket off the floor and carry it over the weir.

1Duty & sludge

Secondary clarification is governed by two different limits at once — how fast water rises (SOR) and how much solids the tank can pass to the bottom (SLR). Set both.

2Sizing criteria & geometry

Area takes the envelope of the hydraulic and solids limits. Length-to-width then sets the plan: long and narrow gives better hydraulic efficiency, but a very long tank needs more scraper power.

Results — area envelope & geometry

Total area
—m²
HRT at avg flow
—h
Horizontal v
—m/min
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3Hydraulics & outlet

The horizontal velocity check has no equivalent in a circular tank — it is the reason a rectangular clarifier can fail even when its SOR is textbook.
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4Sludge balance & collection

The underflow must be able to carry away at least as much solids as arrives. If it cannot, the blanket rises until it spills over the weir — the classic secondary-clarifier failure.
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Operation envelope

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5Instrument schedule

The sludge blanket level is the single most useful measurement on a secondary clarifier — and the one most often missing.
PointInstrumentWhy here

Quote the clarifier instruments at factory price

Blanket level, effluent turbidity, MLSS and RAS flow — all mappable to our catalogue and supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).

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Sources & parameter basis

[1]METCALF & EDDY / EPA — secondary clarifier: peak SOR 30–50 m³/m²·d, average SOR 15–30, SLR 100–250 kg MLSS/m²·d (typ. 120–200), side water depth 3–4.5 m, HRT 2–4 h, RAS 0.5–1.5×, minimum 2 tanks.
[2]Underflow concentration from SVI: CRAS ≈ 10⁶/SVI mg/L (valid SVI 80–150); equivalently RAS ≈ MLSS × 1000/SVI.
[3]Rectangular geometry: L:W 3:1 to 6:1; horizontal velocity limited to about 1.0–2.0 m/min at peak flow to avoid scouring the settled blanket.
[4]Weir loading: 125–250 m³/m·d at average flow, up to 375 m³/m·d at peak. Side-wall weirs give 2L of weir length per tank, end weirs give W.
[5]Chain-and-flight scraper: 0.3–1.2 m/min; flight spacing 3 m; the return run should clear the collected sludge well inside its detention time.
Full derivation and worked example: Rectangular-Secondary-Calculation-Sheet.md. Try switching the weirs from both side walls to end-weir-only — you will watch the weir load roughly quadruple and the design stop working. That is why rectangular tanks run their effluent launders along the long walls.
© 2026 Nanjing JiuYuHui Environmental Technology Co., Ltd. · jiuyuhub.com · Municipal WWTP design manual — Secondary (variant).
⚠ Screening values only. Outputs from this tool are engineering screening estimates. Final equipment sizing, and any construction or permit submission, must be confirmed against the manufacturer’s verified performance curves and the applicable local regulation (e.g., Vietnam QCVN 14 / Indonesia PERMENLHK 68).
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Engineer's FAQ

Frequently asked about rectangular secondary clarifiers

How do I calculate the side water depth of a rectangular secondary clarifier?
Side water depth (SWD) = effective clarifier volume ÷ (length × width). For conventional rectangular secondary clarifiers it is normally 3.0–4.5 m. Depth sets the solids residence time and sludge-blanket stability — too shallow invites short-circuiting, too deep raises civil cost.
What is a typical side water depth for a secondary clarifier?
Rectangular units run 3.0–4.5 m; a few high-rate designs reach 5 m. The chosen SWD balances hydraulic retention, sludge-blanket control and civil cost against the surface overflow rate your effluent limit demands.
How does secondary clarifier design affect effluent quality?
Effluent clarity is governed by surface overflow rate and weir loading more than by depth alone. Size those against your peak flow, then confirm the geometry with a full design check.
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