FREE ENGINEERING TOOL · SECONDARY

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.

Avg flow Q^[1] (1k–300k)m³/d

Peak factor^[1] (1.5–3)×

MLSS^[1] (2–5)g/L

SVI^[2] (80–150)mL/g

RAS ratio R^[1] (0.5–1.5)×

Tanks^[1] (2–8)ea

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.

Peak SOR^[1] (30–50)m³/m²·d

SLR^[1] (100–250)kg/m²·d

Side depth H^[1] (3–4.5)m

L : W^[3] (3–6)—

Weir limit^[4] (125–375)m³/m·d

v_h limit^[3] (1.0–2.0)m/min

Scraper speed^[5] (0.3–1.2)m/min

Weirs per tank^[4]—

Results — area envelope & geometry

Total area

—m²

HRT at avg flow

—h

Horizontal v

—m/min

Item	Value	Basis

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.

Item	Value	Basis

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.

Item	Value	Basis

Operation envelope

Item	Value	Basis

5Instrument schedule

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

Point	Instrument	Why 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).

Request a quote for this duty ← Conventional Activated Sludge Chlorine Disinfection →

Response within 24 hours · Inputs stay in your browser — nothing is uploaded.

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: C_RAS ≈ 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.

Questions about a duty, a specification, or transparent sourcing? Contact our application engineers →