Water tools

Secondary clarifier area by loading rates and solids flux

A secondary clarifier must both clarify (the upflow must stay below the settling velocity) and thicken (the applied solids flux must stay below the limiting flux). Both are checked here, alongside the conventional loading-rate method.

Average flowm³/d

Maximum flowm³/d

Return sludge flowm³/d

MLSSmg/L

Sludge settleabilityDesign for fair or poor unless you have settling data.

Hydraulic loading at average flowm³/m²·hConventional 0.67 to 1.33; extended aeration 0.33 to 0.67.

Hydraulic loading at maximum flowm³/m²·hConventional 1.7 to 2.0; extended aeration 1.0 to 1.33.

Solids loading at average flowkg/m²·hConventional 4 to 6; extended aeration 1 to 5.

Solids loading at maximum flowkg/m²·hConventional 10; extended aeration 7.

Recommended area

898m²

Solids flux theory governs (clarification).

Solids flux for the settleability class

Area, flux theory

898m²

Clarification limit

0.46m/h

Limiting solids flux

3.64kg/m²·h

Area for clarification

898m²

Area for thickening

876m²

Conventional loading rates

Area, loading rates

638m²

Hydraulic, average

511m²

Hydraulic, maximum

445m²

Solids, average

638m²

Solids, maximum

471m²

How it works

Loading rates at average and maximum flow; the largest of the four areas governs. Only the influent flow has an upward component, so the hydraulic loading uses Q alone:

A = \frac{Q}{H L R} A = \frac{(Q + Q_{r}) X}{S L R}

(1)

Solids flux theory. The interface settling velocity and the limiting flux are taken by settleability class (very good to very poor, mapped from the sludge volume index):

v = v_{o} e^{- K C} G_{L} = m (Q_{r} / A)^{n}

(2)

Clarification requires the upflow to stay below the settling velocity, thickening requires the applied flux to stay below the limiting flux:

\frac{Q}{A} \leq v_{o} e^{- K C_{o}} \frac{(Q + Q_{r}) C_{o}}{A} \leq m {(R v_{o} e^{- K C_{o}})}^{n}

(3)

where

$A$: clarifier surface area, m²
$Q$: influent flow, m³/h
$Q_{r}$: return sludge flow, m³/h
$H L R$: hydraulic loading rate, m³/m²·h
$S L R$: solids loading rate, kg/m²·h
$X$: mixed liquor suspended solids, mg/L
$v$: interface settling velocity, m/h
$v_{o}$: settling velocity coefficient, m/h
$K$: settling velocity coefficient, m³/kg
$C, C_{o}$: mixed liquor concentration, kg/m³
$G_{L}$: limiting solids flux, kg/m²·h
$m, n$: flux coefficients by settleability class
$R$: return ratio

with C_o the MLSS in kg/m³ and R the return ratio. The larger area is adopted. Conventional literature loading rates fall between the fair and poor curves. For an overloaded tank, lowering MLSS helps both criteria; raising the return ratio helps thickening only.

Secondary clarifier area by loading rates and solids flux

How it works

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