MLSS And SRT Calculator

Work out mlss and srt instantly with clear inputs, formula shown and shareable results.

About MLSS And SRT Calculator

Solids retention time is the biomass inventory in the system divided by the mass wasted each day. It is the master control variable of activated sludge: below about 4-6 days nitrifiers wash out, 8-15 days gives stable nitrification, and above 20 days gives extended aeration with well-mineralised sludge.

Formulas

Solids inventory

Inventory (kg) = volume (m3) x MLSS (mg/L) / 1000

Solids retention time

SRT (days) = inventory / (waste flow x waste concentration / 1000)

Worked example

With the default values below, MLSS And SRT Calculator returns Solids Retention Time of 16.4063. Change any input above and every figure here is recalculated in your browser.

Inputs used

Aeration Tank Volumeminimum 0 m3: 3,000 m3
MLSS Concentrationminimum 0 mg/L: 3,500 mg/L
Waste Sludge Flowminimum 0 m3/day: 80 m3/day
Waste Sludge Concentrationminimum 0 mg/L: 8,000 mg/L

Results

Solids Retention Time: 16.4063
Solids Inventory: 10,500.00
Solids Wasted Per Day: 640.000
Waste Sludge Dry Solids: 0.8000%

Step by step

1Values used
2Solids inventory
3Solids retention time
4Solids Retention Time= 16.4063
5Solids Inventory= 10,500.00
6Solids Wasted Per Day= 640.000
7Waste Sludge Dry Solids= 0.8000

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is SRT more fundamental than MLSS?

MLSS is the consequence of SRT and loading, not an independent variable. Controlling the plant by wasting to a target SRT gives stable performance; chasing an MLSS target does not.

What happens if SRT is too long?

Excessive endogenous decay, pin floc, poor settling and unnecessary aeration energy. Nitrate build-up can also cause rising sludge in the clarifier through denitrification.