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Activated sludge process -WAS claculation
Azmathulla Zaffrulla
Posted: Tuesday, February 23, 2010 12:48 AM
Joined: 2/17/2010
Posts: 1

 Hi forum members ,

Volume of excess Sludge = (Q x BODin x Yobs x BODRE) ÷ MLSS                                                

Q      Influent flowrate      m3/d
BODinInfluent BODKg/m3
Yobs      Observed yield coeficient %
BODRE BOD removal efficiency %
MLSS      Mixed liquor suspended solidsKg/m3

I was told that the above equation can be used to determine the sludge wasting for a Activated sludge plant.

I would like to know the following

Is it applicable to all types of ASP plants
The source of this equation ( any books references etc.,)

OR is there any other method available

can any one share views with me


Posted: Thursday, October 28, 2010 5:59 AM

This equation is to control the MLSS when wasting from the aeration basin.

All it is a mass balance:


Mass of sludge waste = Daily volume wasted x MLSS

                                        = Mass of sludge produced

                                        = Daily mass of BOD removed x Sludge yield


If you want Sludge age control, and are wasting from the aeration basin, the maths is easier:


Daily volume removed = Aeration tank volume / Sludge age


If you ware wasting from the return sludge line (thicker sludge - less volume) then you need to replace MLSS by RAS solids concentration in the MLSS-based equation; for controlling sludge age by RAS-line wasting the maths is not so straight-forward.


Any good book on wastewater treatment will either include the formulae, or include the basics that allows you to derive the formulae. See the standard, Metcalfe & Eddy, or the Mogens Henze book on wastewater treatment, or that by Ron L Droste. Or search for David Vaccari - his website contains more advanced methods for wastage control, so that you don't try & remove too much in a single go in a (futile) attempt to control some 'gold standard' design criteria.

Posted: Sunday, April 3, 2011 6:07 AM
Joined: 4/3/2011
Posts: 1

Volume of Excess Sludge = (Qin x BODin x Yobs x BODRE) / MLSS

is an approximation with some big assumptions.

1) If Yobs is in units of TSS/BOD (check because this is not always the case) then:

Qin x BODin x Yobs x BODRE is the mass of biosolids grown each day.

Because MLSS measures biosolids and inert solids:

2) You have to assume that no biosolids or inert solids are coming in with the influent.

If you meet these assumptions then the equation is true.

Volume of Excess Sludge = Qin x (BODin x Yobs x BODRE + TSSin) / MLSS

gets you a little closer by including the influent solids.  This is still an approximation assuming that the TSSin is inert. 

To refine the equation more requires very detailed analysis to distinguish dead, active and inert TSSin and evaluate kinetic and stoichiometric parameters and knowledge of the SRT.