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# sludge weight (lb/ft3)

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## sludge weight (lb/ft3)

(OP)
I am trying to find the density of a municipal WWTP sludge at 30% dry solids,lets say in lbs/ft3 (the sludge is a mix of primary and secondary, no digestion).  I also want to land apply it, so it has to be mixed with lime for stabilization.  (I would also like to know the mixing requirements and typical density of that as well)

I have been researching this, but the numbers I am finding are all over the map.

Is there a good table for % dry solids versus lbs/ft3?
Or a good calc?

thanks

### RE: sludge weight (lb/ft3)

The specific gravity of the sludge solids is probably around 1.3. 70% of the sludge is water.

Sludge density = 62.4 lbs/ft3 * 1.3 * 30%  +  62.4 lbs/ft3 * 1.0 * 70%  = 68.0 lbs/ft3

The lime dosage depends on the sludge, but will probably be around 400 lbs lime per lb dry solids.

### RE: sludge weight (lb/ft3)

(OP)
BIMR: I have been searching for numbers on sludge and % dry solids, and found a range. One is sludge 54lbs/ft3(comment: must be very dry?).  One is SG = 1.3  One is lime stabilized sludge is 100 lbs/ft3 (@20% lime per lb dry solids)(I assume your number is per ton dry solids).  Then I had a gallon of wet centrifuged sludge (consistency of, lets say, porridge) weighed.  Result -> one gallon of 6% sludge weighed 10 lbs (74.8lbs/ft3). However, I am leanning toward using SG 1.3 before lime addition.  thanks

### RE: sludge weight (lb/ft3)

RYANTD,

I recommend you take a close look at "Lime Post-Treatment" section in Metcalf & Eddy before going further.  If you use quicklime (which is discussed in this chapter)I bet you'll be dosing in the range of 0.6 to 0.9 lb CaO per lb of 30% sludge.

Best of Luck!

### RE: sludge weight (lb/ft3)

CivEnvEng98,

Metcalf & Eddy shows a dosage of 0.48 lbs quicklime per lb of sludge dry solids for 30% sludge.

### RE: sludge weight (lb/ft3)

(OP)
This worked for me.

The mass of solids in a slurry is related to the volatile and fixed suspended solids content. If Ss = Sp. Gr. of the slurry, Sw = Sp. Gr. of water, Sv = Sp. Gr. of VSS, Sf = Sp. Gr. of FSS, Mw = mass of water, Mv = mass of VSS, and Mf = mass of FSS, then since

Ms/Ss = Mw/Sw + Mv/Sv + Mf/Sf -> Solve for Ss

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