Hi gang,

I got a question on calculating the mass of sludge to be disposed following a belt press.  I've run the calc a couple of ways and get two different numbers.

I am going to clean an oxidation ditch at a municipal wwtp by pumping the wastewater through a press first before going to landfill.  I imagine the sludge in the ww will be primarily biological but will contain some grit as well due to being in service for so long.  Somewhere close to 400,000 gal at 4% solids.

My calcs are as follow:

Tank Volume = 51,000 ft3
Unit weight sludge = 55 lbs/ft3 (assumed)
Total weight = 2,805,000 lbs or 1402 wet tons
Following press % solids = 20%
Total Mass to be disposed = 280 tons

Is that correct?  The press manufacturer told me 67 dry tons and I get close to that by:

400,000 * 4% = 16,000 gal
16,000 gal = 2139 ft3
2139 ft3 * 55 lbs / ft3 = 117647 lbs or 59 tons

So which is it?  I'm concerned the second number is just 100% sludge and doesn't factor in the filter cake will be 80% water.

Thanks!

Ok, I see my error in the second calculation and I can find 67 tons now.  However, which calculation is correct for my disposal?  Is the second calculation just giving me 100% solids?

Are you telling me my error was going from 4% solids to 20% in my mass to be disposed calc?  Should the calc be wet tons * 4% = dry tons to be disposed?

Where I am confused is the terminology of dry tons...does that mean following the press and basically cake?  Dry tons accounts for the moisture content still in the cake?

Just in case anyone comes across this thread and needs to know the answer, the dry tonnage that the press manufacturer and typical calculations give you is 0% moisture.  It is commonly used for land application and polymer dosage.

However, if you are calculating tonnage for disposal at a landfill, you must add in the weight of water that the sludge cake will contain.  In this instance, we assumed the press can get the cake to 20% solids.  At the landfill scales, the 80% moisture will be weighed so you have to factor that in along with the solids.  It will drastically increase your cost so be careful!!!