Sludge Production Calculations and Removal Methods

Any process will generate waste that has to be dealth with. In the softening process the sludge produce must be removed from the system and is usually dried and hauled off site. Stanton Energy Center uses waste sludge from it's softener and other water plants in it's scrubbers to remove sulfur dioxide from it's flue gasses. The resultant calcium sulfate can be then used as a raw material for other useful products.

The material below shows how to calculate the removal of sludge out of a softening basin.

Ca hardness, Mg hardness and NCH are as CaCO3 . Sludge production is as such. For every lb of Ca Carbonate Hardness removed there are 2 lbs of sludge produced. So 1 lb of Ca removed is equal to 2 lbs of dried sludge. This is shown by the equation -- Ca(CO3)2 + Ca(OH)2 --> 2CaCO3 + 2H2O . For every lb of NCH removed you produce 1 lb dried sludge.

The equation is --CaSO4 + Na2CO3 --> CaCO3 + Na2SO4 . Any other anion could be substitute for SO4 . Mg is taken care below . For every lb of Mg removed 2.58 lbs of sludged dried are produced. The equations are --Mg(HCO3)2 + Ca(OH)2 --> MgCO3 + CaCO3 . MgCO3 + Ca(OH)2 --> CaCO3 + Mg(OH)2 . In all of these cases the CaCO3 equivalents are simply added up for each type of sludge produced.

The other two things needed to predict sludge produced and removal are specific gravity and percent by weight of solids in the sludge.To figure specific gravity , tare a 100 ml graduated cylinder. Pour in 100 ml of well mixed sludge sample. Weigh it to nearest one hundredth of a gram. Divide the sample weight by 100. This gives specific gravity.Specific gravity is a factor to tell you how much more or less the liquid weighs than a equal volume of water. The percent by weight is gotten by weighing a volume of sludge then filtering it with a dry tared filter. The filter and sludge cake are dried. The weight of the sludge cake is calculated. The dried sludge cake is divided by the weight of the original sludge sample . This gives the percent solids as a decimal percent.

A Example of Sludge Production Calculations

On one MG of water you remove 120 ppm of Ca Carbonate Hardness, 4 ppm of Mg Hardness and 60 ppm of NCH all expressed as CaCO3 .

1) Ca Carbonate Hardness -- 120 ppm X 2 X 8.34 X 1 MG = 2002 lbs.

2) Mg Hardness -- 4 ppm X 2.58 X 8.34 X 1MG = 86 lbs.

3) NCH -- 60 ppm X 1 x 8.34 X 1MG = 500.4 lbs.

4) Total up the #'s, 2002 + 86 + 500 = 2588 lbs/MG dried.

5) Given by test the sludge is 6 % solids and the specific gravity is 1.03 . Then 1.03 X 0.06 X 8.34 = 0.515 lbs/gal of solids .

6) 2588 lbs/0.515lbs/gal = 5025 gal/MG of sludge that needs to be with drawn per Million Gallons.

Sludge Removal Control Methods

Practically there are two control methods for sludge removal. One is the calculation method just done.

After the % solids in the reaction zone are in the correct range , remove all solids produce there after.

The other is to have your sludge removal pumps come on and off according to the % torque on your sludge rake.

You adjust this according to the % solids in the zone. When the zone solids are correct leave the torque setting at that % .

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