Calculating Sludge Waste

I had an operator call the other day and say, “I finally got my plant producing a good effluent and have calculated the sludge age. Now I need to know how to figure out how much sludge I need to waste per day to keep the sludge age and the food-to-microorganism ratio the same. How do I do that?” Good question.

In wastewater treatment, we assume that for each pound of biological oxygen demand (BOD) that comes in the influent, we make approximately 0.5 pounds of sludge, but each plant is different. To check that and have a good working number, I told this operator to go back to his DMRs for the last year and get the monthly BOD loading in pounds and determine how many pounds of BOD came into his plant last year. I then told him to look at the sludge disposal record and determine how many pounds of sludge were disposed of last year.

By using the following equation, the operator can determine how many pounds of sludge are produced per pound of BOD in the influent.

Pounds of sludge disposed of
Pounds of BOD in the influent

The operator can then use the number of pounds of sludge produced per pound of BOD in the influent daily to tell him how many pounds of sludge to waste.

Let’s work one.

Last year at this facility, they had a total of 341,000 pounds of BOD in the influent, and they disposed of 166,000 pounds of sludge. How many pounds of sludge were produced for each pound of BOD in the influent?

166,000 lbs. Sludge
341,000 lbs. BOD Inf.
=
0.48 lbs. of sludge produced/lb. of BOD Inf

If the operator had 6,780 lbs. of BOD in the influent today, how many lbs. of sludge would he have to waste to keep his sludge age the same?

6,780 lbs. BOD inf. X 0.48 lbs. sludge/lb. BOD inf.
=
3,254.4 lbs. sludge to be wasted

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