ANS:B - 50%

If the moisture content of the sludge decreases from 90% to 80%, it means that the solids content increases proportionately. Initially, if the sludge is 90% moisture, then the solids content is 10%. When the moisture content decreases to 80%, the solids content becomes 20%. Now, let's consider the volume of sludge:

• Initially, if the volume of sludge is 100 units (arbitrary unit), then the volume of solids is 10 units and the volume of water is 90 units.
• After the decrease in moisture content, the volume of solids remains the same, 10 units, and the volume of water reduces to 80 units.

So, the volume of sludge decreased from 100 units to 90 units. To find the percentage decrease: Percentage decrease=(Decrease in volume/Initial volume)×100 Percentage decrease=((100−90)100)×100=(10/100)×100=10% The correct percentage decrease in the volume of sludge is 10%.