The moisture content of sludge is reduced from 90% to 80% in a sludge digestion tank. The percentage decrease in the volume of sludge, is A. 25 % B. 50 % C. 10 % D. 5 %

The correct answer is: C. 10 %

The percentage decrease in the volume of sludge is calculated by the following formula:

Percentage decrease in volume = (100 – new moisture content) / (100 – old moisture content) * 100%

In this case, the new moisture content is 80% and the old moisture content is 90%. Therefore, the percentage decrease in volume is:

Percentage decrease in volume = (100 – 80) / (100 – 90) * 100% = 10 %

This means that the volume of sludge has decreased by 10% after the moisture content has been reduced from 90% to 80%.

Option A is incorrect because it is the percentage decrease in the moisture content, not the volume of sludge.

Option B is incorrect because it is twice the actual percentage decrease in the volume of sludge.

Option D is incorrect because it is half the actual percentage decrease in the volume of sludge.