What is the sludge concentration of a sample containing 7% sludge?

In the context of evaluating the sludge concentration in a sample containing 7% sludge, understanding the percentage as a direct representation of mass can clarify why a specific choice aligns with the calculation. A 7% sludge concentration means that for every 100 units (it could be grams, liters, etc.) of the sample, there are 7 units of sludge.

To calculate the concentration in terms of a more standardized unit, such as milligrams per liter (mg/L), the conversion process typically involves multiplying the percentage by a factor that transforms the percentage into the desired concentration metric. If you assume a total volume of 1,000 liters of wastewater, for example, the calculation would reveal a concentration of 70,000 mg/L of sludge.

This perspective illustrates that the chosen answer reflects a common standard practice within wastewater analysis, allowing professionals to express sludge concentration in a widely understood format. The other options are not consistent with the correct conversion and understanding of sludge concentration.