Tennessee Wastewater Operator Practice Exam

What units are used to report the results of a settleable solids test?

1. g/L

2. ml/L

3. ppm

4. mg/L

The correct answer is: ml/L

The results of a settleable solids test are reported in milliliters per liter (ml/L). This unit is appropriate because settleable solids testing measures the volume of solids that settle out from a water sample over a specified period of time, typically 30 minutes. Using ml/L allows operators to quantify how much of the total volume in a liter of water is made up of solids that have settled to the bottom, which is essential for assessing the performance of wastewater treatment processes. This measurement directly correlates to the sedimentation capacity and efficiency of the treatment system. Other units like grams per liter (g/L), parts per million (ppm), and milligrams per liter (mg/L) are used in different contexts. Grams per liter would measure the mass of solids but not the settling characteristics. Parts per million might be used for very dilute solutions, which can be misleading when dealing specifically with settleable solids. Milligrams per liter is often used for suspended solids but does not accurately reflect settleable solids that have settled out over time.