In what unit is the sludge volume index (SVI) expressed?

The sludge volume index (SVI) is expressed in milliliters per gram (mL/g) because it reflects the volume of settled sludge relative to its mass. Specifically, SVI is calculated by measuring the volume of sludge that settles in a specified time period, usually measured in a graduated cylinder, and then dividing that volume by the mass of the suspended solids that were present at the same initial volume. This unit effectively indicates how well the sludge settles and helps in assessing the settling characteristics of the sludge in wastewater treatment processes.

Other units provided in the question do not appropriately convey the relationship between sludge volume and mass. For example, grams per milliliter (g/mL) would indicate density rather than the settling characteristics of sludge; cubic feet per day (ft³/day) measures flow or volume over time, which is not relevant to the characteristic being assessed; and milligrams per liter (mg/L) represents concentration, which is also unrelated to the volume-to-mass ratio needed for SVI. Thus, expressing SVI in mL/g provides a clear understanding of the sludge's settling capability relative to its mass.