Sludge Volume Index (SVI)

Fill a one-litre graduated cylinder with mixed liquor, wait thirty minutes, read the sludge interface, and divide that settled volume by the solids concentration: SVI is the number of millilitres one gram of sludge occupies once it has settled. Settle 200 mL/L with an MLSS of 2500 mg/L — that is 2.5 g/L — and the SVI is 80 mL/g. The solver takes the settled volume as a volume percentage, so 200 mL/L is entered as 20%, and returns SVI in the conventional mL/g.

Below about 100 the sludge is dense and settles beautifully; 100–150 is normal; above 150 you have bulking, and above 200 the clarifier is on borrowed time. High SVI almost always means filamentous organisms — Nocardia, Microthrix, type 021N — thriving because of low dissolved oxygen, low F/M, septic influent or nutrient deficiency, and a microscope will identify which within ten minutes, which is far more useful than another calculation. Two cautions on the test itself: a thick sludge above roughly 3000 mg/L hinders its own settling in a narrow cylinder and reads artificially high, which is why the stirred SSVI test and dilution to a standard concentration exist; and the reading must be taken at the sludge blanket interface, not at the point where the water merely looks cloudy.