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.