Enter your Settled Sludge Volume (V₃₀), Mixed Liquor Suspended Solids (MLSS), and Sample Volume into the Sludge Volume Index Calculator to find your SVI value along with a Sludge Condition assessment and Settleability Rating that tells you how well your activated sludge is actually settling.
Sludge Volume Index (SVI)
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Sludge Condition
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Settleability Rating
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SVI is a measure of the settleability and compactability of activated sludge. It represents the volume in milliliters occupied by 1 gram of sludge after 30 minutes of settling.
SVI is calculated using the formula: SVI = (Settled Volume × 1000) / MLSS Concentration. The settled volume is measured after 30 minutes of settling, and MLSS is the mixed liquor suspended solids concentration.
Generally, SVI values between 80-120 mL/g indicate good settling characteristics. Values below 80 may indicate pin floc or old sludge, while values above 150 suggest poor settling and potential bulking issues.
SVI helps operators monitor and control the activated sludge process. It indicates settling problems early, helps optimize return activated sludge rates, and ensures efficient solid-liquid separation in clarifiers.
SVI should be measured daily in most activated sludge systems. More frequent monitoring may be necessary during process upsets or when making operational changes to the treatment system.
High SVI values are typically caused by filamentous bulking, low food-to-microorganism ratio, nutrient deficiency, low dissolved oxygen, or pH imbalances. Identifying the root cause is essential for proper treatment.
SVI can be controlled by maintaining proper F/M ratio, adequate dissolved oxygen levels, appropriate pH, sufficient nutrients, and proper return activated sludge rates. Chlorination may be used to control filamentous bacteria.