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Sludge Volume Index (SVI)

Sludge Volume Index (SVI) is a measurement used in wastewater treatment to show how well sludge settles in the aeration tank. It helps operators understand the health and performance of the biological treatment process. SVI tells you how much space the sludge takes up after 30 minutes of settling. A lower SVI usually means the sludge settles well and can be separated easily, while a higher SVI may point to poor settling or bulking issues.

SVI is calculated by dividing the volume of settled sludge (in millilitres per litre) by the concentration of solids in the tank, known as MLSS (Mixed Liquor Suspended Solids), and multiplying by 1000. This gives a value in mL/g. For example, if the sludge doesn’t settle well and floats or stays suspended, the SVI number will be high, often a sign of filamentous bacteria causing problems in the system.

Operators and engineers use SVI daily to keep treatment processes stable and efficient. It’s a quick, reliable way to check whether the system works well or adjustments are needed. Automating this test with the right analyser can save time and improve consistency across multiple sites.

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Products for this application

Suspended Solids Analyser – Accurate MLSS & SVI Monitoring

Measure key wastewater parameters with our Suspended Solids Analyser, including MLSS, SVI, supernatant clarity, and floating sludge. Designed for reliable, automated performance in wastewater treatment and process control.

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