Sludge Volume Index (SVI) is a critical indicator of activated sludge settling characteristics, essential for plant operation and control.

What is SVI?

SVI is the volume occupied by 1 gram of sludge after 30 minutes of settling, expressed as mL/g. It indicates how well biomass settles in the secondary clarifier.

SVI Interpretation

• SVI < 80 mL/g: Poor settling, pin floc
• SVI 80-120 mL/g: Good settling (ideal range)
• SVI 120-150 mL/g: Fair settling, some bulking
• SVI > 150 mL/g: Bulking sludge, settleability problems

Causes of High SVI

Filamentous Bulking

Excessive filamentous organisms interfere with settling. Caused by low DO, low F/M, nutrient deficiency, or sulfide presence.

Non-Filamentous Bulking

Viscous, non-filamentous bulking due to high polysaccharide production. Often caused by high F/M or specific industrial compounds.

SVI Control Strategies

• Maintain proper DO levels (2-4 mg/L)
• Optimize F/M ratio (0.05-0.15 lb BOD/lb MLSS-day)
• Ensure adequate nutrients (N, P)
• Control return sludge rate
• Selector zones for bulking control

Feiran EP Monitoring Solutions

We supply laboratory equipment and online analyzers for SVI monitoring and control.

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