Sludge Volume Index (SVI) Calculator

Q: What is ashing and why does sludge float?

Ashing or "clumping" occurs when secondary sludge stays too long in a clarifier bottom without oxygen. Facultative bacteria reduce nitrate into nitrogen gas (denitrification). Microscopic N2 gas bubbles adhere to the sludge flocs, floating large brown clumps to the surface.

Q: How can operators quickly control severe filamentous bulking?

Emergency short-term mitigation involves dosing chlorine (typically 2 to 4 lbs Cl2 per 1,000 lbs MLSS per day) into the RAS return line to selectively burn exposed filamentous tentacles without penetrating dense floc centers. Long-term correction requires resolving dissolved oxygen or F/M deficiencies.

Compute SVI in mL/g from 30-minute settleometer jar tests and mixed liquor suspended solids (MLSS), diagnose filamentous bulking, and estimate RAS density.

30-Minute Settled Sludge Volume (SS30 - mL / L) Measured in 1,000 mL settleometer jar

Mixed Liquor MLSS (mg / L) Total suspended solids lab test

Supernatant Visual Clarity

Sludge Volume Index (SVI)

112 mL / g

Optimal Good Settleability (80 - 150 mL/g)

Max RAS Concentration

8,929 mg / L

0.89% Underflow Solids

Volume Compaction

72.0% Clear

Blanket compacted to 28%

Sludge Settling Velocity: Rapid & Steady (Good Floc)

Recommended RAS Flow Range: 35% - 50% of Q

Filamentous Bulking Risk: Low (Balanced Biology)

Clarifier Blanket Behavior: Predictable 1.5 - 2.5 ft depth

Evaluated according to Standard Methods for the Examination of Water and Wastewater (Method 2710D).

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Sludge Volume Index (SVI) & Clarifier Settleability

The Sludge Volume Index (SVI) is the standard laboratory metric used worldwide to assess the settling and compacting characteristics of activated sludge in secondary clarifiers. It represents the volume in milliliters occupied by one gram of dry mixed liquor suspended solids after 30 minutes of undisturbed quiescent settling in a 1-liter graduated cylinder.

SVI Governing Formulation

SVI (mL/g) = [ Settled Sludge Volume SS30 (mL/L) × 1,000 ] / MLSS (mg/L)

Clinical Interpretation of SVI Ranges

SVI < 70 mL/g (Rapid / Pin Floc): Sludge settles too fast. Individual small floc particles ("pin floc" or "stragglers") are left sheared behind in the supernatant, causing cloudy effluent.
SVI 80 – 150 mL/g (Ideal Operating Window): Flocs form broad golden-brown blankets with ragged edges, trapping fine particles like a mesh net and leaving a crystal-clear supernatant.
SVI 150 – 250 mL/g (Slow Settling / Incipient Bulking): Floc particles are voluminous and light. Clarifier sludge blanket rides high, requiring elevated RAS pumping rates.
SVI > 250 mL/g (Severe Filamentous Bulking): Hair-like filamentous bacteria physically push flocs apart. Sludge will not compact below 600-800 mL/L, threatening catastrophic blanket washout.

Frequently Asked Questions

How does SVI determine the theoretical maximum RAS concentration?

The maximum density of sludge compacted at the bottom of the clarifier is inversely proportional to SVI: RAS_max (mg/L) = 1,000,000 / SVI. For an SVI of 100 mL/g, the maximum theoretical underflow concentration is 1,000,000 / 100 = 10,000 mg/L (1.0% solids).

What causes filamentous sludge bulking (SVI > 200)?

Common causes include low dissolved oxygen in aeration basins (DO < 1.5 mg/L), low F/M ratio (<0.10), septic incoming wastewater containing sulfides, nutrient deficiency (nitrogen or phosphorus), and low pH (<6.5).

What is ashing and why does sludge float?

Ashing or "clumping" occurs when secondary sludge stays too long in a clarifier bottom without oxygen. Facultative bacteria reduce nitrate into nitrogen gas (denitrification). Microscopic N2 gas bubbles adhere to the sludge flocs, floating large brown clumps to the surface.

How can operators quickly control severe filamentous bulking?

Emergency short-term mitigation involves dosing chlorine (typically 2 to 4 lbs Cl2 per 1,000 lbs MLSS per day) into the RAS return line to selectively burn exposed filamentous tentacles without penetrating dense floc centers. Long-term correction requires resolving dissolved oxygen or F/M deficiencies.