Sludge Volume Index (SVI) & Settleability Calculator
Model activated sludge settleability: calculate Sludge Volume Index (SVI) in mL/g, diagnose filamentous bulking, and determine required RAS recycle flow rates.
Settleometer & MLSS Test Data
SVI Benchmark Ranges
- SVI < 80 mL/g: Rapid settling / Old sludge / Pin-point floc
- 80 - 150 mL/g: Ideal settling floc (crystal clear supernatant)
- 150 - 250 mL/g: Slow settling / Moderate filamentous expansion
- SVI > 250 mL/g: Severe filamentous bulking (blanket washout)
Sludge Volume Index & Settleability
Compaction & Underflow Dynamics
Operational Troubleshooting Guide
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
What is the Sludge Volume Index (SVI) and how is it tested in the lab?
SVI is the volume in milliliters occupied by 1 gram of activated sludge after 30 minutes of undisturbed settling. A 1,000 mL mixed liquor sample from the aeration basin outlet is poured into a 1-liter graduated cylinder (or Mallory settlometer), the 30-minute settled volume (SSV30) is recorded, and divided by the MLSS concentration in g/L: SVI = (SSV30 × 1,000) / MLSS.
Why does an SVI between 80 and 150 mL/g represent the operational ideal?
In this range, heterotrophic bacteria produce the optimal balance of extracellular polymeric substances (EPS) and structural filamentous backbone. The sludge forms large, golden-brown flocs that settle rapidly within 10 to 15 minutes, compacting into a dense sludge blanket and leaving a crystal-clear supernatant.
What causes filamentous sludge bulking (SVI > 200 mL/g)?
Filamentous bulking occurs when long, thread-like bacteria outgrow floc-forming bacteria, bridging flocs apart into an uncompactable sponge. Primary triggers include: (1) low dissolved oxygen in aeration, (2) low F/M ratios (starvation), (3) septic septicity or high hydrogen sulfide in incoming sewage, and (4) nutrient deficiency (lack of nitrogen or phosphorus).
How is SVI used to set Return Activated Sludge (RAS) pumping rates?
The theoretical maximum underflow solids concentration that can be drawn from the clarifier bottom is roughly 1,000,000 / SVI (mg/L). When SVI rises (sludge fluffs up), the underflow concentration drops, forcing operators to pump a much higher percentage of RAS flow to prevent the sludge blanket from backing up over the weirs.