Formula & Calculator
Sludge Volume Index
Measures how well activated sludge settles in a wastewater treatment plant, indicating whether the biological process is healthy.
Interpretation
SVI = (Settled Sludge Volume (mL/L) × 1000) / MLSS (mg/L). Measures the settling characteristics of activated sludge. Used to diagnose bulking and to control solids separation.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| SVI | Sludge volume index | mL/g |
| Settled Sludge Volume | Sludge volume after 30 minutes settling | mL/L |
| MLSS | Mixed liquor suspended solids concentration | mg/L |
What it means
The Sludge Volume Index (SVI) is a key parameter in activated sludge wastewater treatment. It indicates the compactness and settling ability of the sludge. SVI is calculated by measuring the volume of settled sludge (in mL) after 30 minutes in a 1‑L cylinder, then multiplying by 1000 and dividing by the mixed liquor suspended solids concentration (MLSS in mg/L). A typical SVI for well‑settling sludge is between 50 and 150 mL/g. Values above 150 indicate bulking sludge, which settles poorly and can lead to solids carryover in the effluent. The SVI is used by operators to adjust sludge age, return sludge flow, and aeration rates to maintain good settling. Example: A 1‑L sample settles to 250 mL after 30 minutes, and the MLSS is 3000 mg/L. SVI = (250 × 1000) / 3000 = 250 / 3 = 83.3 mL/g, indicating good settling. An SVI of 200 would indicate bulking, requiring corrective action.
Worked example
Sludge Volume Index – Two Examples
Real‑World| Parameter | Value |
|---|---|
| SSV | 250 mL/L |
| MLSS | 2,500 mg/L |
| Parameter | Value |
|---|---|
| SSV | 400 mL/L |
| MLSS | 2,000 mg/L |
Common mistakes
- Settled sludge volume: Measured in a 1‑L graduated cylinder after 30 minutes of settling – in mL/L.
- MLSS: Mixed liquor suspended solids concentration in mg/L.
- Units: SVI = (SV30 in mL/L) * 1000 / MLSS (mg/L) – the result is in mL/g.
- Interpretation: SVI < 100 indicates good settling; 100‑200 is fair; >200 indicates bulking (poor settling).
- Variability: SVI can change with sludge age, temperature, and feed characteristics – monitor regularly.
Applications
Sludge volume index (SVI) is the volume occupied by 1 gram of mixed liquor suspended solids (MLSS) after 30 minutes of settling, expressed as mL/g. It is a key parameter for monitoring the settling characteristics of activated sludge in wastewater treatment. A low SVI (50‑80) indicates good settling, while high SVI (>150) suggests bulking problems due to filamentous organisms. Plant operators use SVI to detect upsets, to adjust return sludge rates, and to optimise clarifier performance. SVI also guides the design of secondary clarifiers and the control of sludge age. By tracking SVI, engineers can maintain stable biological treatment and prevent solids loss from the system.
- Monitoring of activated sludge settleability in wastewater treatment
- Detection and diagnosis of sludge bulking or foaming
- Optimisation of return activated sludge (RAS) and waste activated sludge (WAS) rates
- Design of secondary clarifiers for activated sludge processes
- Process control for nutrient removal systems (nitrification/denitrification)
Frequently Asked Questions
SVI is a measure of the settling characteristics of activated sludge. It is calculated as SVI = (Settled Sludge Volume (mL/L) × 1000) / MLSS (mg/L). The settled sludge volume is the volume occupied by the sludge after 30 minutes of settling in a 1‑litre graduated cylinder. MLSS is the mixed liquor suspended solids concentration.
- Good settling: SVI < 100 mL/g.
- Moderate: SVI 100‑150 mL/g.
- Poor settling (bulking): SVI > 150 mL/g.
- Very high SVI (> 200) often indicates filamentous bulking.
Interpreting a high SVI as always indicating a plant upset. It commonly signals filamentous bulking, but the appropriate response depends on the specific cause (e.g., low dissolved oxygen, nutrient deficiency, or toxic shock). A high SVI requires investigation, not just a single corrective action.
SVI is monitored regularly to assess sludge settleability. A sudden increase in SVI may indicate bulking; a decrease may indicate good settling. Operators use SVI to adjust return sludge rates, wastage rates, and aeration to maintain a healthy biomass.
DSVI is used when the sludge is very concentrated (MLSS > 2000 mg/L). The sludge is diluted with supernatant to 1/3 of the original concentration, and the test is run. This prevents the sludge from "bridging" and gives a more accurate index.
- MLSS concentration (higher MLSS can affect settling).
- Sludge age (SRT).
- Dissolved oxygen level.
- Nutrient (N/P) balance.
- Presence of filamentous organisms.
- Hydraulic loading and mixing.
Take a 1‑litre graduated cylinder, fill it with well‑mixed mixed liquor sample. Allow it to settle for 30 minutes. Read the volume of settled sludge (mL) at the bottom. Divide by 1000 to get mL/L.
A lower SVI means better settling, allowing a higher return sludge concentration. This improves the efficiency of the clarifier and allows a higher sludge recycle ratio. Conversely, a high SVI leads to low return sludge concentration and may cause solids washout.
- Increase dissolved oxygen (if low DO is the cause).
- Add nutrients (if deficient).
- Increase sludge wasting to reduce SRT.
- Add chemicals (e.g., chlorine, hydrogen peroxide) to control filamentous growth.
- In severe cases, use a selector tank to promote floc‑forming bacteria.
SVI = (300 × 1000) / 2500 = 300,000 / 2500 = 120 mL/g. This indicates moderate settling.