Calculate sludge volume index (SVI) from settled sludge volume, sample volume, and MLSS, or solve for missing settleometer values.

Use the 30-minute settled volume and MLSS from the same mixed liquor. SVI alone does not diagnose a process fault.

Sludge Volume Index Formula

SVI = WV / DWC

Variables:

  • SVI is the Sludge Volume Index (mL/g)
  • WV is the wet volume of settled sludge (mL/L)
  • DWC is the dry weight concentration (grams/L)

Divide the 30-minute settled volume in mL/L by MLSS in g/L. For a measured sample, first normalize the settled volume: 1000 × settled volume / total sample volume, using matching volume units. Select the value to solve for in the calculator.

How to Calculate Sludge Volume Index?

The following steps outline how to calculate the Sludge Volume Index.

  1. First, determine the wet volume of settled sludge (mL/L). 
  2. Next, determine the dry weight concentration (grams/L). 
  3. Next, gather the formula from above: SVI = WV / DWC.
  4. Finally, calculate the Sludge Volume Index.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

wet volume of settled sludge (mL/L) = 75

dry weight concentration (grams/L) = 3. SVI = 75 / 3 = 25 mL/g. Interpret this value with plant-specific observations; a low SVI alone does not establish good treatment.

FAQs

What is the importance of calculating the Sludge Volume Index (SVI)?
The Sludge Volume Index is crucial for wastewater treatment processes as it helps in determining the settling characteristics of sludge. A lower SVI indicates a smaller settled volume per gram; very low values can also accompany poorly flocculated sludge. Interpret SVI with plant trends and other process observations.

How often should the Sludge Volume Index be calculated in a wastewater treatment facility?
The frequency of calculating SVI can vary depending on the specific requirements of the wastewater treatment facility. Follow the facility’s approved sampling and process-control procedure; monitoring frequency depends on the plant and operating conditions.

Can the Sludge Volume Index vary for different types of sludge?
Yes, the SVI can vary significantly depending on the type of sludge and the conditions within the treatment process. Factors such as microbial composition, sludge age, and operational conditions can all influence the SVI.

What are the implications of a high Sludge Volume Index?
A high SVI indicates poor settling characteristics of the sludge, which can lead to issues such as cloudy effluent and increased sludge volume. This can compromise the efficiency of the wastewater treatment process and may require operational adjustments or further investigation.