The distinction matters because two samples can occupy the same settled volume while containing different masses of suspended solids.
What is SV30?
SV30 is the volume occupied by settled sludge after 30 minutes in a defined settlement test. In a conventional one-litre procedure it may be recorded in millilitres per litre or as a percentage of the sample volume.
It describes how much space the settled layer occupies at the observation time. It does not, by itself, account for how much mixed liquor suspended solids were present in the original sample.
The S3A uses the term settled sludge level for its automated result from the compact 210 ml sample rather than presenting a manual one-litre SV30 reading.
What is Sludge Volume Index (SVI)?
SVI normalises the settled sludge volume by the MLSS concentration of the same sample. It is normally expressed in millilitres per gram (mL/g).
For a conventional test, SVI is calculated from settled sludge volume in mL/L multiplied by 1,000 and divided by MLSS in mg/L.
Because MLSS is included, the same settled volume can produce different SVI values when the solids concentration differs.
The most useful comparison is the treatment works own consistent trend, produced from the same sampling point and method.
Do not confuse a sample result with clarifier blanket depth
SV30 or settled sludge level describes a sample in a settlement vessel. Sludge blanket depth is a full-scale clarifier measurement. They may be reviewed together, but they are not interchangeable measurements.
Keep operational and compliance roles separate
SV30 and SVI are activated-sludge process-control observations. They do not replace required final-effluent sampling, suspended-solids laboratory analysis or regulatory reporting.
Use a consistent method for the trend
Compare results only when the sample location, preparation, vessel, timing and stirred or unstirred method are identifiable. A method change can move the result even when the process has not changed.
How the S3A connects settled level and SVI
The S3A measures MLSS first, then continues the 30-minute settlement cycle. This connects concentration and settled-sludge information from the same representative sample.
Key differences between SV30 and SVI
SV30 is a raw settled-volume observation after 30 minutes
SVI is a volume-per-mass index expressed in mL/g
SVI requires the MLSS concentration from the same representative sample
The same SV30 can produce a different SVI when MLSS changes
Both results depend on sampling, timing and the selected test method
Stirred and unstirred methods must remain separately labelled
A sample settled level is not the same as a full-scale clarifier blanket measurement
Neither result diagnoses the cause of a settling change on its own
How the S3A reports the complete cycle
The S3A uses a representative 210 ml sample and provides an MLSS result in approximately 10 seconds across its specified 50 to 6,500 mg/L range.
After 30 minutes, it records SVI, settled sludge level, supernatant clarity and floating sludge. The optional stirrer supports SSVI and stirred settled sludge level.
Use those connected results as operational evidence. The S3A is not a continuous online analyser and does not replace required compliance sampling or laboratory analysis.
Follow the measurement definitions
Read the Settled Sludge Level parameter guide and the SVI parameter guide for the two S3A results.
Keep the raw settled-level result beside MLSS and SVI in the operating record. That preserves the evidence needed to see whether a change came from concentration, settlement volume or both.
Build a consistent interpretation route
The SVI vs SSVI guide separates stirred and unstirred methods. The high-SVI article explains how to review an unexpected index value.
For the complete instrument role and limits, use the S3A product page or the activated-sludge application guide.
Authoritative references
Environment Canada: Sludge Volume Index defines SVI and shows how settled volume and MLSS are combined in the calculation.
UK government-hosted effluent-treatment guidance describes the 30-minute settled-volume and MLSS basis of SVI.
For help matching the S3A workflow to a treatment works procedure, contact PMA with the sampling and measurement requirement.
