Do not combine the two methods into one trend. Record which method was used, follow the site procedure and compare like with like.
What is Sludge Volume Index (SVI)?
SVI expresses the volume occupied by one gram of activated sludge after a defined 30-minute settlement test. It combines settled sludge volume with the MLSS concentration of the same representative sample.
In the conventional unstirred method, the sample settles without continuous mixing. The result is useful for trending when sampling, timing, vessel and reading practice remain consistent.
The standard S3A cycle reports unstirred SVI together with MLSS, settled sludge level, supernatant clarity and floating sludge.
What changes in the SSVI method?
1. Gentle stirring continues during settlement
SSVI uses a defined low-speed stirring method while the activated-sludge sample settles. This makes it a different procedure rather than an alternative label for the same result.
2. The method must be identified
Write SVI or SSVI with the result. If a procedure specifies a particular stirred method or sample concentration, retain that information as part of the record.
3. Suitable equipment is required
The stirred method requires a suitable settling vessel and stirrer operated according to the chosen procedure. For the S3A, SSVI and stirred settled sludge level require the optional stirrer.
4. Results need separate baselines
A site may use SVI, SSVI or both, but each method needs its own consistent trend. A change caused by switching methods should not be interpreted as a process change.
5. The site procedure decides the purpose
Use the method required by the treatment works procedure, process-control programme or technical adviser. The presence of a stirred option does not make it the automatic choice for every application.
6. Representative sampling still applies
Stirring does not correct an unrepresentative or delayed sample. Use the same sampling point, preparation and timing discipline applied to any activated-sludge settlement test.
Key differences between SVI and SSVI
SVI is produced from an unstirred settlement test
SSVI is produced with defined gentle stirring during settlement
Both use settled sludge volume and MLSS from the relevant sample
Each method needs its own label and comparison trend
Stirring can reduce some vessel-wall and quiescent-test effects, depending on the specified method
The selected procedure should match the treatment works monitoring purpose
The S3A optional stirrer is required for SSVI and stirred settled sludge level
Neither method identifies the cause of a settling change by itself
How S3A supports both methods
The S3A measures MLSS from a representative 210 ml sample in approximately 10 seconds, then continues the 30-minute settlement cycle.
Without the optional stirrer, the cycle reports SVI and settled sludge level. With the stirrer fitted and the appropriate procedure selected, it supports SSVI and stirred settled sludge level.
Both routes also provide connected settlement evidence. Review the index with supernatant clarity, floating sludge, sampling context and wider treatment-works information.
Which method should you use?
Use the method defined by the site procedure and the purpose of the trend. If a change of method is planned, document the transition and avoid treating the new readings as directly continuous with the old series.
Read the SVI parameter guide and SSVI parameter guide for the separate S3A routes.
Keep method changes visible in the trend
The representative sampling guide covers sample consistency. The high-SVI article explains why an elevated result is a signal rather than a diagnosis.
Use the SV30 vs SVI guide to distinguish settled volume from the normalised index, or review the complete S3A product workflow.
Authoritative references
Standing Committee of Analysts: Determination of SSVI or SSD describes representative sampling and the stirred settlement method.
UK government-hosted effluent-treatment guidance distinguishes SVI from the low-speed stirred SSVI method.
To confirm whether the optional S3A stirrer suits an application, contact PMA with the site method and monitoring requirement.
