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parameter Precise

measurement Easy to measure

Rapid measurement

Immediate measurement

1. Objective and general principle The Sludge Volume Index (SVI) of mixed liquor is the ratio of the sludge volume settled in 30 minutes (Vs30’) divided by the suspended solids of this mixed liquor (called MLSS). It is expressed in ml/gTSS. When the volume of settled sludge after 30 min exceeds 300 ml/l, the mixed liquor is diluted beforehand by a settling test, so as to drop below this value. Historically, the Mohlman Index (IM) was more common: this is the same test, the only difference being that the volume read in the test tube was divided by the MLSS concentration, regardless of the volume of sludge measured after 30 min (no mixed liquor dilution). In this case, the IM depends more on the MLSS than on its thickening capacity. It is therefore recommended to no longer use this Mohlman Index. Note: IM is sometimes referred to instead of IB (always verify the presence or absence of dilution). These indexes are used to characterize the settling ability and thickening capacity of biological sludge.

2. Sampling, packaging and conservation of the samples, analysis deadline Bottling Volume

Recommended conservation conditions

Maximum non-evolution time

Plastic or glass 2 l

Do not fill to the brim 24 hours at 4°C

Note: mixed liquor must be sampled when the aerators of the aeration tank have been in operation for at least 15 min so as to obtain a representative mixed liquor sample of the activated sludge tank.

3. Water quality indication to control the analysis, detection limit No measurement limit

4. Procedure Determination of the volume occupied by sludge after 30 min settling of 1 liter of mixed liquor. The MLSS content is determined simultaneously. If the volume of settled sludge after 30 min exceeds 300 ml/l, the mixed liquor is diluted until Vs30’ < 300 ml/l.

ctorDilutionFalgTSS

lmlVsSVI

×=

)/(][

)/('30

TTeecchhnniiccaall DDeeppaarrttmmeenntt

SSlluuddggee VVoolluummee IInnddeexx ((SSVVII))

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22//22

1111//1100//0099 1177

”This document includes internal and confidential information belonging to Veolia Water – It cannot be reproduced or used without Veolia Water’s written authorization”

5. Possible interference and measurement errors

For VS30’ � The diameter of the test tube used must be roughly 8-8.5 cm to limit the edge effects.

Ideally, an Imhoff cone should be used. � The mixed liquor must be sampled when the aerators are in operation to obtain a

representative sample. � To maintain sludge flocculation, clarified water must be used as dilution water (not

drinking water). If low-mineralization water is used for dilution, there will be an osmotic shock on the biomass, causing cell lysis and consequently reducing the settling ability of sludge. In addition, the test tube or Imhoff cone must be stored out of sun light.

For the MLSS content of mixed liquor, refer to analysis sheet ref. 01.

6. Estimates SVI (and possibly IM) is used to characterize the settling ability of biological sludge: � IB > 200 ml/g of SS: poor

settling ability (bulking phenomenon)

� 50 <IB < 200 ml/g of SS: suitable settling ability

� IB < 50: ”very quick” settling ability (risk of deposit at the bottom of the tanks if stirring is insufficient)

Source: Biolog (internal VWS application)

The variations over time must be monitored more closely than the absolute value. In case of high sludge index, microscopic observation is recommended to highlight the potential presence of filamentous bacteria.