Digested sludge / Sludge Volume Index (SVI) test

The sludge volume index of activated sludge is defined as the volume in milliliters occupied by one gram of activated sludge after settling for 30 minutes. From this definition it may be observed that the lower the SVI, the better is the settling quality of the aerated mixed liquor. Also, high SVI values (greater than 100) indicate poor settling qualities and possible bulking problems in the secondary clarifier. Sludge with an SVI of less than 100 is considered a good settling sludge. Ideally you are looking for a value that ranges from 50 to 75 for most conventional activated sludge plants. You should be aware of the fact that SVI or SDI are classical measurements of sludge quality, but may or may not always reflect the true sludge conditions and must be compared with other process parameters, such as sludge units.



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