30-Minute Settling Test of Activated Sludge Systems

30-Minute Settling Test of Activated Sludge SystemsThe 30-minute settling test is used to determine the settled sludge volume of mixed liquor samples in activated sludge systems. It is useful in the routine monitoring of biological processes and should be run at least once per shift.  A thirty-minute settled sludge volume (SSV30) can be used to determine the returned-sludge flow rate and when to waste the sludge. It is also used to calculate sludge volume index (SVI), which helps determine the health of the floc as well as the severity of poor settling (bulking) episodes. The sludge volume index is calculated using the sludge volume and the mixed liquor suspended solids value.

The formula is as follows:

30-Minute Settling Test

Variations in temperature, sampling and agitation methods, dimensions of settling column, and time between sampling and start of determination can significantly affect the results. A settleometer is the preferred vessel for the settling test, but a 1000 milliliter wide mouth beaker can be used as a substitute if you do not have a settleometer. Samples should be well mixed by rotating the samples in the settleometer with lid upside down and back gently so as not to break up the floc particles. Start the timer immediately.
During the first five minutes of the 30-minute settling test, it can be observed how the sludge particles agglomerate while forming the sludge blanket. If the sample does not settle well after 30 minutes, another test can be run on a diluted sample. For the diluted test, add one-half MLSS and one half effluent to a settleometer and run the same procedure. It is important to use effluent or tap water because deionized water can lyse the bacterial cells and breakup the floc. If the diluted sample settles better than the non-diluted, it may be an indication that removing some of the solids will help settling. To accomplish this, sludge needs to be wasted.

Please contact Environmental Business Specialists for a complete procedure at 985-674-0660.

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