Settled sludge volume (SSV) is determined at specified times during sample testing (e.g., 30- and 60-minute observations).
• Normal operation—When the process is operating properly, the solids will settle as a "blanket" (a mass), with a crisp or sharp edge between the solids and the liquor above. The liquid over the solids will be clear, with little or no visible solids remaining in suspension. Settled sludge volume at the end of 30 to 60 minutes will be in the range of 400 to 700 mL.
• Old or overoxidized activated sludge—When the activated sludge is overoxidized, the solids will settle as discrete particles. The edge between the solids and liquid will be fuzzy, with a large number of visible solids (e.g., pin floc, ash floc) in the liquid. The settled sludge volume at the end of 30 or 60 minutes will be greater than 700 mL.
• Young or underoxidized activated sludge—When the activated sludge is underoxidized, the solids settle as discrete particles, and the boundary between the solids and the liquid is poorly defined. Large amounts of small visible solids are suspended in the liquid. The settled sludge volume after 30 to 60 minutes will usually be less than 400 mL.
• Bulking activated sludge—When the activated sludge is experiencing a bulking condition, very little or no settling is observed.
Milliliters of Settled Sludge x 1000 mL/L Milliliters of Sample
Milliliters of Settled Sludge x 100 Milliliters of Sample
Note: Running the settleability test with a diluted sample can assist in determining if the activated sludge is old (too many solids) or bulking (not settling). Old sludge will settle to a more compact level when diluted.
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