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SVI is best defined as?

  1. The volume of sludge settled

  2. The tendency of activated solids to thicken during the sedimentation process

  3. The specific gravity of sludge

  4. The volume of sludge over the duration of aeration

The correct answer is: The tendency of activated solids to thicken during the sedimentation process

SVI, or Sludge Volume Index, is effectively a measure that indicates the tendency of activated solids to thicken during the sedimentation process. It is calculated by the volume of settled sludge in a given time and is often measured in milliliters per gram for the suspended solids present in a particular sample of wastewater. A higher SVI value typically suggests poorer settling characteristics of the sludge, meaning that the sludge is fluffier and does not compact well. This characteristic is crucial for wastewater treatment as it affects the efficiency of the separation processes in clarifiers. The context of the other options is important, as they relate to aspects of sludge and sedimentation but do not accurately define SVI. The volume of sludge settled and the volume of sludge over the duration of aeration merely describe measurable quantities rather than the behavior of the sludge during sedimentation. Similarly, specific gravity of sludge pertains to the density and mass relationship rather than the tendency of sludge to thicken or the operational characteristics that SVI aims to represent. Thus, the option defining SVI as the tendency of activated solids to thicken during sedimentation captures the primary principle that SVI is intended to illustrate in wastewater treatment processes.