What is the acceptable range for the Sludge Volume Index (SVI)?

The Sludge Volume Index (SVI) is an important parameter used to assess the settleability and performance of activated sludge systems in wastewater treatment. The SVI is calculated by dividing the volume of settled sludge by the concentration of suspended solids in the mixed liquor. An acceptable SVI range indicates optimal settling characteristics, which are crucial for maintaining effective treatment and preventing issues such as poor effluent quality or sludge bulking.

The correct range for the SVI is typically considered to be 70 to 150 mL/g. This range indicates that the sludge is settling well, which is key for efficient separation of solids from the treated effluent. Sludge within this SVI range ensures that there is a good balance between the biomass concentration and the volume of sludge produced, which allows for effective removal and minimal interference during the treatment process.

Values outside this range can signify problems; for instance, an SVI below 70 may indicate that the sludge is too dense and not settling well, while an SVI above 150 can suggest that the sludge is experiencing bulking, which can lead to operational challenges and reduced treatment effectiveness. Therefore, the choice reflecting an SVI of 70 to 150 is aligned with best practices in wastewater treatment operations.