Two parameters commonly used to quantify the settling of the sludge are the zone settling rate and the sludge volume index (SVI). The SVI is the volume occupied by 1g of sludge after 30 min of settling in a 1000 mL unstirred cylinder. Many researchers recognize SVI as the best parameter characterizing the sludge settling properties. The SVI is also a good indicator of sludge bulking. Practically, the SVI can vary from 30 to 400 mL/g where 30 mL/g indicates a sample with very good settleability and 400 mL/g a sample with poor settling properties . In other words, if the SVI is less than 60 the sludge is settling too fast and if above 150 the sludge is bulking. It was obtained by dividing the settled volume of sludge (SV30 in mL/L) with the concentration of sludge or TSS (g/L). The volume of the sludge after 30 min of settling was used to calculate the SVI value using the Equation where MLSS is the concentration of the sludge, or the mixed liquor suspended solids: SVI (mL/mg) = settled sludge volume (mL/L)/MLSS (mg/L) (2)