Seasonal variability of SVI in the secondary sedimenter of an Italian WWTP

This study presents a number of empirical evidences suggesting that the Sludge Volume
Index (SVI) in an Italian WWTP (Fusina, Venice), operating at high sludge age to
allow, nitrification, is strongly affected by Mixed Liquor Suspended Solids (MLSS)
concentrations. The dependence of SVI on MLSS is detected only within peculiar
periods of the year (winter-spring), and seems not to be affected by the use of Al3+ to
counteract the growth of Microtrix parvicella, as this filament type is substituted by
type 0041. It is shown that because of this time-varying dependence between SVI and
MLSS, the manager of the WWTP gains more meaningful information from SVI than
from diluted SVI (DSVI), which could hide the effects of MLSS on the performance of
the secondary settling tanks.