Abstract
Excess activated sludge is very difficult to be filtered, but its solid–liquid separation characteristics can be improved by a freezing-and-thawing process. In this study, centrifugal and the gravitational settling experiments were done by using unfrozen and frozen activated sludges.
An equation for the average solid compressive pressure ((ps)av) in the sediment zone was derived, based on the general equation for the local compression characteristics of a solid–liquid mixture. Also, the relationship between (ps)av and the average porosity ((εs)av) in the sediment zone (1 – (εs)av) = E(ps)avβ; E and β = constants) was confirmed by centrifugal and the gravitational settling experiments.
The order of magnitude of (ε)av was unfrozen sample < fast-frozen sample < slow-frozen sample. This order was consistent with that of the average specific filtration resistance obtained from the constant-pressure filtration experiments reported in a previous paper.
