UNIDIRECTIONAL FREEZING OF WASTE ACTIVATED SLUDGES - EFFECTS OF FREEZING SPEED

In this work, we employ unidirectional freezing of waste-activated slu dge to investigate the effects of freezing speed on the sludge perform ance including filterability, zone settling, and flee density versus s ize relationships. With a high freezing speed, no global particle migr ation occurs, but the sludge filterability is markedly improved. Howev er, the flee density and the flee morphology are only slightly changed ; therefore, the zone settling characteristics are those of the origin al sludge. With a low freezing speed, however, global particle migrati on occurs. Consequently, both the sludge filterability and settleabili ty are enhanced. The flee density and the morphology are changed signi ficantly. With an intermediate freezing speed, a gradual transition of sludge characteristics occurs between the two extremes, resulting fro m an average behavior of the constituting flocs. No precise definition for the critical freezing speed can thereby be defined. If improvemen t of sludge filterability is the only concern, a high freezing speed i s acceptable. Furthermore, a low freezing speed is necessary if both s ludge settleability and bound-water content must be changed.