Visual experiments were conducted to investigate freezing behavior of activated sludge. Experimental observations indicate that there existed strong interactions of advancing ice-liquid interface with sludge agglomerates during freezing treatment. Agglomerates would be pushed to move by advancing ice-liquid interface in the cases of lower advancing speeds while no needle-like ice-crystals forming at the interface. For faster freezing processes, the ice front interface line turned needle-like, while ice-crystals forming/generating at the interface, and crystals pierced into the sludge agglomerates rather than pushing them. The crystals served as roles holding the agglomerates. It seems that there was a critical speed below which repulsion occurred and above which the particles were entrapped without obviously pushing. The transition of these two cases is still needed to identify.

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