Interaction between wastewater-sludge floc and moving ice front

Freeze/thaw treatment is an effective sludge-dewatering technique. The efficiency of dewatering of freeze/thawed sludge declines as the approaching ice front engulfs flocs at a high freezing rate. The forces exerted on a wastewater-sludge floc by the approaching ice front and the surrounding fluid field determine whether the ice front engulfs the floc. Two wastewater flocs of sizes 2170 and 2240 μ m were hung on a cantilevered beam which underwent constant-speed freezing (5 or 8 μ m s^{- 1}). Beam deflection and the shape of sludge floc were recorded to estimate the force exerted by the ice and the elasticity of the sludge flocs. The force exerted by an approaching ice front on the floc ranged from 1.6 to 1.8 × 10^{- 6} N, and was attributable primarily to the interaction between the gas bubbles formed between the floc and the ice front. When the ice had partly engulfed the floc, the corresponding force continuously increased as freezing continued, compressing the unfrozen part of the floc and pulling apart the network of the frozen part of the floc. The change in floc shape was tracked to estimate the interior elasticity of 0.0063 - 0.025 N m^{- 1}, and the elongation coefficient of 0.0030 - 0.0089 N m^{- 1} for the flocs herein. © 2006 Elsevier Ltd. All rights reserved.