Ultra-rapid freezing of water treatment residuals

An alum water treatment residual (“sludge”) was conditioned by freezing at ultra-rapid rates. Ultra-rapid rates were obtained by freezing on liquid nitrogen. The purpose of the experiments was to determine the conditions necessary for such rapid freezing to improve dewaterability. Contrary to results reported in the literature, ultra-rapid freezing worked extremely well. Although the conditioned flocs (“zots”) were not as dense or granular as obtained at slower freezing rates, dewaterability was still greatly improved. Dewaterability, as measured by cake solids content (CSC), capillary suction time (CST), filterability coefficient (χ) and aggregate volume index (AVI), was best for samples with an initially high solids content and for samples that had been cured or stored at sub-freezing temperatures, for long times. Zot size analyses showed that zots decreased in size following ultra-rapid freezing. The results are explained using a proposed conceptual model of the residuals freezing process.