Predicting Freezing Design Depth of Sludge‐Freezing Beds

Abstract

A sludge‐freezing bed is a new unit operation for dewatering sludges produced by water and wastewater treatment plants in cold regions. Sludge is applied to the bed in layers, where it is allowed to freeze naturally during the winter. Freezing causes the solid and liquid fractions to separate during ice crystal formation. After the sludge thaws in the spring the water is drained away, leaving the solids that can be handled with conventional earth‐moving equipment. This paper presents the development of a mathematical model for predicting the total depth of sludge that can be frozen in the bed. An analysis of this model indicates that the time needed to cool the sludge to the freezing point is insignificant compared to the time needed to freeze it. Data from a pilot‐scale freezing bed and other freezing experiments were used to evaluate the model. The average wind velocity at the site was found to be a primary factor in determining the freezing depth. An example of how to use the model for design is also presented.