Natural Sludge Dewatering. II: Thawing-Drying Process in Full-Scale Sludge Freezing Ditches

Abstract

The thawing and drying process in full-scale sludge freezing ditches was studied. The ditches are located in northern Sweden. The treated sludge derived from a small wastewater-treatment plant with mechanical and chemical treatment. Due to the relatively warm winter of 1994–1995, it became possible to study the effect of incomplete freezing. The results show that frozen and thawed sludge has a porous and humus-like structure, whereas unfrozen sludge has a compact and smeary consistency. The study also shows that incomplete freezing decreases the dewaterability of the sludge, but that the result measured as solid content is satisfactory even if all the sludge is not well frozen. Moreover, the study indicates that the thawing depth could be estimated by a simple equation based on the number of positive degree-days.