| Sludge dewatering is a difficult and expensive process in water and wastewater treatment. Mechanical techniques, such as centrifuges and filters, are difficult to operate and maintain, and drying beds require large land areas. All dewatering operations require expensive chemicals for sludge conditioning. Freeze-thaw technology, which is an alternative method of sludge conditioning has gained considerable popularity during the last decade. Particularly in cold regions, where freeze-thaw can be achieved by taking advantage of the natural climatic conditions, freeze-thaw conditioning has emerged as an effective and low-cost method of assisting sludge dewatering.; Freeze-thaw conditioning works very well for alum sludge, however it does not work as well for activated sludge. Apparently, the organic, inorganic, and biological constituents of activated sludge, which are not found in alum sludge, hinder the freezing process. With activated sludge, the improvements achieved after freeze-thaw conditioning are temporary and reversible. If, however, the sludge chemistry and the floc structure of activated sludge are changed prior to freezing, it may be possible to improve the results of freeze-thaw conditioning.; In this study, the sludge chemistry and floc structure were changed by removing and adding extracellular polymers and cations to the sludge matrix, and by employing other methods such as elutriating the sludge, varying the freezing temperature, and storing the sludge under aerobic and anaerobic conditions. Cations were removed by dilution, chelation and ion exchange processes, and extracellular polymers were removed by NaOH, EDTA and EGTA extraction, heating, blending, and centrifuging. The concentration of extracellular polymers was quantified by measuring protein and carbohydrate concentrations. The concentration of the cations was determined by using a direct coupled plasma (DCP) spectrometer. The results of this study show that high protein, carbohydrate, and cation concentrations present in activated sludge hinder complete freezing of sludge, thereby decrease the dewaterability, settleability, and compactibility of sludge after freeze-thaw conditioning. Removal of extracellular polymers and cations from sludge matrix improves the outcome of freeze-thaw conditioning. In a treatment plant, the effectiveness of freeze-thaw conditioning can be significantly increased simply by employing dilution and elutriation during the treatment process. |
