Dewatering sludge by osmotic technique - A comparative experimental study

The present paper aims to introduce a novel and environment-friendly sludge dewatering method based on the osmotic technique, and to examine the efficiency of this method from macro- and micro-points of view. Intact sample, taken from the primary settler of a local wastewater treatment plant in Guangzhou (China), was firstly dewatered by mechanical methods (i.e. vacuum filtration and centrifugation), then by the osmotic technique. Macroscopic experimental results show that: the vacuum filtration and centrifugation can only dewater the sludge from 94.5 to 78.3% and to 78.8%, respectively, whereas, using the osmotic technique, water content of the sludge can be decreased to 44.2% in 12 h and to 35.9% in about two days. The micro-structure, obtained by scanning electron microscope (SEM), shows that: the sludge samples after vacuum filtration and centrifugation have a porous structure and water can be drained either through the cracks that have been developed under the vacuum pressure or by the micro-pores of several micrometers that have been created under the disturbance of the centrifugal force; on the other hand, in the case of the osmotic technique, the samples show a much denser structure, where both the macro- and micro-pores are significantly decreased or even closed at high concentration of the polyethylene glycol (PEG) solution. At last, the future application of the osmotic technique in practice is discussed from the viewpoints of deep dewatering and energy consumption.