Application of filtration aids for improving sludge dewatering properties – A review

The difficulties in sludge dewatering are associated with the high organic content and colloidal materials in sludge solids. Chemical conditioning by flocculation improves sludge filterability by reducing the specific resistance to filtration. However, achieving final cake solids of a significantly higher solids concentration is hindered by the high compressibility of the flocculated sludge during the compression stage of mechanical dewatering. Sludge cake particles can be easily deformed under pressure following cake growth causing cake void closure and a subsequent reduction in sludge filterability. Physical conditioners, including minerals and carbonaceous materials, have been used to aid in the mechanical dewatering of sludge. These materials, acting as skeleton builders, are able to enhance sludge dewaterability and cake properties by adding more rigid and incompressible structures to the sludge solids providing water passages. Measurement of the change in cake properties, such as specific resistance to filtration, net sludge yield, cake permeability and porosity, helps to identify the beneficial role of the physical conditioners in improving sludge dewaterability. The surfaces of the sludge solids particles and physical conditioners as well as the chemical conditioners are often charged. Hence, interactions between polyelectrolyte, the physical conditioner and sludge colloids may occur, leading to the formation of a homogeneous mixture of the solids with strong and porous structure. The interactions can be identified by measuring the variations of particle surface charge, particle size and population before and after the addition of physical conditioners.