Dewatering of pulverized coal-conditioned municipal sewage sludge and heat recovery of the product

Advanced wastewater treatment requirements are resulting in the production of larger volumes of wastewater sludge which present problems for dewatering processes and final disposal. This study addresses these problems by investigating the use of pulverized coal as a conditioning agent in municipal sludge dewatering processes. The effects of variations in coal dosage and coal particle size fraction on the filter yield from vacuum filters and pressure filters were studied by utilizing laboratory scale filter-leaf and pressure filter apparatuses. In addition, calorimetric tests were conducted on the filter cake from several coal-sludge mixtures to examine the resulting heat content values. The objective of this process is to provide a product which can be effectively burned in a combustion process.

For the coal and sludge samples used in this study, both the total filter yield and the sludge filter yield were found to increase with increases in coal dosage. The coal dosage was also found to be indirectly dependent on coal particle size since the laboratory mixing apparatus was only able to maintain the smaller coal particles in suspension. When a coal dosage of greater than 1.0 gram of coal for each gram of sludge total solids is used, the heat content of the resulting filter cake was found to be at least 75 percent of the value for the pure coal.

The study indicates that the small increases in filter yield achieved by coal addition without other conditioning agents are not significant enough to warrant the use of coal for sludge conditioning prior to dewatering. However, coal additions will increase the heat content of the n m filter cake and may help to minimize problems associated with wide variations in the heat content of boiler feed materials.