| Examined herein are the engineering properties of flue gas desulfurization (FGD) sludges with emphasis on double alkali sludges and sludge-additive (primarily fly ash) mixtures. Extensive test data are compiled on the index properties, viscosity, sedimentation characteristics, compactability, compressibility, time rate of consolidation, and strength of these sludges, and these data are utilized to assess the feasibility and difficulty of different disposal methodologies.;In general, double alkali FGD sludges are silt-like materials composed primarily of calcium sulfite and calcium sulfate. Their apparent viscosity at about 20% solids content (by weight) is on the order of 200 centipoise at a shear rate of unity per second. The sedimentation rate of a sludge slurry at 5% solids content is on the order of 50 cm/hr, and it is influenced only minimally by an additive.;As-produced sludges can not be effectively compacted due to their high water content; field dry densities usually lie between 30 and 60 pcf, depending on the water content and applied compactive effort during placement. The permeability of plain sludge is on the order 10('-6) to 10('-4) cm/sec; this enhances the rate of consolidation, but is not particularly desirable for minimizing the quantity of leachate that permeates the material. Adding fly ash and/or lime to sludge changes the permeability somewhat, but it usually remains within the same order of magnitude. The strength of sludge is governed by a frictional phenomenon when tested under drained conditions, with a friction angle on the order of 35 degrees. The unconfined compressive strength of compacted fresh plain sludge is on the order of 0.4 kg/cm('2), but in aged sludge-fly ash-lime mixtures the development of pozzolanic bonds increases the unconfined compressive strength ten-fold or more relative to that for plain sludge. The strength of sedimented sludge is very low.;The most economically feasible way to dispose of sludge at an existing landfill site, where sludge and fly ash are handled separately, is to blend dry fly ash with the sludge to increase its handleability during disposal and long-term performance after disposal. Usually the early stages of disposal in a landfill operation are the most critical, because stability generally increases with curing time. |
