Low Temperature Drying of Sewage Sludge Using a Fluidized Bed Dryer
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Currently, there are over 16,000 wastewater treatment plants (WWTPs) in the United States that produce 8 million dry metric tons of biosolids annually, with 66,000 dry metric tons being produced in the state of Nevada alone. These biosolids contain a high moisture content, in excess of 75%, and are commonly seen as a waste product requiring disposal, typically through landfilling, incineration, or land application. The treatment and disposal costs of these biosolids are over $1 billion annually. Dry biosolids contain an energy content of approximately 20 MJ/kg, similar to that of wood, making the solids a viable energy source if the water content can be cost effectively reduced to a level that would be expected in a solid fuel, ranging between 10-20%. With the biosolids produced in the US, as much as 1,500 MW of renewable energy could be produced at WWTPs, enough to power nearly 1.2 million US homes. This work describes the design, construction, and operation of a continuous fluid bed dryer for removing the moisture from sludge produced at a local WWTP as a pretreatment for gasification and eventual onsite power production. The results of this work show the fluidized bed dryer operates well at varying operating conditions and for extended periods of time, producing dried solids well below the desired goal of 10-30% moisture content. The dryer was operated at gas velocities above the minimum fluidization velocity, Umf, with velocities ranging from 1.21Umf to 4.04Umf and bed temperatures from as low as 34°C to as high as 74°C. It produced solids with moisture contents below 30% at all operating conditions. The dryer sustained continuously for as long as 6.25 hours.