Biosolids Engineering
Authored by a diplomate of the American Academy of Environmental Engineers, this expert guide offers professional help for designing and managing a biosolids program and decreasing the burden on landfills.
TABLE OF CONTENTS Biosolids Engineering
3.5: Heat Drying
3.5 Heat Drying
Heat drying is the process of evaporating water from sludge by thermal means. Under normal conditions, heat drying is used to remove additional moisture from sludge following mechanical dewatering. Reasons for employing heat drying include the desire to (1) market the final sludge product in bags, (2) increase the fuel value of sludge, and (3) reduce sludge transportation costs. The minimum sludge moisture content achievable with heat drying depends on the design and operation of the dryer, moisture content of the feed, and its chemical composition. The typical range of solids content achieved by this process is in the range of 90 to 99 percent [33,38,48]. In heat drying, the temperature difference between the heating medium and the sludge-air interface provides the driving force for heat transfer. Dryers are classified on the basis of the predominant method of transferring heat to the wet solids.
3.5.1 General design of heat dryers
Mechanical dewatering is an important pretreatment step in conventional heat drying because it reduces the volume of water that must be removed in the dryer. In the dryer, water that has not been removed mechanically is evaporated without decomposing the organic matter in the sludge. This means that the sludge temperature must be kept between 140 and 200 F (60 and 93 C). A large portion of the dried sludge is blended with the wet sludge fed to the dryer, making the drying operation more efficient by reducing solids agglomeration (formation of larger chunks of sludge)...
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