In dry lime stabilization (also known as postlime stabilization), dry lime (hydrated lime or quicklime) is mixed with dewatered sludge cake to raise the pH of the mixture. The process requires adequate mixing to avoid pockets of putrescible material and to produce a homogeneous mixture. An effective mixer is a pug mill in which two screw conveyors or paddle mixers rotate in opposite directions. Other types of mixers include plow blender, paddle mixer, and screw conveyor. Figure 6.3 shows the schematic of a dry lime stabilization system. The system consists of a dry lime storage silo, a volumetric feeder and conveyor to transfer the lime, a dewatered sludge cake conveyor, and the mixer described above. The stabilized sludge is granular or crumbly in texture, depending on the moisture content of the dewatered sludge cake, and can be stored for long periods or distributed on land using a conventional manure spreader.
Quicklime, hydrated lime, or other dry alkaline materials, such as lime kiln dust or cement kiln dust, can be used for dry lime stabilization. Hydrated lime is typically limited to small installations. Quicklime is less expensive than hydrated lime and is easier to handle in large-scale facilities using lime in excess of 3 to 4 tons/day. Additionally, the heat generated from the exothermic reaction of quicklime and the water in sludge cake can enhance pathogen destruction.
Significant advantages of dry lime stabilization include no additional water added to sludge in the form of lime slurry, no special requirements for sludge dewatering equipment, and no lime-related abrasion and scaling problems to the dewatering equipment.
Dewatered Sludge l_k
Blosollds to Beneficial Use
Figure 6.3 Typical dry lime stabilization process schematic.
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