1. Feed Preparation
- Sludge is first dewatered (via centrifuge, belt press, or filter press).
- The dewatered cake typically contains 65–80% water.
- This pre-dewatered material is conveyed into the dryer.
2. Indirect Heat Transfer
- Most municipal sludge dryers use indirect heating rather than direct flame.
- Heat is delivered through hollow walls, tubes, or paddles made of stainless steel.
- Common heating media: steam, thermal oil, hot water, electricity, or natural gas-heated jackets.
- Indirect heat avoids combustion gases contacting the sludge directly, preventing contamination and fire risk.
3. Agitation and Mixing
- Inside the dryer, slow-speed paddles or screw conveyors continuously turn and move sludge through the heated zone.
- This ensures uniform heat distribution, prevents crust formation, and maximizes surface area for evaporation.
4. Moisture Evaporation
- Heat drives water out of the sludge as vapor/steam.
- Dryers typically reduce sludge moisture from 65–80% down to 10–20%.
- Vapor is captured by an off-gas handling system, which may include condensers, scrubbers, or biofilters for odor control and emission compliance.
5. Discharge of Dried Solids
- Dried biosolids exit the dryer in a granular, pelletized, or powdery form, depending on design.
- Final dryness and particle size can be controlled by adjusting residence time, temperature, and mixing speed.
How It’s Used in Biosolids Drying
- Volume & Weight Reduction
- Biosolids dryers shrink sludge mass significantly, reducing trucking and disposal costs.
- A ton of wet sludge might become only a few hundred pounds of dry product.
- Stabilization & Pathogen Reduction
- High heat exposure in the dryer destroys pathogens, enabling compliance with EPA 503 biosolids regulations.
- Depending on conditions, dried sludge can qualify as Class A (safe for land application).
- Odor & Handling Improvements
- Dry, pelletized biosolids are far easier to store, transport, and spread than sticky sludge cake.
- Odors are greatly reduced once water is evaporated and volatile organics are driven off.
- End Uses
- Land Application: Soil amendment or fertilizer.
- Energy Recovery: Co-firing in cement kilns, boilers, or waste-to-energy plants.
- Landfill Disposal: With much lower tipping fees due to reduced weight and volume.
✅ In short: A sludge dryer in biosolids applications is essentially a thermal drying system that applies indirect, controlled heat and mixing to evaporate water, stabilize the solids, and transform wastewater sludge into a lighter, drier, and more versatile product.