Paddle Sludge Dryer vs. Thin Film Sludge Dryer - Technology Comparison

Paddle Sludge Dryers (also referred to as Screw Dryers) and Thin Film Sludge Dryers have both successfully dried municipal wastewater sewage sludge for many years. These dryers operate on a similar drying principle but have a different method of operation. Learn about the advantages and disadvantages of each and which is best for your application.

Paddle Sludge Dryer

No Complex Feed System	✓
Small Airflow	✓
Single Rotating Auger	Χ
Slow Turning Auger	✓
Auger Designed for Life	Χ
No Heating Medium in Auger	Χ
No Auger Synchronization	Χ
Less Electrical Usage	✓
Lower Thermal Energy Usage	Χ
Single Drive for Air/Vapor	✓
Continuous Operation	✓
Lower Cost of Maintenance	Χ
Easier to Maintain	Χ
Higher Frequency of Maintenance	✓
Small Horizontal Maintenance Space	✓
Small Vertical Maintenance Space	Χ
Automatic Sludge Cleanout	Χ

Thin Film Sludge Dryer

No Complex Feed System	✓
Small Airflow	✓
Single Rotating Rotor	✓
Slow Turning Rotor	Χ
Rotor Designed for Life	✓
No Heating Medium in Rotor	✓
No Rotor Synchronization	✓
Less Electrical Usage	Χ
Lower Thermal Energy Usage	✓
Single Drive for Air/Vapor	✓
Continuous Operation	✓
Lower Cost of Maintenance	✓
Easier to Maintain	✓
Higher Frequency of Maintenance	Χ
Small Horizontal Maintenance Space	Χ
Small Vertical Maintenance Space	✓
Automatic Sludge Cleanout	✓

Principle of Operation

Paddle Sludge Dryers and Thin Film Sludge Dryers both use a heated surface to transfer heat indirectly to the sludge and evaporate the water using little airflow (known as indirect drying/conduction). The principle is like boiling water in a pot or cooking in a frying pan. Both dryers typically use steam or thermal oil as their heating medium.

Using this drying principle allows both dryers to have many of similar advantages compared to other technologies including:

Low complexity
Fewer rotating parts
No complex feed systems
Small footprint
Small exhaust vapor flows
Low thermal energy usage
Partial or full drying of sludge

Dual Augers vs. Single Rotor

Paddle Sludge Dryer

The biggest difference between the Paddle Sludge Dryer and the Thin Film Sludge Dryer is the Paddle Dryer’s Dual Auger configuration versus the Thin Film Dryer’s Single Rotor configuration.

The Paddle Dryer uses a heated dual auger system (also called screws or agitators). The augers must be heated to increase the heated surface area in contact with the sludge, since the dryer has a “thick” sludge layer inside the dryer. Without heated augers the dryer would have to be much larger in size to dry the sludge.

Additionally, the dryer cannot clean sludge out automatically. It requires additional sludge to be fed to push sludge through the dryer. The sludge must be manually removed for servicing.

Advantages of the Dual Auger include:

Slow Turning
Electrically Efficient
Small horizontal maintenance footprint

Thin Film Sludge Dryer

The Thin Film Dryer uses a single rotor with blades. The purpose of this rotor system is to create a “thin film” of sludge for an efficient heat transfer without heating the rotor.

Advantages of the Single Rotor include:

Rotor designed to last the life of the equipment
No need for auger synchronization
No need for heating fluid in the rotating rotor
No special shaft seals for heating fluid
Automatic cleanout of sludge
Quick startup and shutdown
Small vertical maintenance space
Simple/inexpensive maintenance

Maintenance

The Thin Film Sludge Dyer and Paddle Sludge Dryer also significantly differ in maintenance requirements. The Paddle Dryer has two main wear items, the augers, and the shaft seals. To service the augers on the paddle dryer:

The residual sludge is removed
The drive system is removed
The feed system is disconnected
The vapor ductwork is disconnected
The top housing is lifted off
The heating fluid is drained
The augers lifted out of the housing
System reassembled with new augers
Augers resealed and filled with heating fluid
The augers are synchronized

This requires:

High vertical maintenance space
Heavy lifting equipment
Factory technicians for proper synchronization of augers

Thin film sludge dryer maintenance components

In contrast to the Paddle Dryer, the Thin Film Dryer has simple bolted-on blades as wear parts. The process to replace the blades is as follows:

The dryer end plates are removed
The rotor is pulled out on a preconfigured trolley
The blades are replaced as needed at the operational floor level
The system is reassembled

The maintenance does not require:

The drive system to be removed
The feed pipe disconnected
The vapor duct work disconnected
The heating fluid to be removed
Heavy parts to be lifted out vertically
The rotor to be synchronized

While the Paddle Dryer auger’s useful life can vary, they must be replaced. They are expensive to stock at a facility and have a long lead-time to procure. This could force a long-term alternative disposal method with increased disposal costs. In contrast, while it is recommended to inspect and replace Thin Film Dryer worn blades once per year, they are small, easy to replace, and inexpensive to stock at the facility.

Conclusion

Paddle (Screw) Sludge Dryers and Thin Film Sludge Dryers have many features in common. They are both indirect dryers which use similar heating methods, and are less complex, have lower air flows, simpler maintenance and smaller footprints than other drying technologies.

The biggest difference between these dryers is the way the sludge is conveyed, either by a dual auger system with the paddle dryer or a single rotor with the Thin Film dryer. The positives and negatives of each system should be considered when deciding on the right dryer for your application.