Full drying
of Sludge

Drying of sludge to 90-95% DS. Ideal for manufacturing of fertiliser supplement, soil conditioner or fuel.


Thermal sludge drying is a process where the dewatered sludge from a wastewater treatment plant is dried using external heat to produce a Class A sludge. The dried product is a sterile granular bio solid with 90-95% solids content. The dried product can be used as a fertilizer supplement, soil conditioner or a sustainable fuel.

Haarslev offers two complete drying systems for full drying of municipal and industrial sludge.

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Full drying of sludge

Rotary Drying System

The Flo-Dry Rotary Dryer is the latest development in continuous rotary drying offering a simple, economical and efficient solution for the processing of wet by-products.

Wet material is continuously fed through a water-cooled screw feeder into one end of the rotating drum, which cascades and distributes the material along the drum. The material is gently dried as it cascades through the hot air flow.

Moisture content can be very accurately controlled by the outlet air temperature, maintaining the balance of air temperature and speed of rotation. Dust is kept to a minimum by the low air velocities. The meal is screw conveyed to the mill for a final grinding and transferred into a bagging hopper or bulk storage bin.

Simplicity in design and manufacture is the key to the success of the Flo-Dry Rotary Dryer. It has a minimum of moving parts which means reduced wear, less maintenance and increased operating life. Construction materials are customized to suit product type. For example mild steel for non-corrosive, non-food material, stainless steel for food and/or corrosive material.

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Full drying of sludge

Drying System

Haarslev Rotadisc Drying System is a disc dryer with oil or steam heating. The highly efficient system of the Rotadisc dryer is ideal for adhering materials that are difficult to dry.

A system of scrapers and paddles prevent accumulation on heating surfaces by gentle mixing of the product. High thermal effectiveness is achieved by uninterrupted contact between the material to be dried and the heating surface of the dryer.

The Rotadisc stands out due to its ease of use, low maintenance and ideal volume and length ratio.

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Full drying of sludge

Drying System

Flo-Dry Enersaver is a two-stage dryer system, where the first stage is a direct fired hot air rotary dryer which dries sludge to 40-45 % dry solids. The partially dried product from the first stage dryer is passed to the final drying in the second stage dryer. The second stage is a belt-type dryer with multiple belts and drying chambers.

Heated air blows down through the product and out at the bottom of the belt, where it is recirculated through the heat exchanger to the top again. The heat exchanger uses hot water from the first stage dryer as heat source. The water is recirculated between the first and second stage dryers for the required heat transfer.

Energy costs for a two stage system can be reduced by more than 20 % compared to a conventional system. Product slowly passes through the chambers and belts to dry fully. The product is dried gently and minimizes dust, which makes it a safe process. The two stage process eliminates the need to recycle the product.

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Full drying of sludge


The wet mechanically dewatered product is continuously fed in top of the dryer through a distribution hopper that separates the product evenly on the belt. Products such as sewage sludge must be structured first, in order to increase the surface area of the material to obtain more efficient heat transfer and evaporation.

A perforated stainless belt transports the product gently through the dryer to avoid friction of the product and thereby reducing dust explosion risk.

As the product passes through the successive chambers, the gas temperature increases progressively heating up the sludge to the desired temperature (approximately 60-80°C). At the end of the top belt the product transfers to the second belt where the evaporation process is completed. The product is cooled down as it passes back to the exit of the dryer.

The temperatures inside the dryer chambers are constantly monitored, these measurements automatically control the speed of the belt and the feeding devise, and this gives the most efficient drying time and consistent quality of product.

Process is

Haarslev Global Services makes sure your business keeps on going

Downtime is the worst for all companies. You just have to avoid it at all costs. Our systems have been designed to ensure maximum uptime.
Haarslev Global Services department helps keep your plant running reliably, efficiently and profitably every day of the year.

Highly trained experts are available to give you the best advice when it comes to mastering your processes and by implementing the right procedures for maintenance and spare parts stock, you will be able to:

  • guarantee your throughput
  • obtain more yield per product
  • achieve a reduced Total Cost of Ownership (TCO)
Harald Petersen
Mechanical Engineer