The Background

Municipal sludge consists of all the organic and inorganic compounds extracted from the wastewater during the wastewater treatment process. Wastewater treatment plants are becoming increasingly efficient in eliminating these compounds from the wastewater which leads to more ending up in the sludge. Thus, the sludge contains many unwanted components that are harmful to the environment. The following unwanted components are usually found in municipal sludge:

Current situation

In addition to the unwanted compounds the sludge also contains useful fertilizers such as phosphorus and nitrogen which can be utilized once the contaminants are removed. With current sludge handling technologies, such as composting and biogas production the contaminants are not eliminated and seep back into the environment through landfills causing environmental problems.

Our solution treats the sludge thermally at a constant temperature of 850 ˚C to ensure the destruction of unwanted organic compounds. The heavy metals that cannot be destroyed thermally are separated from the hygienic ash, that can be used as a fertilizer in forestry or farming.

Transport emission reduction

Municipal sludge created in wastewater treatment plants has a high water content and is therefore mechanically dewatered before transported to further treatment. Even after mechanical dewatering the usual sludge dry solids content is still under 30% which means you mostly transport water, creating unnecessary transport emissions.

Due to our compact design the facility can be placed in connection to the wastewater treatment plant and eliminate the need of sludge transport. Only under 10 % of the original sludge mass needs to be transported away from the site as ash.


The main working principles of our solution called the PAKU process is the following. Mechanically dried sludge is fed to the circulating mass dryer where the sludge is dried to over 95% dry matter content before being fed to the circulating mass reactor where the temperature is held at 850 ˚C. The flue gases created in the reactor is then cleaned by removing the product and byproduct ash and by scrubbing before releasing into the stack.

Thermal energy is extracted from the PAKU process both from condensing the vapor from the dryer and from the thermal process in the reactor. This energy can for example be used in district heat production.

PAKU is the most affordable commercial solution for sludge thermal treatment.


Benefits of the PAKU process

  • Simple design which make it compact and cost efficient
  • Efficient energy recovery
  • Operates fully automated and continuously
  • Recovers valuable nutrients
  • Eliminates contaminants from the circulation
  • Reduces sludge transport pollutions
  • Requires no additional fuel in normal operation
  • Suitable also for handling of biogas production residues

Future options

  • Recovery of nitrogen from the dryer condensate
  • Electricity generation
  • Ennergy and nutrient recovery from industrial waste streams such as pulp mills and bioethanol production sites