Description of Sludge Drying Process

Sewage sludge from digestion towers or sedimentation tanks is pressed using a dewatering screw or filter press to achieve approximately 25% dry matter. Wheel loaders, conveyor belts, or screw conveyors pass the pressed sewage sludge into the 40 m3 push-floor container of the dryer. The push-floor doses the wet material into a screw feeder in a bridge-free manner. Wet sewage sludge from the push floor and dry sewage sludge from the return feed are mixed in a downstream mixing screw to approximately 45% dry matter and conveyed into the first chamber of the dryer.


In the first drying chamber, the treated sewage sludge falls onto quickly rotating scoop blades and generates a swirling cloud of material with a maximum surface area in the drying chamber.


The hot exhaust gases from the CHP, from the gas combustion chamber, or a combination of the two energy sources leads directly to the dryer in an exhaust gas pipe. In a mixed air unit, the 550°C (1022°F) exhaust gasses are mixed with fresh air to 300°C (572°F) and distributed symmetrically into the individual drying chambers. The impact of the extremely high temperature in the material vortex leads to immediate turbo drying. This procedure achieves an evaporation rate of approximately 600W per liter of water, creating an efficiency which is not yet available on the market.

A free and effortless side effect is produced through the impact of the hot gas. The sewage sludge and the evaporating water volume are completely sanitized during the drying process.

The sewage sludge is conveyed through the individual chambers in a process controlled and fully automated manner. The degree of drying can be adjusted and varied between 80 and 95% dry matter, depending on the users requirements. The high level of efficiency means that the average retention time in the dryer is less than 5 minutes.

The exhaust air is treated in the closed dryer system. Due to the impact of the hot air, the ammonia in the sewage sludge is immediately removed upon insertion into the drying chamber. In this process. sulphuric acid at a ratio of 1:3 is introduced and the chemical binding of the volatile ammonia into crystalline ammonium sulphate occurs. There is no need for downstream stripping or other exhaust gas purification.

All the exhaust gas flows through an integrated fine filtering system and virtually all the dust is cleaned as a result. If the exhaust gas has odorous substances, it can then be completely oxidized by a biofilter.

The arising particulate matter in the filter chamber drops into a screw feeder, is conveyed to the holding chamber, admixed to the dry sewage sludge there and then conveyed in a sealed state to an interim container.

Pelletization is carried out immediately following the process. Dust-free pellets are produced from the dried sewage sludge. The highly compacted pellets can be stored in open containers and transported using standard bulk freight logistics. The sewage sludge pellets are then suitable for phosphorous recovery and for gasification, combustion or for use as a spreadable fertilizer.