Biomass/sewage sludge from digestion towers or sedimentation tanks is pressed using a de-watering screw or filter press to achieve approximately 25% dry matter. Wheel loaders, conveyor belts, or screw conveyors pass the pressed biomass/sewage sludge into the 1400 ft3 (40 m3) push-floor container of the dryer. The push-floor doses the wet material into a screw feeder in a bridge-free manner. Biomass/sewage sludge from the push floor and dry biomass/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 1022°F (550°C) exhaust gasses are mixed with fresh air to 572°F (300°C) 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 2271W per gallon (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 biomass/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 controlled and fully automated manner. The degree of drying can be adjusted between 80% and 95% dry matter, depending on the user’s 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. In this process, sulfuric acid at a ratio of 1:3 is introduced and chemically bonds with the volatile ammonia into crystalline ammonium sulfate. There is no need for downstream stripping or other exhaust gas purification.
All the exhaust gas flows through an integrated fine filtering system, cleaning virtually all the dust as a result. If the exhaust gas has odorous substances, it can then be completely oxidized by a biofilter.
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.
