Pelleted sewage sludge is increasingly being demanded in tenders as the transport of a compacted, dust-free product, for example, is much more economical and safer.
For pelleting the pre-treated sewage sludge, a solids content of 70 to 95 % is required. This content is achieved by means of dehydration and subsequent drying (or a combination of both). The pre-treatment of the sewage sludge in a digestion tower has a positive effect on the capacity of the pellet mill as well as the wear of the pan grinder rollers and die. For example, at a solids content of 70 % less solid pellets are produced than at 90 %. The percentage required depends on the use of the sewage sludge.
The bulk density of the sewage sludge pellets is approx. 800 kg/m³.
In case of a high percentage of organic substances the dried sewage sludge has a high volume and can only be proportioned into the press by means of special devices due to its bulk density of 150 - 200 kg/m³. Besides, grinding will be required before pelleting. The design of our sewage sludge pelleting plant therefore includes a pre-bin which is followed by our multiple screw. The product is then mixed and finally fed to our flat die pellet mill, where it is processed by means of the pan grinder rollers and the die. Our system is particularly appropriate for sewage sludge, as it can be adapted without problems to varying organic contents.
Applications and Advantages:
- The service life of the pelleting tools is between 1,500 and 2,000 hours under regular operating conditions.
- The dust content is very low with < 1 % referred to a dust particle size of < 0.5 mm.
- The maximum circumferential speed is 2.5 m/s and the surface temperature is far below 120 °C.
- Due to the automatic operation operating phases of about 3 months are possible. During this time the pelleting plant is operated unattended. After three months, the specified and preventive repairs are carried out before the plant is put back into operation.