The sludge drying plant of the District Water Control Board of Holland North District (Hoogheemraadschap Hollands Noorderkwartier – HHNK) in Beverwijk processes waste 'sewage treatment sludge' into fuel for the HVC (Huisvuilcentrale) waste incineration plant in Alkmaar. They were said to have been battling for years with high maintenance costs, a high breakdown frequency and unsafe situations due to overheating, dust and odour nuisance. In cooperation with °Celsius BV from Drunen HHNK has upgraded its sludge drying plant into a safe, trouble-free and energy-saving plant.
During the treatment process the sewage treatment plant dewaters sludge to produce dry sewage treatment sludge. The dry matter content of this sewage treatment sludge is then only 18 to 23% and is not yet suitable as a secondary fuel for HVC. In the sludge drying plant the dewatered sludge first undergoes a drying and cooling process using a screw heat exchanger. The dried sludge leaves the drying process in pellet form and has then reached a dry matter content of 90 to 93%. Each year the HHNK sludge drying plant processes around 100,000 tonnes of dewatered sludge into 22,000 tonnes of fuel pellets.
The crucial improvement in the sludge drying process was to replace the long conveyor belt by a screw heat exchanger (or screw cooler) and two connected screw conveyors. The former conveyor belt could only carry pellets from the dryer to the storage silo. The screw cooler on the other hand transports and also cools the pellets by 30° to 40 °C. After this a long vertical tubular conveyor with an auger and a horizontal tubular conveyor with an auger transports the pellets to the storage silo. This improved set-up removes the danger of overheating in the storage silos and the pellets do now meet the HVC temperature requirement. www.celsius.nl