Celsius, Drunen, the Netherlands, has helped upgrade the sludge drying plant of the district water control board of Holland North District (Hoogheemraadschap Hollands Noorderkwartier – HHNK) in Beverwijk. Here sewage treatment sludge is processed into fuel for the HVC (Huisvuilcentrale) waste incineration plant in Alkmaar. 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 by which time it has reached a dry matter content of 90-93%. Each year the HHNK plant processes around 100,000t of dewatered sludge into 22,000t of fuel pellets.
The crucial improvement in the sludge drying process was to replace a long belt conveyor with a screw heat exchanger and two connected screw conveyors.  The previous belt conveyor system was susceptible to malfunction and caused dust as well as odour problems. By contrast, the enclosed stainless steel heat exchanger is dust-tight and, apart from consuming far less electricity, has achieved annual maintenance cost savings of about €30,000. It consists of a double screw in a trough with a capacity of 2000kg/h. As soon as the pellets reach the cooler, they flow slowly through the cooled screws and jacket. From the inlet to the outlet the sludge covers a distance of around 9m, during which time its temperature is reduced from 85° to about 40°C. The jacket of the screw cooler has a double wall and the screws are hollow so the effluent water can flow through them. www.celsius.nl

The Celsius screw heat exchanger being craned into position at the HHNK plant

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