Case story: Sludge drying plant safe and durable due to cooling screw technology

°Celsius Screw Heat Exchangers
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Sludge drying plant safe and durable due to cooling screw technology



De slibdrooginstallatie van Hoogheemraadschap Hollands Noorderkwartier

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 had been battling for years here 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. In the coming years °Celsius BV will support the cost-effective management of this plant with bold Asset Management.

Dewatered sludge turned into fuel

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.


Adaptation of sludge drying process gives considerable saving

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. Installatie schroefwarmtewisselaar slibdroger 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.


The old conveyor belts were very susceptible to malfunctions and were high maintenance and the leakage meant there was dust and odour nuisance. For reasons of good Asset Management the HHNK carried out a “Life Cycle Costs analysis” which showed that screw conveyors were a better solution in every respect. After all a screw conveyor requires little maintenance and in principle oper-ates trouble-free. The dust and odour nuisance is also a thing of the past because of the enclosed tubular screw conveyor. °Celsius BV has solved all the problems and achieved a saving in maintenance costs for the HHNK of around 30,000 Euros per year. In addition the whole plant consumes much less electricity.


A perfectly functioning plant due to synergy

Mr Kramer, Tactical Manager of the Sludge drying plant at HHNK, says. ‘How great it is when in the design phase of a plant you can already name all the risks and adapt your design to this. Add the knowledge and experience of °Celsius in the area of screw heat exchangers together with our knowledge of the product being processed and you have a perfectly functioning plant! Client and supplier carrying out a risk analysis together, for both parties a new experience that has proved very satisfactory. From design to commissioning °Celsius BV has assisted us as a partner. We look forward confidently to the joint management and maintenance in the coming years’.


Technical specifications of cooling screw

Ingebouwde slibdrooginstallatie celius

The screw heat exchanger is made of SS 316L and consists of a double screw in a trough with a ca-pacity of 2000 kg/hour. As soon as the pellets reach the cooler, they flow slowly through the cooled screws and jacket. From the inlet to the outlet the sewage treatment sludge covers a distance of around 9 metres. During this time the temperature of the sewage treatment sludge is reduced from 85 degrees °Celsius to around 40 degrees. The jacket of the screw cooler has a double wall and the screws are hollow so the effluent water can flow through them, or treated wastewater from the sewage treatment. The effluent cooling water is around 30 degrees °Celsius and this water temperature rises during the process by around 2.5 degrees. The design of the machine complies with PED standards.



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Installatie schroefwarmtewisselaar slibdrogerThe 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 had been battling for years here
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