Persistence pays off

Persistence pays off

In the autumn of 2004, Alfa Laval Nordic was awarded a contract for a one-off ALDEC G2-50 decanter centrifuge for dewatering sewage sludge at the Høllen waste water treatment plant in the municipality of Søgne, in Norway. This contract was won in the face of stiff competition from companies such as Noxon and Westfalia, and included an annulment clause should the equipment not live up to the client’s expectations. These requirements were quantified in terms of performance figures related to capacity and separation efficiency, pegged at levels guaranteed to outperform the plant’s existing Hysep HD44 set-up. Soon after the ALDEC G2-50 unit had been commissioned in the spring of 2005, it became evident that it was unable to achieve the designated performance targets. In addition, it was the cause of downstream operating difficulties due to the sticky, plastic cake that it produced. There were other problems, too. Sludge would build up under the hood, to the point where it actually slowed down the bowl’s rotation speed. This in turn resulted in excessive power consumption as well as erosion of the outside of the bowl. In practice, the plant operator had to open the hood several times a week in order to open up the downwards sludge duct. These interruptions cut into both processing time and throughput capacity. Initial studies seemed to indicate that the unusual behaviour of this specific sludge was due to waterworks sludge being “dumped” into the sewer upstream from the waste water treatment plant. However, this practice was also used before the ALDEC G2-50 was installed. This meant it could not be used to explain or excuse the performance deficiencies.

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