The water company Vitens N.V. is responsible for the supply of water on the Wadden Sea islands of Terschelling and Ameland. Vitens decided to renovate the obsolete facilities at the West Terschelling and Hollum drinking water production sites. The goals of the renovation project are to replace worn-out installations, to reduce the hardness of the drinking water and to improve its colour and biological stability. Royal Haskoning was asked to prepare the general specifications and the final design for the structural, mechanical and electrical works. The final design incorporated all the relevant design choices and the level of detail was sufficient to permit execution of the works. The project is currently being implemented.
Substantial fluctuations in the demand for drinking water
One of the most important features of water production on the Wadden Sea islands is the substantial variation during the year. The population doubles during the spring and summer because of the influx of tourists. Over the course of a year the demand ranges from 6 to 75 m3/h on Terschelling and 2 to 58 m3/h on Hollum. Prior to the renovation this variation in demand was met by a pipeline that supplies drinking water from the mainland. The dependence on this pipeline during high season makes it the Achilles heel of the islands’ drinking water supply. Presently it is not possible to supply enough drinking water during high season if the pipeline is shut down for maintenance or because of operational problems. The drinking water production sites therefore need to have the capacity to temporarily replace the quantity supplied by the pipeline.
Purification steps
The specific features described above call for a special approach to drinking water production on the islands. The large variation in output translates into very tough requirements for the purification steps. It was decided to use nanofiltration as a purification step in order to achieve the desired pH reduction and colour improvement. After nanofiltration the water has such a low pH that mixing with unsoftened water is necessary. In order to achieve the desired biological stability the existing post-filtration in the slow sand filtration section will be replaced by low pressure UV disinfection.
Approach saves money and space
It was decided to use the space being made available in the slow sand filtration section as the location for the nanofiltration, the extra sand filter vessels and supporting equipment. This enables the existing buildings to be retained and there is no change to the outward appearance of production sites. The renovation has a tight schedule so that production can be guaranteed during execution of the project and operational installations can be guaranteed for the 2007 high season.
Aad van Duyvenvoorde
T +31 (0)10 2865655
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Rob Schotsman
T +31 (0)10 2865695
E Rob Schotsman