About half of the drinking water in the Netherlands is softened in pellet reactors. This technology generates a residual in the form of lime pellets, which consist of a sand nucleus (seed material) and a shell of calcium carbonate (lime). This residual product is reused as a secondary raw material in various sectors such as construction, agriculture and mineral commodities. By replacing the sand nucleus with one made of calcite (calcium carbonate), one produces a ‘pure’ Dutch lime pellet, which consists of a single component and has a lower iron content. One thus creates a product with high-value application possibilities in industries such as glass, carpeting and paper. The returns on the lime pellets increase accordingly. The production of an own Dutch calcite as a seed material from ground and sieved lime pellets is, under certain circumstances, more advantageous than the extraction and delivery of an imported product from marble quarries abroad: it can thus generate a sustainability benefit. The portion of the Dutch lime pellets that is not processed into seed material can be employed as lime in Dutch industries.
Pilot installation research demonstrated that it is possible to have pure Dutch lime pellets meet seed material specifications by having them dried, ground and sieved. The softening reaction using recovered Dutch calcite as seed material works well. Some important unknown aspects concerning the application of this seed material were examined more closely in this research. The processing of Dutch lime pellets and the application of Dutch calcite in drinking water production must be conducted with the assurance of safeguarding the microbial and chemical safety of the drinking water. The lime pellets must efficiently and reliably meet specifications by means of drying, grinding and sieving. The sustainability impact and the economic feasibility of the reuse concept depend on a number of factors, including scale. It is important that the softening in the pellet reactors using recovered calcite be stable over the long term, and that the water softened in this manner meet all regulatory and company-specific requirements.
The project began in 2014 with the switching of seed material from garnet sand to Italian calcite at Waternet’s Weesperkarspel production site. This resulted in lime pellets consisting entirely of calcium carbonate. The lime pellets were then processed (dried, ground, sieved) by different companies. The ground lime pellets were then successfully used as seed material by Waternet in two durability tests, one during the summer and one during the winter period. The low water temperature during the winter test is critical for the softening process. The remaining pure lime pellets were used in carpet tiles. Quality measurements were conducted throughout the entire logistics chain, from the processing of the lime pellets up until, and including, the seed material application. In addition, the economic and sustainability-technical feasibility of different implementation variations of the concept was studied; these ranged from the production and application at the level of the individual softening reactor, to the centralised processing by an independent party working for several water companies. The need and possibility of certification was also assessed.
The intended results provide an understanding of the possibilities of having lime pellets meet specifications through drying, grinding and sieving, and of the quality risks associated with the application of ground pellets as seed material in drinking water softening processes. Moreover, insight was gained into the certification-related, economic and sustainability-related feasibility of different technological and market-associated concepts for the processing of lime pellets into lime products of various specifications, including those for seed material for water softening.