In The Netherlands, 50% of the drinking water is treated with pellet softening for various reasons: i) public health (heavy metal solubility), ii) costs (warm water device maintenance, energy and soap requirement), iii) environmental benefits (energy and soap requirement) and iv) customer comfort (scaling) [2]. Calcium carbonate crystallizes on a seeding grain of garnet or sand and calcium carbonate pellets are produced as a by-product. These pellets are applied as a (secondary) raw material in industries such as the construction, agricultural and mineral-resource sector. The sand grain inside the pellet inhibits reuse as seeding material and application in high potential market segments such as glass, paper, food and feed. Pellet quality significantly improves by replacing the sand grain with a grain of calcite (calcium carbonate) since in such way the pellet exists of one component only and the iron content is reduced. Earlier work showed that it is technologically possible and in some cases economically feasible to replace sand grains with commercially available calcite grains [3], obtained from limestone quarries (e.g. Italy or Germany). In order to further increase sustainability by reducing transportation, the drying, grinding and sieving of pellets and the reuse of this calcite as seeding material for softening was investigated on pilot scale at Waternet. Currently, the calcite reuse concept is tested at full scale at the Weesperkarspel facility of Waternet in a TKI project1. The objective is to investigate the feasibility of this reuse concept. An important aspect is the risk of contamination of the drinking water due to the reuse of calcite material which is processed externally.