About 50% of the drinking water in the Netherlands is centrally softened by the drinking water companies in a process known as pellet softening. In this process a base and seeding material are mixed in an upflow reactor, where subsequently CaCO3 precipitates on a seed core as pellets. The seeding material is usually sand, but recently CaCO3 was introduced as seeding material at several full scale plants. The pure calcite pellets that are produced as a by-product in these plants can be reused as seeding material, after grinding and sieving part of the produced pellets. The main advantages of this reuse are an expected significant decrease of the ecological footprint of both calcite as raw material and the drinking water treatment plants using pellet softening, and increased valorisation of the pellets. However, the handling and processing of the pellets and seeding material should be such, that microbiological and chemical contamination risks of the drinking water are negligible. This research shows that grinded and sieved CaCO3 outperforms commercial CaCO3 and that microbiological risks can be adequately controlled.

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.