Optimizing CaCO₃ Reaction Kinetics during Drinking Water Softening

Since the 1980’s in the Netherlands, drinking water softening is applied on the basis of robustness. However, in more recent years, significant changes have been made at the drinking water softening location of Weesperkarspel, which is responsible for providing drinking water to Schiphol and Amsterdam. A few of those examples are the exchange of pellet material, where garnet sand has been replaced by calcite pellets, the way of mixing of NaOH and raw water, the target value of water hardness and the acid addition, where HCl has been replaced by CO₂. Yet, these changes have not led to a change from a robust way to a more flexible way of drinking water softening. For that reason, batch and column experiments have been performed at the pilotplant of Weesperkarpel to study the CaCO₃ crystallization kinetics during drinking water softening. For the batch experiments, the seeding material or specific surface area (i.e. 0, 1, 5, 10, 30 and 60 g/L CaCO₃) and temperature (5, 20 and 40 °C) were varied, while for the column experiments, the amount of seeding material, temperature conditions, the linear velocity and the NaOH addition rate were varied to investigate their impact in regard to the Ca²⁺ removal rate (i.e. CaCO₃ precipitation rate) from the raw water matrix. In addition, the experiments were performed in winter and summer to investigate the temperature influence. The pH, (electrical) conductivity (i.e. EC) and turbidity (i.e. TUB) were measured for both the batch and column experiments and the bed height was monitored as well. From the pH, EC and TUB measurements, the CaCO₃ precipitation rate was determined. Batch results show that at least 30 g/L of seeding material is needed for ≥ 20°C, but up to 60 g/L is required for < 20°C for optimal softening. The initial column results show that for ideal drinking water softening at Weesperkarspel, an optimum exists in the NaOH dosage and its value depends on the temperature, amount of seeding material and linear velocity.