Conditioning of Aggressive Water

Worldwide limestone (CaCO3) filtration is used in many treatment plants for the conditioning and (re)mineralization of drinking water. The goals are to increase the concentrations of Ca2+ and HCO3-, the pH and the Saturation Index (SI), thereby improving the quality of the water regarding corrosion control, buffering and taste. Typical applications include (very) soft groundwater with (very) low alkalinity and desalinated water. In Norway, some plants use a product made of ground natural limestone, called Micronized CaCO3 Slurry (MCCS), which is dosed as a slurry of fine particles (order of 1-2 ?m) into the raw water. In this research a study is made of the potential of MCCS as an alternative to limestone filtration. Experiments were performed to determine the dissolution kinetics of MCCS and other CaCO3-products, including natural limestone grains (1-2 mm) and two precipitated CaCO3 (PCC) powders (Heyer test powder and NanoPCC powder), that consist of even finer particles than MCCS (down to 0.088 ?m). The experimental setup consisted of jars containing water in which the CaCO3-product was introduced. An increase in conductivity as a function of time was observed, which could be used to describe the dissolution kinetics. As expected from theory, the dissolution kinetics are strongly influenced by the particle size of the CaCO3 and the driving force towards the chemical equilibrium. However, all CaCO3-products needed substantial detention times (30 minutes and more) to dissolve completely. This severely hampers the feasibility of MCCS as an alternative to limestone filtration, which can be operated with an Empty Bed Contact Time in the same order of magnitude. This finding can be explained by the fact that the available mass/volume of CaCO3 in a limestone filter is much higher than the available mass/volume of particles in the dosed slurry, thus compensating for the difference in particle size. It is concluded that MCCS is generally not a feasible alternative for limestone filtration as a stand-alone option for the conditioning and (re)mineralization of drinking water. Applications of MCCS will be limited and should either be found in combinations with coagulation/filtration (which is the actual mode of operation in Norway and limits the problems associated with non-complete dissolution of the CaCO3) or in combination with other conditioning and (re)mineralization methods (by dosing a limited amount of MCCS to the raw water, which limits the problem of the slow kinetics).Sanitary EngineeringWatermanagementCivil Engineering and Geoscience