Calcium Carbonate Formation in Groundwater-Supplied Drinking Water Systems: Role of CO2 Degassing Rate and Scaling Indices Applicability

Abstract

CaCO3 precipitation is a ubiquitous and vital process with far-reaching implications for various natural systems. In drinking water supply networks, it creates malfunctions in the system, especially by pipes clogging. This is a common problem in Tunisia, particularly for systems supplied with groundwater. This work attempts to highlight the effect of dissolved CO2 degassing kinetics and determine the most reliable scaling index to predict scaling. For this, a diagnosis of two drinking water circuits is followed by a laboratory study. Results of the field study show that the network scaling is controlled by the dissolved CO2 content, which is significantly affected by the water/atmospheric air contact. The scale formed is mainly CaCO3–calcite. A laboratory-scale simulation of the natural phenomenon using an experimental setup of the fast-controlled precipitation method (FCP) was performed. The result shows that a low CO2 content is a necessary condition for a supersaturated system regarding calcite but not sufficient for precipitation to take place. The precipitation can occur at very low supersaturations if time is allowed for stable nuclei to form, explaining the scaling of drinking water networks. The fundamental and applied study of the scaling indices shows that the Ryznar stability index (RSI) is the most adaptable index for predicting scale formation.