In order to limit the leaching of asbestos cement and the cement contained in pipes, the Dutch Drinking Water Act includes the Saturation Index (SI), a measure of calcium carbonate saturation in drinking water. The literature indicates that a carbonate coating can form on the inner walls of pipes through which enough sufficient saturated water flows. A layer of this kind can provide protection against the degradation of pipes containing cement as a result of the leaching of calcium hydroxide. It is important to prevent this leaching because it results in the weakening of the pipe and possibly to the release of asbestos fibres (in the case of asbestos cement).
The aim of this study is to establish a clearer picture of the effect of drinking water composition on the process of leaching over time. This is done by simulating the process with PHREEQC: a package that is ideally suited to the evaluation of the complex interplay between different substances in drinking water and cement.
Support for SI as a normative indicator
There is only limited support for the effectiveness of the SI as a normative indicator for leaching. This is firstly due to the complexity of the formation of coatings on the inside of pipes because this process depends not only on calcium carbonate but also on the entire composition of the drinking water. Secondly, research has shown that the protective effect of the coatings against the leaching of calcium hydroxide is not the same in all cases.
Some water companies currently need to add chemicals to keep the SI at the statutory level. The amounts of chemicals added should preferably be kept to a minimum for both financial and sustainability reasons. It is therefore important to establish a clearer picture of the effectiveness of the protection afforded by the SI. The PHREEQC model is ideally suited to modelling and evaluating leaching processes for this purpose.
The study consists of two modelling phases. In the first phase, the different stages of leaching are described chemically step-by-step with PHREEQC. This is a crucial basis for modelling leaching correctly. It will be possible to apply the PHREEQ model successfully to obtain valuable (but qualitative) information about the effect of ion concentrations (and therefore acidity, SI etc.) on the level of leaching.
In the second phase, we draw up a diffusion model and combine it with the chemical model. This results in a model of the entire leaching process that can be used to predict how, for a given water composition, leaching will behave quantitatively over time.
The results of this study will provide water companies with a better picture of how to manipulate drinking water composition in order to manage leaching in their pipes containing cement. This makes for a more future-robust mains network. The modelled link between drinking water composition and leaching can make an essential contribution to reconsidering index parameters for the risk of leaching that are derived from the drinking water composition such as SI or CCPP (calcium carbonate precipitation potential). This gives water companies the tools they need to enter into discussions about a possible amendment of the standard in the Dutch Drinking Water Decree.