Criteria for the existence of self-sustaining water softening and desalination processes for a basic two-component system

Abstract

The idea of reagentless softening and purification of water appeared in the first half of the 20th century. The idea implies that the regeneration of the column after softening is performed using a solution concentrate obtained after removing the purified water. The possibility of the implementation of such processes is accounted for by the so-called electroselectivity effect. It means that when using the same ratio of equivalent concentrations of multi-charge cations in the solution, the selectivity of the cation exchanger to ions with smaller charges increases with an increase in the total concentration of cations. The interest to self-sustaining softening and purification processes has been induced by new technological capabilities for the creation of the next-generation water treatment systems, for instance mobile systems or systems ensuring a high degree of extraction of fresh water and brine concentration.

Our theoretical study demonstrated that the process is possible under certain conditions. To determine these conditions we conducted a numerical experiment using a basic neutral two-component solution containing calcium and sodium cations and chlorine anions. The study determined the formulae for three criteria for a self-sustaining process.

Our calculations demonstrated that self-sustaining processes are only possible in solutions containing a limited concentration of calcium ions, which should be below one tenth of the volume of the sorbent used. The characteristics of self-sustaining processes are also affected by the total concentration of ions and the degree of concentration in the distiller. These conditions are presented as a criterion which can be assessed using the suggested formulae as well as based on the results of the practical study of the first cycle of the actual process. The results of the study can be generalised and propagated to more complex systems.