Clogging of the electrodeionization chamber

Abstract

To achieve the limits given by power engineering standards (e.g. VGB-R-450 Le, EPRI) of make-up water for high pressure boilers and steam generators, the power industry has been utilizing mixed bed units in the majority of cases (fed by the water treated with the demineralization units). But, nowadays, the mixed bed units are more often substituted by electrodeionization (EDI) units. The basic advantage of EDI units is the reduction of operating expenses (OPEX). The OPEX decreases because of the regeneration by electric current instead of using regenerating chemicals. Unlike classical mixed bed units, these EDI units require better quality of feed water (quality of the reverse osmotic permeate and better). Operational experience has shown irreversible clogging of EDI chamber during continuous operation, in several days, in case of using feed water without reverse osmosis treatment. The aim of this work was to simulate clogging (also fouling of membranes and clumping of mixed bed) of EDI chambers on the laboratory scale to prove the presumption that the origin of clogging is represented by colloidal substances (which can not be removed from surface water without the use of reverse osmosis or nanofiltration unit). However, the work shows that the model solution of colloidal silica nanoparticles could pass through the EDI chamber without any clogging, regardless the diameter of the particles. Thus, these results refuted the hypothesis about the key influence of colloidal particles on so-called clogging of EDI chambers. To get a deeper understanding of the general clogging process in EDI chambers, a series of experiments using very low quality feed water was carried out. The qualitative analysis of clogged components proved that the clogging is principally caused by dissociated ions (e.g. iron, calcium, magnesium, etc.). The experiment also revealed that the sorption of silica in EDI unit is given by the steady state of the dissociated and colloidal form.