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Figure 7 Bromide removal with a 2 stage and 3 stage EDR system

The 2 stage EDR system showed a bromide removal ranging from 75% to 85%. This means that especially at low temperatures bromide removal will not be sufficient to restrict the bromate concentration to 5 jj.g/1, and in addition acid has to be dosed during the ozonation to comply with the bromate standard of 5 p.g/1. At a low water temperature (4 °C) the relationship between ozone dose, pH and bromate formation was established for the water treated with the 2 stage EDR system. Figure 8 shows the results. By lowering the pH to 6.5 the bromate formation can be restricted to 5 |ig/l, even at an ozone dose of 2.3 mg/1.

The 3 stage EDR system showed a bromide removal ranging from 90% to 95% (Figure 7). This implies that bromide removal is sufficient to meet the bromate standard of 5 p.g/1 during ozonation. Under all circumstances, even without an acid dose, the bromate concentration after ozonation was in compliance with the standard (results not shown). From these results it is clear that in EDR-IMS 2, applying EDR as first process unit, both with a 2 stage and 3 stage EDR system the bromate requirement can be fulfilled, albeit that with a 2 stage EDR system an additional acid dose is required. The acid dose (30% HC1) to control the bromate formation is 30 g/m3pr<K|uct.

4.3 Fouling of the EDR system

Scaling and fouling can seriously affect the EDR performance. Scaling can be controlled by dosing HC1 in the concentrate up to a LSI of 1.2-1.5, by restricting the recovery to 93% and by using a reversal time of 20-30 min.

To avoid fouling, Ionics gives guide values for the EDR feedwater and advises to use cartridge filters preceding the EDR unit. The Ionics guidelines and the actual concentrations in the EDR feedwater in scheme EDR-IMS 2 are compared in table 3.

Table 3 Guidelines of the EDR feedwater quality and the actual feedwater quality (pre-treated Rhine River water) in EDR-IMS 2

Parameter

Ionics guidelines

Actual concentrations

pH

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