Rejection of Conductivity

The raw water conductivity decreased 5 to 10 % during microfiltration. No rejection of salt is expected with MF, however, the ions may be associated with the particulates or retained organics. The RO permeate was very low in conductivity, as expected, and is not very concentration dependent. During the concentration experiment, the permeate conductivity increased from 3 to 10 pS. 

After one month of experiments, the MF conductivity rejection did not increase significantly. The layer formed on the MF membrane which increased DOC rejection, had no major influence on salt rejection. This is also shown in the backwash efficiency section. 

The raw water and MF filtrate pH were relatively constant ranging between 5.5 and 6.0. Both RO permeate and concentrate pH values increased with the concentrate pH increasing from 5.8 to 7.3. These increases are most probably related to the behaviour of carbonate species, since in the concentrate tank the development of gas was observed. Also, changes in ions which permeate through the membrane and thus change the solution chemistry may influence pH. 

A change in temperature was not observed and can therefore not be an explanation for the pH variation (calcium carbonate is more soluble at low pH). However some precipitation may have occurred. 

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Figure A 1.14 Rejection of conductivity and permeate conductivity as a function of concentrate conductivity (25 10196).

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