Removal of nitrate in semi and fully continuous-flow Donnan dialysis systems

Abstract

Removal of nitrate by Donnan dialysis was investigated in semi- and fully- continuous flow systems. The performance of the two different operating modes was compared. In the semi-continuous mode, a nitrate solution was continuously dosed into the feed compartment while the receiver compartment was operated in a batch mode. In the fully continuous system, both feed and receiver solutions were continuously dosed into the corresponding compartments. Similar removal efficiencies of nitrate were obtained in both modes of operation, with a maximum of 80% reached at residence time of 4 h, based on the flow into the feed compartment. Differences between the two modes of operation were manifested by inhibition of nitrate transfer following a pseudo steady-state in the semi-batch system and by nitrate accumulation in the receiver solution. In the fully continuous-flow Donnan dialysis system, the nitrate concentration in the receiver reached a saturation level corresponding to the nitrate removed from the feed. As in the case of batch dialysis, nitrate was adsorbed by the ion exchange membrane also in continuous flow. The amount of adsorbed nitrate depended on both the concentration of the receiver solution and on the feed nitrate concentration. The mass transfer coefficients, obtained in the semi- and the fully- continuous systems, were found to be very close to the values obtained in batch operation.