Ph.D Thesis

Ph.D StudentBreytus Anna
SubjectDonnan Process for Nitrate Removal from Contaminated
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Raphael Semiat
Full Thesis textFull thesis text - English Version


Nitrate contamination of drinking water supplies is an acute public health concern. Accepted potable water treatment methods for nitrate removal include: anion exchange, reverse osmosis and electrodialysis reversal. Due to the production of high-strength brine residuals, sustainable application of these three technologies is often limited by a lack of local residual disposal options. Donnan dialysis has a high potential for purifying waters from nitrate contamination due to its simple operation and low energy requirement. In Donnan dialysis, an ionic contaminant migrates across an ion exchange membrane to a concentrated receiver vessel holding a stripping solution.

The general objective of this research was to develop a process for nitrate separation based on Donnan purification of water while minimizing high salinity brine disposal. The following parameters were tested: effects of solution chemistry on nitrate, bicarbonate and sulfate removals; adsorption of nitrate ions by anion exchange membranes; effects of design parameters on the efficiency of nitrate removal; effect of the anion exchange membranes properties on the performance of the process.

Investigation of nitrate ion removal by Donnan dialysis using a stripping solution with three different anion exchange membranes revealed that part of the nitrate was adsorbed by the ion exchange membrane. The adsorption was found to be related to the concentration of the feed and stripping solutions, the counter ion species, and the membrane type. A Donnan dialysis model incorporating the adsorption process was developed and experimentally verified.

Another aspect investigated was the impact of accompanying ions on nitrate transport. Nitrate transport in multi-ionic mixtures, simulating three types of nitrate polluted groundwater, was tested. It was found that water quality had only negligible effect on nitrate and sulfate transport. Additionally, very close nitrate and sulfate removal rates were obtained. Bicarbonate transport was the least efficient compared to the nitrate and sulfate, being strongly impacted by both the water quality and the membrane type.

The performance of semi- and fully- continuous flow systems 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. 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.

Donnan dialysis separations of nitrate, bicarbonate, and sulfate ions using homogeneous and heterogeneous anion exchange membranes were compared under mass transfer, diffusion, and combined mass transfer and diffusion mechanisms. Under mass transfer-control conditions, separation of the three anions was similar with both membranes. Under diffusion control conditions, superior transport was observed with the homogeneous ion exchange membrane. The economic evaluations performed indicate the viability of the Donnan dialysis process, especially with the use of heterogeneous membranes.

The overall results of the study lend support to the high potential of Donnan dialysis for nitrate removal from contaminated water.