|M.Sc Student||Oded Porat|
|Subject||Identifying diffusion of organic pollutants across Ionic|
Exchange Membranes in Microbial Desalination Cells
|Department||Department of Civil and Environmental Engineering||Supervisor||Full Professor Dosoretz Carlos|
|Full Thesis text|
This thesis focused on the previously identified problem of contamination of the permeate stream in a Microbial Desalination Cell (MDC), which is a novel bio electrical system that uses recovered energy from wastewater to drive saline water desalination. More specifically this study examined the type of molecules, reactor conditions, and reactor configurations (rectangular- rMDC or tubular- tMDC) which lead to contamination of the permeate stream. To accomplish this goal model compounds were selected (acetate, Paracetamol (PCM), Ibprofuen (IBP), Phosphate) and tracked under abiotic and biotic conditions.
In abiotic experiments, results were compared under three conditions: no voltage with 0 g L-1 or 10 g L-1 salt solution in the desalination compartment, or 10 g L-1 salt solution with voltage. Under 0 g L-1 conditions marker concentration gradient was the sole driving force for leakage while under 10 g L-1 conditions Donnan exchange and osmotic pressure were added as additional driving forces. The addition of an electrical potential via a DC power supply allowed for the investigation of current effects on leakage in abiotic reactors. The results from the experiments where no salt was present showed that marker concentration gradient between anolyte and salt solutions caused 15.6±2.8%, 3.1±0.3%, and 0.7±0.7% leakage for acetate, (PCM) and (IBP) respectably. When 10 g L-1 salt was added to salt solution without voltage, the leakage increased to 20.7±6.9% (acetate) and 1.3±0.8% (IBP); while no noticeable difference was observed with PCM, 2.8±1.0%. The percent leakage of acetate, IBP, and PCM under the influence of an electrical potential was 17.2±2.1%, 0.6±0.12%, and 1.9±0.29% respectively.
The aforementioned results showed that acetate was most prone to leakage followed by PCM and IBP, and the trend is relevant to their molecular weight (acetate< PCM < IBP) and diffusion coefficient (acetate> PCM > IBP). PCM and IBP most probably got stuck in the membrane due their larger size and hyrdrophobicity. Additionally it was found that all markers had a higher percent leakage in the rMDC, most probably due to the higher organic loading rate implicit to this reactor configuration.
In biotic experiments reactors were run in open circuit (no current) or closed circuit (current) modes. The open circuit tests showed leakage percentage of 16±0.5%, 18.42±2.4%, 1.3±0.8%, and 2.8±1.1% for acetate, phosphate, PCM, and IBP, respectively. While the closed circuit tests, reduced the percent leakage of acetate, phosphate, and IBP slightly to 12.1±3.2%, 15.6±2.3% and 2.1±1.5%, while no reduction was observed with PCM; 2.9±1.7%.
In conclusion, this study has shown that there is a potential for contamination of product water due to concentration gradients (organic and salt) between the anode and desalination chambers which lead to molecular diffusion, osmotic transport, and Donnan exchange (charged molecules). Small negatively charged-polar molecules are most prone to leakage. To reduce leakage of pollutants a reactor configuration with low organic loading rates, small salt concentration gradient between anolyte and salt solutions (brackish water), and high bioelectricity production should be favored. These findings provide useful information for further development of the MDC.