Electrochemical removal of nitrate from a Donnan dialysis waste stream.

The objective of this work was to investigate electrochemical removal of nitrate from a high salinity waste stream generated by Donnan dialysis. Donnan dialysis for nitrate removal is a promising technique. It produces a distinctive composition of a high salinity waste stream of NaCl or Na2SO4 that requires a viable disposal method. The waste stream has the full anionic composition of contaminated groundwater, but the only cation is sodium. Experiments were conducted in a batch system setup. A copper cathode was chosen over brass, aluminum and graphite cathodes. A dimensionally stable anode (DSA), Ti/PbO2, was selected over a Ti/Pt anode. Electrochemical denitrification of high salinity Donnan dialysis nitrate wastes was successfully achieved, with different behavior exhibited in high salinity NaCl solution than in high salinity Na2SO4 solution. NaCl inhibited nitrate removal at high salinities while Na2SO4 did not. The maximum removals after 4 h operation in the high salinity wastes were 69 and 87% for the NaCl and Na2SO4 solutions respectively.

Modeling Remineralization of Desalinated Water by Micronized Calcite Dissolution.

A widely used process for remineralization of desalinated water consists of dissolution of calcite particles by flow of acidified desalinated water through a bed packed with millimeter-size calcite particles. An alternative process consists of calcite dissolution by slurry flow of micron-size calcite particles with acidified desalinated water. The objective of this investigation is to provide theoretical models enabling design of remineralization by calcite slurry dissolution with carbonic and sulfuric acids. Extensive experimental results are presented displaying the effects of acid concentration, slurry feed concentration, and dissolution contact time. The experimental data are shown to be in agreement within less than 10% with theoretical predictions based on the simplifying assumption that the slurry consists of uniform particles represented by the surface mean diameter of the powder. Agreement between theory and experiment is improved by 1-8% by taking into account the powder size distribution. Apart from the practical value of this work in providing a hitherto lacking design tool for a novel technology. The paper has the merit of being among the very few publications providing experimental confirmation to the theory describing reaction kinetics in a segregated flow system.

Rotating cylinder technique for assessing the effectiveness of anti-scalants.

A novel technique for evaluating the relative inhibition effectiveness of different anti-scalants is presented. The technique is based on the ability of anti-scalants to modify the scale deposition mechanism from mass transfer control to surface control. A rotating cylinder system which is known to provide well controlled mass transfer conditions is used to determine the scaling rate of a supersaturated solution dosed with various feed concentrations of an anti-scalant. Mass transfer conditions were characterized by turbulent flow at hydrodynamically smooth surface. In the absence of an anti-scalant, scale deposition is mass transfer controlled and scaling rate increases with rotation speed. With sufficient anti-scalant dosage, precipitation is modified to surface control and rotation speed has no effect on the scale deposition rate. Determination of the critical anti-scalant dosage enabling surface controlled precipitation provides a sensitive technique for comparing the relative effectiveness of different anti-scalants.