(299j) An Innovative, Environmentally Friendly, Low-Cost Desalination System

The lack of clean drinking water is rapidly becoming one of the most important international health issues. Many remote regions in developing countries face water shortages and lack of potable water causing significant hardship to local people. One popular method used is the desalination of brackish water. The most widely used and commercially proven desalination technologies consist of thermal (evaporative), membrane based methods such as electrodialysis (ED) and reverse osmosis (RO). Unfortunately, these processes require a large amount of energy, which translates to being expensive. People in rural areas of developing countries do not have the means or resources for expensive water treatment systems

To this end, a multi-stage hybrid ED-CON (Electrondialysis/Concrete) process was designed to separate salt from seawater.  The typical setup of an electrodialysis (ED) cell is adopted for the proposed ED/CON hybrid cell. However, rather than using a series of anion and cation exchange membranes that are arranged in an alternating pattern between anodic and cathodic electrodes, two concrete slabs of prescribed thickness are used as permeable membranes separating the anodic, cathodic, and desalination chambers. When a DC potential is applied between the two electrodes, positively charged cations (Na⁺) move toward the cathode passing thorough the cathodic concrete membrane, and negatively charged anions (Cl⁻) move toward the anode passing through the anodic concrete membrane therefore, the concentration of salt decreases as the salt water flows through each stage in the series of desalination cells.  This design is unique and environmentally friendly because it can be produced using locally available recycled material and can be powered by a renewable energy source such as a solar panel.

Concrete is a mixture of Portland cement, water, and aggregate. It can be produced locally from abundant natural or recycled materials.  As the water-cement ratio in a concrete mixture increases, the porosity of the cement paste increases. Concrete pores are interconnected and dissolved ions can move through concrete with water if there is a pressure gradient, or by diffusion due to a concentration gradient when a pressure head is absent.

Extensive testing was performed to optimize the process step by step.  The parameters chosen for study were concrete porosity, the type of anodic solution, and potential used in the test. These variables were adjusted as the research progressed. The suitability of the parameters was judged based on the conductivity of the solution in the desalination chamber, which was used as a surrogate measure for the salinity.  Preliminary results were promising and they show that the conductivity of salt water can be reduced by 10 ms/cm in 6 hours, which corresponds to about a 27% reduction in the salinity of water, in an optimized system.  The optimized system used pervious concrete slabs, lime water in the anodic chamber, and a 10 V potential between the electrodes. The idea of using concrete as a membrane in water filtration is novel and addresses the issue of sustainability. This concept can be broadened and applied not only to the desalination of water, but also to the removal of other contaminants in ground or surface water by designing the concrete filters with suitable aggregate material and size gradations.