Using microbial desalination cells to reduce water salinity prior to reverse osmosis
Energy & Environmental Science
A microbial desalination cell (MDC) is a new method to reduce the salinity of one solution while generating electrical power from organic matter and bacteria in another (anode) solution. Substantial reductions in the salinity can require much larger volumes of the anode solution than the saline water, but any reduction of salinity will benefit the energy efficiency of a downstream reverse osmosis (RO) desalination system. We investigated here the use of an MDC as an RO pre-treatment method
... eatment method using a new type of air-cathode MDC containing three equally sized chambers. A single cycle of operation using a 1 g L À1 acetate solution reduced the conductivity of salt water (5 g L À1 NaCl) by 43 AE 6%, and produced a maximum power density of 480 mW m À2 with a coulombic efficiency of 68 AE 11%. A higher concentration of acetate (2 g L À1 ) reduced solution conductivity by 60 AE 7%, and a higher salt concentration (20 g L À1 NaCl) reduced solution conductivity by 50 AE 7%. The use of membranes with increased ion exchange capacities further decreased the solution conductivity by 63 AE 2% (20 g L À1 NaCl). These results demonstrate substantial (43-67%) desalination of water is possible using equal volumes of anode solution and salt water. These results show that MDC treatment could be used to substantially reduce salt concentrations and thus energy demands for downstream RO processing, while at the same time producing electrical power. Current desalination technologies such as reverse osmosis (RO) or distillation require a large amount of energy, ranging from 3.7 to 650 kW h per cubic metre of water treated. The amount of energy needed for RO increases in proportion to the initial salinity of the water. Recently a new device, called a microbial desalination cell (MDC), was shown to desalinate water without the use of any external electrical power. Electrical energy produced directly by the degradation of organic matter in water by bacteria was used to achieve desalination. However, large volumes of water were used in the anode chamber containing the bacteria, relative to volume of water desalinated. It is shown here that using equal volumes of water in the anode and desalination chamber it is possible to remove up to 60% of the salt in the water. No external power source was needed, and up to 480 mW was produced per square metre of electrode surface area. Removing over half the salt can substantially reduce energy requirements needed for subsequent treatment using RO.