Microwave heating combined with activated carbon reaction bed: an energy-saving approach to convert seawater into freshwater

Abstract

Water shortage occurs worldwide due to a lack of clean potable water supply and inland freshwater resources. Desalination via osmosis and distillation are energy intensive and produce brine as a by-product with highly concentrated salt that undesirably undermines the ability of soil and vegetation to produce crops and forage. In this study, microwave irradiation with a unique heating mechanism that provided fast and energy-efficient heating, combined with a reaction bed of activated carbon, was compared and investigated as a technique to enhance freshwater recovery from seawater. The experiment was performed over a range of microwave power (500–700 W) and the ratio of activated carbon (g) to the volume of seawater (mL) to investigate the influences on the yield and quality of freshwater. The addition of activated carbon to seawater with the ratio of 1:20 yielded up to 32 vol% of freshwaters at the microwave power of 700 W for 38 min. The activated carbon enhanced the heating rate of seawater 2-fold, which greatly reduced the energy consumption to 1.34 MJ compared to those without additional activated carbon (1.87 MJ). The recovered freshwater was also detected with low turbidity of 0.2 NTU, total dissolved solid of 0.2 g/L and salinity of 0.2 ppt that is within Class I of the National Water Quality Standards of Malaysia, and it is considered safe to be used in agriculture and industry. The production cost of freshwater was estimated to be about RM 2.75/L including the transportation cost, pre-treatment and electricity usage which is comparatively close to that of treated or mineral water in the market. The results demonstrate that this microwave approach shows an exceptional promise to recover freshwater from seawater without producing brine that would otherwise lead to environmental pollution.