Deep Concentration Of Various Aqueous Solutions Of Inorganic Salts And Reverse Osmosis (RO) Brine By Using Multiple-effect Membrane Distillation

Membrane distillation technology (MD) has many advantages includingoperating at the mild operating conditions, high retention rate, utilizing low grade heatsources, dealing with highly concentrated salt solutions, and is thus widely used invarious researches, such as treatment of wastewater and desalination of sea water andconcentrating aqueous solutions of inorganic salts. Multiple-effect membranedistillation (MEMD), based on hollow fiber air gap membrane distillation (AGMD)module with a function of recovering internal latent heat, has many advantages, suchas being operated at ambient pressure, less possibility of membrane leak, extendingthe life of membrane module, and achieving a high gained output ratio(GOR) of6-15.MEMD process makes the industrial application of membrane distillation becomereality and represents future development of membrane distillation.In this study MEMD was used to deeply concentrate aqueous solution ofinorganic salts. The influence of feed-in concentration of0.5~40wt%on theperformance of MEMD process such as trans-membrane flux (J) and GOR wasstudied experimentally. The experimental data indicated that both J and GORdecreased with the increase of feed-in concentration. The factors responsible for suchphenomena included variation of vapor pressure、viscosity、diffusion coefficient、specific heat capacity and thermal conductivity with increase of feed concentration,however, the decrease of vapor pressure played a major role. The increase ofconcentration of potassium salts, ammonium salts, sulfate and nitrate had less effectson J and GOR, however，the concentration of chloride and magnesium salts hadgreater effects on J and GOR. Nevertheless, for all feed-in concentration of18%, thevalue of J was above2.30（L m-2h-1）and the value of GOR was above6.5respectively, which was similar with the thermal-efficiency of nine-effect evaporator.During the experimental period, membrane module demonstrated excellentperformance as indicated by the rejection rate of>99.9%for all the inorganic saltstested.MEMD process is limited because of scaling problem caused by calciumdeposition when feed stream, e.g., brine drained from desalination plants, containscalcium and sulfate. In this study, a novel approach with adding sodium oxalate was investigated to selectively remove calcium ions from RO brine. The experimentalresults indicates that: calcium removal efficiency was95%at45℃and dosage ofNaC2O4of a stoichiometric value (1.2), meanwhile the loss of magnesium ions duringthe entire experiment was almost zero. The decalcified RO brine was successfullyconcentrated4-fold to a final concentration of24°Bé by MEMD process, highrejection rate and GOR were attained within the deep concentrating process, whichwould improve the water recovery ration, reduce the discharge of concentrated waterand increase comprehensive benefits in following utilization process of chemicalrecourses in the concentrate.This study indicates that MEMD can be applied in deep concentrating aqueoussolutions of inorganic salts and RO brine, which could provide fundamental data forindustrial application of concentrating varieties of brine streams by MEMD.