Supported-Gas-Membrane Process For Removal/Recovery Of Ammonia From Wastewater By Using Dilute Nitric Acid As Absorbent

Membrane desorption, membrane absorption or supported-gas membraneprocess (marked by microporous membrane wall space was occupied by gaseousphase) has been used for a variety of liquid or gas separation, including removal,recovery, enrichment and purification of NH3, CO2, SO2, H2S, Br2, I2, HCN, amines,phenols, volatile organic compounds and so on. However, in some cases, it needs todeal with the system containing the oxidizing substance (such as nitric acid, bromine,bromide, hypochlorite, chlorine, hydrogen peroxidation and ozone), wihle PP, PVDFand PE membrane have poor oxidation resistance; PTFE membrane having excellenthydrophobic performance and developed pore structure is not eroded by acid, baseand oxidant in a wide range of temperature. In this study, to simulate oxidizingenvironment, dilute nitric acid was used as absorbent in a supported-gas membraneprocess when aqueous ammonia solution was used as feed. The feasibility andlong-term operational stability of PTFE hydrophobic microporous hollow fiber-basedsupported-gas membrane process was demonstrated to remove/recover ammonia andproduce ammonium nitrate as a by-product. Under similar operation conditions, theseparation performance of three kinds of membrane contactors made from PTFE, PPand PVDF were compared; the influence of feed-in velocity, feed-in concentration offeed solution and absorbent solution, and temperature on the mass transferperformance of PTFE, PP and PVDF contactors was also studied. Mass transfermodel was established and MATLAB was used to solve the membrane mass transfercoefficient kMand the lumen-side mass transfer coefficient kL. Experimental resultsshowed that the lumen-side diameter and wall thickness of the hollow fibers were themost dominant factor influencing the mass transfer process, and that the overall masstransfer coefficient K, kM, kLand removal rate R increased significantly with theincrease of temperature. The long-term experimental results showed that the PVDFmembrane contactor had a leakage problem7days later, the mass transfer coefficientof the PP membrane contactor had dropped notably20days later, while the PTFEmembrane contactor still had a stable performance after running for30days.When PTFE hollow fiber membrane contactor was used to treat thepharmaceutical wastewater, oxidizing sulfuric acid solution with Fe3+ions asabsorbing liquid, first the oil sick, phenyl acetic acid and other pigment substance were removed by macroporous resin absorption, microfiltration and ultrafiltration,which also obviously decrease the COD. After treated by PTFE supported-gasmembrane process, more than96%ammonia was removed from the wastewater.Finally, the Fe3+ions in the ammonium sulfate solution were removed by cationexchanging process. Shaker test showed that the damages of ammonia nitrogen andFe3+ions to the biochemical treatment were removed basically. When PTFEmembrane module was used to treat actual pharmaceutical wastewater, it could runfor more than one month and no leakage was found, it also had a stabile mass transferperformance.PTFE hollow fiber membrane contactor overcomes the faults of traditional ones,like poor nitric acid oxidation resistance and short service life. PTFE membraneshowed good performance for ammonia-nitrogen removal, more acceptableby-product and good operation stability. It provides a basic gurantee forremoval/recovery of ammonia from wastewater (or chemical feed solution) streams asa by-product of ammonium nitrate by using gas-supported membrane separationprocess.