Study of Stabilized Gas Membrane Process for Removal/Recovery/Enrichment of Ammonia from Chemical Feed and Wastewater Streams
|Keywords||ammonia-containing wastewater stabilized gas membrane masstransfer operational stability|
The ammonia-containing wastewater is being paid more and more attention afterthe ammonia and nitrogen compounds were brought into the overall control indexsystem for the first time in the China Twelfth Five-year Program.Supported-gas-membrane-based separation was an effective method to removeammonia and other volatile compounds form their dilute aqueous solutions because ofits some advantage, such as larger specific surface area, ease of operation, the lack ofproduction of any secondary pollutions and the possibility of recover pure by-product.Supported gas membrane also exists some disadvantage, such as short life, the shellside non-uniform flow, low concentration ammonium salt in the by-product，especially is not suitable to discharge landfill leachate and other dirty feed. Stabilizedgas membrane separation process using modules of two sets of hollow fibers could beused to overcome these shortcomings. The aims of this paper were studying theinfluence of temperature, the velocity of feed, the initial concentration on the masstransfer performance and the influence of salts on the mass transfer performanceaccording to the fact that several salts exist in practical wastewater; studying theadvantage of stabilized gas membrane in the recovery/purification/enrichment ofammonia. On this basis, we used the membrane to discharge landfill leachate.The experimental results of discharging the simulative wastewater by stabilizedgas membrane showed that the temperature was the most important factor on the masstransfer performance compared to the feed concentration and the velocity, the totalmass transfer coefficient increased with the increase of temperature nearly in theforms of exponent. The sodium salts and ammonium salts such as NaCl, Na2SO4,NaNO3,NH4NO3,NH4Cl performed different premiums salting-out effect and（NH4）2SO4effected the strongest salting-out effect, when its concentration increasedfrom0to25%, a2.1-fold increase in the membrane mass transfer coefficient and25%increase in the total mass transfer coefficient. CaCl2revealed powerful salting-ineffect, the increasing viscosity of aqueous feed solution induced by salt concentrationled to the significant decrease of lumen side mass transfer coefficient. When using thesupported gas membrane process and stabilized gas membrane process to removeammonia in the same condition, the concentration of ammonium salt in theby-product stream of stabilized gas membrane process was7%higher than that of thesupported gas membrane process.When using the stabilized gas membrane discharged landfill leachate, effectual pre-treatment was an important step in the overall procedure. The slick oil and largemacromolecules that did harm to the hydrophobic microporous membrane could beremoved on the whole and COD decreased obviously, some heavy metal ions such asCd and Cu was less than or near the detection limit after flocculation-coagulation, airfloating and membrane ozone oxidation processes. The circulating and continuousammonia removal experiments revealed that when the feed liquid was continuousbumped into the stabilized gas membrane module for four times or being dischargedby four-stage membrane modules, the ammonia removal rate could be up to96%, sothe harm or inhibited effect of ammonia to biochemical treatment was almosteliminated. The membrane modules were not found any leakage after running forabout one month continuously, and the mass transfer coefficient were near to the labexperimental data.The stabilized gas membrane process overcame disadvantages of conventionalsupported gas membrane process, and performed well in the ammoniaremoval/recovery/enrichment, and demonstrated a higher operational stability,therefore it is a prospect method for treatment of ammonia-containing wastewater.