Study Of Membrane Absorption Process On Separating Ammonia From Wastewater

Ammonia, presenting in many industrial wastewaters, is a common pollutant to induce eutrophication of the receiving water body and to destroy water environment. Currently, the most widely used method for removing ammonia from industrial wastewater is air stripping which has several drawbacks such as high energy consumption and difficult in control, etc.Membrane absorption has a lot of advantages over traditional separation processes, such as high efficiency, low investment and energy consumption, convenient application and simple operation. In addition, membrane absorption also has some merits including surprisingly high interfacial area, absence of emulsions, no unloading at low flow rates and no flooding at high flow rates. Therefore, membrane absorption process was used for removal of ammonia from wastewater in this thesis.The laboratory-made hydrophobic microporous poly-vinylidene fluoride (PVDF) hollow fiber membrane was applied and sulfuric acid was used as the absorbent in the experiments. The influence of operation parameters in the process of ammonia/water separation using membrane adsorption was investigated experimentaly. The results indicated that increasing the circulation velocity, pH and temperature of the feed facilitated the mass transfer evidently and above 90% removal of ammonia from the wastewater was obtained. Whereas, the variation in operating factors of the absorbent had negligible effect on the efficiency of the membrane absorption process, except that the pH should be maintained below 4 for an effective ammonia removal. In addition, higher ammonia removal was obtained by applying outside-in module instead of inside-out module by utilizing the larger surface area of the former. Further, experimental results showed no obvious effect of the module orientation on the mass transfer.The effect of membrane and module properties were also discussed in this thesis based on the experimentalal data. The results indicated that the mass-transfer coefficient increased with the module packing density but followed by a decreasing trend. Longer modules favored the removal of ammonia but the mass transfer coefficient decreased due to the decreased mass transfer driving force along the longer modules. Moreover, mass-transfer coefficient was found to be linearly related to fiber thickness. As the fiber thickness increased from 0.12mm to 0.4mm, the mass-transfer coefficient decreased 69%. In addition, increasing the fiber inner diameter from 0.6mm to 1.2mm resulted in 62.5% enhancement in the mass-transfer coefficient.Finally, this paper introduced a field study of ammonia removal from an industrial wastewater using a pilot-scale membrane absorption system. The results in influence of liquid circulation velocities, pH of sulfuric acid, module orientation and different operating modes on ammonia removal and mass transfer confirmed the conclusion obtained from the laboratory study. The ammonia removal rate achieved 95% at the optimal operating condition.