Membrane Distillation Process For Treating High Concentrated Ammonia Wastewater

The treatment of ammonia bearing wastewater is a hot topic at present. Because of the biological toxicity of high concentrated ammonia nitrogen, physicochemical method is usually used for the treatment of this type of wastewater, including air stripping, adsorption, chemical precipitation, and reverse osmosis. Membrane filtration is an important area in water treatment field and membrane distillation（MD） process has advantages in separation of volatile substances from water. In this study, MD process is used for high concentrated ammonia wastewater treatment. The research includes three parts: determination and optimization of configurations and parameters; influences of different components in feed and permeate side; influences of structure parameters controlled by membrane preparation conditions. This study aims to provide reference and guidance for industrial application on high concentrated ammonia wastewater.Different from the traditional membrane separation processes, target compounds transfers through porous hydrophobic membranes only in vapor phase. One of the most important steps is adjusting p H of feed water to convert ammonia exist into the volatile molecule state. Since water can also pass through the membrane pore in volatile forms. Consideration of selectivity becomes an issue as important as the mass transfer of ammonia.The experiment adopts ammonium chloride solution to simulate high concentrated ammonia wastewater and uses hollow fiber membrane and flat sheet membrane modules. The membrane materials were polypropylene（PP）, polytetrafluoroethylene（PTFE）, and polyvinylidene fluoride（PVDF）. The results show that MD process has a good effect on treatment of simulated ammonia bearing wastewater. When PP hollow fiber membrane modules was used, with the membrane area being 9m2, feed volume being 20 L and ammonia nitrogen concentration being 600 mg/L, temperature being 25℃,ammonia nitrogen removal rate reached above 98% within 20 min, which has a good prospect for future application.To study the influencing factors on ammonia wastewater treatment, three kinds of MD configurations were considered: direct contact membrane distillation（DCMD）, sweep gas membrane distillation（SGMD） and vacuum membrane distillation（VMD）. The factors include feed temperature, permeate temperature, feed initial p H, feed velocity and permeate velocity. Results indicated that temperature has a big impact on ammonia mass transfer in three MD configurations. Meanwhile, water flux was also significantly improved, so the separation efficiency of ammonia and water flux decreased sharply. For DCMD, water flux is related to not only feed water temperature but also the temperature difference between feed and permeate side. Feed p H value has a significant effect on ammonia treatment efficiency, too. When the feed p H reached 12, the removal rate increased obviously, meanwhile, water flux was inhibited to cetain extent. So it is an important control measures. Feed and permeate velocity influence differently on different MD processes. For DCMD and VMD, rising feed flow is effective in promoting ammonia transfer, but it was not very abvious in the case of SGMD. On the contrary, increasing permeate velocity has a critical impact on ammonia transfer in case of SGMD, but there is little impact in the case of DCMD. In addition, a comparative study was carried on three types of MD processes. In the same experimental conditions, DCMD has the best effect with the ammonia nitrogen removal rate of 84%, using PTFE flat sheet membrane.Different components in feed solution were also been investigated on ammonia treatment efficiency. It shows that increasing ammonia concentration in feed solution did not affect the treatment efficiency, but can improve the mass transfer and separation efficiency. When feed concentration increased to 1800 mg/L, ammonia removal rate reached more than 87% within 20 min. Na+ and Ca²⁺ in feed solution can inhibit water flux. When 50 g Na Cl was added to 1L feed, the water flux decreased 50% and ammonia removal rate increased. However, excessive salt concentration aggravated concentration polarization on the membrane surface, leaded to salt precipitation. Excessive Ca²⁺ concentration can also cause membrane fouling and increased membrane resistance. Small molecular polyol alcohol in feed water has inhibition effect on the water flux, but there is little impact on ammonia removal efficiency. Excessive concentration can also cause polarization effect. Solution viscosity increased in the boundary layer, and hindered ammonia mass transfer. In this study, it is observed that ethylene glycol or glycerol content in feed solution should not be more than 1%.Different types of absorbing solution were investigated in this study. The increase of sulfuric acid concentration had little contribution to ammonia removal rate, but promoted water flux more than 1.6 times when sulfuric acid concentration increases from 0.5% to 10%, and greatly reduced the separation efficiency. As the absorbing solutions do not require high concentration, acetic acid and carbonic acid were considered as permeate phase. Results showed that, acetic acid and carbonic acid were not suitable to be used for membrane distillation. The concentration should not exceed 2% when using acetic acid as asbsorbing solution. Increasing Na+ concentration can promote membrane water flux, but have no impact on ammonia transfer process. The present of Mg²⁺ in absorbing solution can also improve membrane water flux but meanwhile improved the mass transfer coefficient of ammonia due to the reaction between Mg²⁺ and ammonia. But high Mg²⁺ concentration will produce Mg（OH）2 attached to the membrane surface, and decrease the mass transfer coefficient.In this study we also prepared PVDF flat membranes using phase inversion method. Different polymer concentrations, different Li Cl contents and different membrane thickness were evaluated in studying MD process for ammonia wastewater treatment. The results showed that, increased polymer concentration inhibited the development of large pores and reduced porosity, while low concentration of polymer lead to low surface hydrophobicity of membrane. From the experimental results, optimal concentration of polymer is 16%. Li Cl addition to casting solution improved the porosity of membrane. But excessive Li Cl brings granular nodes to the membrane structure, which is not beneficial to mass transfer of ammonia. The results showed that the optimum dosage of Li Cl is about 4%. Increasing membrane thickness significantly reduced membrane permeate flux and separation performance and the sponge structure of membrane layers is a serious obstacle to mass transfer and the separation of ammonia. With regards to this aspect, the membrane should be as thin as possible.