Study On Ammonia Removal By Gas-Membrane Absorption And On Salt Effect On Ammonia Mass Transfer

Ammonia is a frequent contaminant in wastewater streams. The accumulation of ammonia in water body can result in serious environmental and health problems. Some traditional physical or chemical processes for ammonia removal are still in vogue but do suffer with known limitations, such as secondary pollution, high energy consumption, large occupancy area et al. Gas Membrane-Based Separation Process, a hybrid process combined with physical and chemical processes, has been widely explored since 1980s because of their attractive advantages compared to the traditional methods.However, the membrane absorption process is still in the research stage because of the imperfect of transfer theory, the dispute of operating conditions and the high cost and short life time of the membrane material. In this paper, we have a systematic study on the impact of operating conditions and salt effects on the ammonia removing process using two membrane contactors made by different methods. With resolving the transfer equation, it can explain the mechanisms of the impact of operating conditions and salt effects on each sub-transfer coefficient.The experimental results demonstrate that: the temperature has a huge impact on the ammonia mass transfer. When the system temperature is up to 45℃from room temperature,the overall transfer coefficients of two modules have increased to 4 times and 2 times than before respectively. While the flow rate and initial concentration do not have a significant impact on this process, all the influence of coefficients values are less than 3%. In the salt effect experiments we found that: sodium chloride, sodium nitrate, sodium sulfate and ammonium sulfate have a salting-out effect on ammonia. Sodium sulfate has the most obvious effect that the membrane transfer coefficients of two modules are 1.4 and 2.6 times in the 120g / L concentration than that in pure water solution. Ammonium chloride and ammonium nitrate have a slight salting-in effect on ammonia. Ammonium chloride of 160g / L leads to a reduction of 14% and 21% of two modules'membrane transfer coefficients, while the same concentration of ammonium nitrate makes a reduction of 28% and 13% respectively. Calcium chloride shows a significant effect of salting-in. When the concentration was up to 160g/L, the mass transfer rate of module 1 has decreased 30%, and the module 2's decreases to 1/6 of none-salt aqueous solution. The temperature has a significant effect on salt effects. The salting-out effect of ammonium sulfate will be covered by the increasing viscosity at a high temperature. In 50℃,the overall transfer coefficient will begin to decrease after the concentration of 80g / L. And with increasing temperature, the salting-in effect of calcium chloride will be weakened. For example, In 50℃, the membrane transfer coefficient in concentration of 160g / L is 30% higher than that in 120g / L.The research method and experimental results will provide a guide and theoretical foundation for the theory study and industrialization of gas membrane-based process for removal of ammonia, especially for the selection of operating conditions dealing with ammonia wastewater containing inorganic salts.