Evaluating the potential of nanofiltration membranes for removing ammonium, nitrate, and nitrite in drinking water sources

Advanced drinking water treatment process using nanofiltration (NF) membranes has gained attention recently because it removes many challenging constituents in contaminated surface waters, such as dissolved organics and heavy metals. However, much literature has reported high variations and uncertainties of NF membranes for removing nitrogen compounds in the contaminated water—ammonium (NH4+), nitrates (NO3−), and nitrites (NO2−). This study aimed to identify the ability of commercial NF membranes to remove NH4+, NO2−, and NO3− and clarify the mechanisms underlying their transport through NF membranes. This was examined by evaluating their rejection by three commercial NF membranes using artificial and actual river waters under various conditions (variable permeate flux, temperature, pH, and ionic strength). Ammonium commonly showed the highest removal among the three nitrogen compounds, followed by nitrites and nitrates. Interestingly, ammonium removal varied considerably from 6% to 86%, depending on the membrane type and operating conditions. The results indicated that the selected nitrogen compounds (NH4+, NO2−, and NO3−) could be highly rejected depending on the clearance between their hydrated radius and the membrane's pore walls. Further, the rejection of the lowest molecular-weight nitrogen compound (NH4+) could be higher than NO2− and NO3− due to its highest energy barrier and larger hydrated radius. This study suggests that compliance with the drinking water regulations of NH4+, NO2−, and NO3− can be reliably achieved by selecting appropriate membrane types and predicting the range of their removal under various feed water quality and operating conditions.