Why is reverse osmosis (RO) ineffective at removing ammonia nitrogen from water?

Reverse Osmosis (Ro) Technology has a high removal rate for ionic pollutants in water, but its removal effect on ammonia nitrogen is relatively limited. This phenomenon is mainly determined by the following factors:

1. The special existence form of ammonia nitrogen in water
Ammonia nitrogen does not exist in a single form, but rather in a dynamic balance between ammonium ions (NH₄⁺) and free ammonia (NH₃) depending on the pH value.

In typical neutral or slightly acidic water bodies, ammonia nitrogen mainly exists as positively charged ammonium ions; when the pH increases (especially >8), the proportion of free ammonia increases significantly. The difference in the physicochemical properties of these two forms directly affects the retention efficiency of the Ro Membrane.

2. Limitations of the retention mechanism of reverse osmosis membranes
RO membranes mainly separate substances through pore size sieving and dissolution-diffusion mechanisms. In actual operation, the surface charge of the membrane may weaken due to fouling or pH changes, and the hydration radius of ammonium ions is small (approximately 0.36 nm), close to the membrane pore size threshold, making leakage likely.

Free ammonia is electrically neutral, with an extremely small molecular size (approximately 0.26 nm), and exhibits some lipid solubility, readily dissolving and diffusing through membrane materials, leading to a significant decrease in its removal rate. Experiments show that when pH > 9, the overall ammonia nitrogen removal rate may drop below 50%.

3. Key Influences of Operating Conditions
Influent pH directly determines the distribution of ammonia nitrogen speciation. For example, at pH ≈ 6, the ammonia nitrogen removal rate can reach 80-90%, while it may drop sharply in alkaline industrial wastewater.

Organic fouling or scaling alters the membrane surface charge and hydrodynamic environment, potentially creating preferential channels for ammonia nitrogen permeation.

The diffusion and mass transfer of free ammonia is significantly affected by temperature; increasing temperature exacerbates permeation; while changes in operating pressure have a limited impact on the diffusion mechanism.

4. Competition with Other Pollutants
High concentrations of sodium, calcium, and other cations in the water compete with ammonium ions for electrostatic repulsion sites on the membrane surface, indirectly reducing the ammonium ion rejection rate. Simultaneously, ammonia nitrogen may form neutral complexes with certain organic substances, further increasing the risk of permeation.

In practice, water treatment projects often use combined processes to compensate for the shortcomings of RO, such as: pre-treatment pH adjustment (acidification to convert ammonia nitrogen into ammonium ions); adding biological nitrification or stripping processes to the pretreatment; and then following up with ion exchange or breakpoint chlorination.