Rejection of small solutes by reverse osmosis membranes for water reuse applications: A pilot-scale study

• Small solute (N-nitrosamine & boron) rejections were evaluated by a pilot RO plant.
• Results validate findings from lab-scale experiments wrt small solute rejection.
• Permeate flux and temperature could considerably influence small solute rejection.
• Boron & NDMA rejections responded to changes in operating condition in a similar way.
• A strong correlation (R2 = 0.86) between boron and NDMA rejections was obtained.

N-nitrosamines and boron are small solutes of particular concern during water recycling applications. Here, we evaluated the rejection of seven N-nitrosamines and boron under a range of operating conditions and feed solution characteristics. The evaluation was conducted using a pilot-scale reverse osmosis (RO) system to appropriately simulate hydrodynamic conditions of full-scale RO installations. The rejection of seven N-nitrosamines by the pilot RO system varied significantly in the range from 31 to 94%, and rejection increased in the increasing order of their molecular weight. Rejection values obtained from this pilot-scale study were lower than those previously reported in laboratory-scale studies. These discrepancies were attributed to a difference in RO system operating condition (i.e. recovery) between the pilot-scale study (25%) and laboratory-scale study (< 0.1%). Nevertheless, rejection data reported here validate the recent findings from laboratory-scale studies with respect to the impact of permeate flux, feed temperature and feed pH on separation efficiencies of N-nitrosamines. Data obtained from this pilot-scale study also validate the strong correlation between boron and NDMA rejection at or below pH 8 regardless of operating conditions and feed solution characteristics. The results suggest that boron rejection can be used as a surrogate for NDMA rejection in full-scale RO installations.