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

Abstract

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 pH8 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.