Impact of heat modification conditions on the removal of N-nitrosodimethylamine by polyamide reverse osmosis membranes

Abstract

Advanced wastewater treatment using a reverse osmosis (RO) membrane is a key separation process for ensuring the removal of chemical hazards so that treated wastewater can be used for potable purposes. This study systematically assesses the effects of heat modification conditions of three commercial RO membranes on the removal of a challenging chemical of emerging concern–N-nitrosodimethylamine (NDMA). The RO membranes were modified in pure water at the heat treatment temperature between 70 and 100 °C; 90–100 °C achieved the highest NDMA rejection during separation tests. A lower pH during heat treatment generally resulted in higher NDMA rejection, however a solution pH lower than four can reduce NDMA rejection depending on the presence of a protective layer on the RO membrane surface. Overall, a linear tradeoff between NDMA rejection and water permeance was commonly observed among the heat-treated RO membranes. The stability of a heat-modified membrane in water permeance and NDMA rejection was demonstrated over a week-long test using treated wastewater. The enhanced removal of NDMA by heat-treated RO membranes was speculated to occur due to the shrinking of the passage of solutes–free-volume holes. This study demonstrates the importance of heat modification conditions on RO membranes to achieve a high NDMA rejection.