Spacer-induced forward osmosis membrane integrity loss during gypsum scaling

• FO membrane integrity was compromised during gypsum scaling with spacer.
• Gypsum preferentially accumulated adjacent to the spacer.
• Measuring membrane transport parameters cannot detect FO membrane integrity loss.
• Fluorescent Rhodamine challenge test revealed decrease of log removal value to 3.5.
• Latex nanoparticle challenge test show wider and larger particle size in the draw.

We demonstrated forward osmosis (FO) membrane integrity loss during gypsum scaling with the presence of membrane spacer. The gypsum scalant had preferential accumulation adjacent to membrane spacer where the needle-shape gypsum potentially compromised polyamide thin-film composite FO membrane integrity. However, the loss of FO membrane integrity cannot be sensitively detected by in situ measurements of membrane water and salt (NaCl) permeability coefficients. We, for the first time, employed membrane integrity challenge tests to reveal the impaired FO membrane integrity by fluorescent Rhodamine WT tracer and amine-modified latex nanoparticles, respectively. Challenge tests using Rhodamine WT tracer showed that membrane log removal value decreased to 3.5 after three scaling–cleaning cycles, which corresponded to a pinhole size of 0.06 μm2 on the FO membrane. This result was further corroborated by challenge test using latex nanoparticle where the particle size distribution in the permeate became wider and the average particle size increased over the three scaling–cleaning cycles. Both challenge tests were sensitive enough to identify impaired FO membrane integrity. Results reported here have significant implications for achieving better membrane spacer and module design, as well as demanding periodical monitoring of FO membrane integrity in water reuse.