Perimeter artefacts, such as test cell induced membrane damage and unrepresentative hydrodynamics, can significantly affect bench scale performance evaluation using membrane coupons. By comparing separately collected center and perimeter permeate, the effect of perimeter artefacts can be identified. The additional salt passage in the perimeter area (i.e., along the edges of the membrane coupon) depends non-linearly on pressure, and is implicitly affected by flux, temperature and salinity (osmotic pressure), leading to a potential increase of the measured salt passage by a factor two. We observed a very slow dynamic response (τ≈12h) to changes in operational conditions, suggesting that in our case the perimeter artefact was diffusive in nature. Due to this slow response, the perimeter salt passage trails previously evaluated operational conditions, possibly resulting in a false trend, hysteresis or high variance, depending on the sequential ordering and duration of test condition intervals. The effect of perimeter artefacts can be largely mitigated by measuring salt rejection from only the central area of the coupon.

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