Biofouling development is affected by a variety of factors that change over the length of reverse osmosis (RO) membrane modules in pressure vessels. Spatially resolved biofouling formation was studied under conditions representative to practice using four one-meter Long Channel Membrane Test Cells (LCMTCs) in series, simulating an industrial pressure vessel. Biofouling was induced by dosing an easily assimilable substrate to the feed water. The impact of biofouling on the sequential decline of RO membrane performance indicators (feed channel pressure drop, permeability and salt rejection) was investigated. Also, the temporal organic carbon (DOC) consumption was assessed spatially over the four test cells. Results showed that all membrane performance indicators were impacted by biofouling formation. The feed channel pressure (FCP) drop increase was impacted earliest and strongest followed by permeability and salt rejection decline, underlining that FCP drop is a sensitive and early biofouling monitoring indicator. Spatially resolved biofouling investigations revealed that most biofouling was formed in the lead sections of membrane installation with a decreasing gradient over length, linked to DOC availability in the system. In this study, FCP drop played a crucial role: the FCP drop increase at the lead test cell of the membrane installation caused performance losses for the downstream test cells. Minimizing the effect of biofouling on membrane performance should be pursued by a combination of strategies involving (i) early detection and preventive cleaning, (ii) substrate limitation for delaying biofouling built-up and (iii) optimized (early) cleaning procedures for more effective biofilm removal.

A long-channel membrane test cell (LCMTC) with the same length as full-scale elements was developed to simulate performance and fouling in nanofiltration and reverse osmosis spiral-wound membrane modules (SWMs). The transparent LCMTC enabled simultaneous monitoring of SWM performance indicators: feed channel pressure drop, permeate flux and salt passage. Both permeate flux and salt passage were monitored over five sections of the test cell and were related to the amount and composition of the accumulated foulant in these five sections, illustrating the unique features of the test cell. Validation experiments at various feed pressures showed the same flow profile and the same hydraulic behaviour as SWMs used in practice, confirming the representativeness and suitability of the test cell to study SWM operation and fouling. The importance to apply feed spacers matching the flow channel height in test cell systems was demonstrated. Biofouling studies showed that the dosage of a biodegradable substrate to the feed of the LCMTC accelerated the gradual decrease of membrane performance and the accumulation of biomass on the spacer and membrane sheets. The strongest permeate flux decline and the largest amount of accumulated biomass was found in the first 18 cm of the test cell. The LCMTC showed to be suitable to study the impact of biofilm development and biofouling control strategies under representative conditions for full-scale membrane elements.