Shear induced detachment of laboratory biofilms and full-scale RO fouling layers before and after clean-in-place (CIP) in spacer-filled channels

In this study, we benchmarked the response of laboratory-grown biofilms against fouled membranes from a full-scale desalination reverse osmosis (RO) module under spacer-filled channel conditions. Using membrane fouling simulators, we compared (i) enhanced hydraulic shear stress by stepwise increased crossflow velocity (CFV) (up to 1.5 m/s), (ii) conventional alkaline/acid clean-in-place (CIP) at baseline CFV, and (iii) CIP followed by enhanced hydraulic shear to evaluate whether chemical exposure altered the extent of removal of residual foulants. Cleaning effectiveness was assessed by hydraulic performance response, fouling layer volume fraction using optical coherence tomography (OCT), total organic carbon (TOC), and metabolic activity (ATP). Increased hydraulic shear removed laboratory biofilms substantially, achieving similar to 85% reduction in biofilm volume fraction with strong recovery of pressure drop and permeability. CIP strongly inactivated laboratory biofilms (similar to 99% ATP reduction) and removed most biomass (similar to 77% TOC reduction), yet spacer-proximal residues persisted and limited performance restoration. In contrast, the investigated full-scale fouling layer showed limited structural response under the applied conditions. Elevated shear produced only partial ATP reduction (similar to 68%) with modest TOC removal (similar to 19%), while CIP caused near-complete inactivation (>99%) with comparable TOC reduction and minimal OCT-detectable volume change and partial permeability recovery (similar to 36%). Post-CIP shear improved extent of removal for laboratory biofilms but not for full-scale fouling layers, indicating that chemical inactivation did not translate into enhanced removal under the tested conditions. These differences highlight that cleaning responses depend on fouling layer characteristics and history and suggest that observations from laboratory biofilms may not fully represent the behavior of operationally aged fouling layers under the investigated conditions.