Spatial analysis of compositional and mechanical heterogeneity in fouled membrane elements from a full-scale SWRO plant: Implications for clean-in-place (CIP) performance

Fouling in full-scale seawater reverse osmosis (SWRO) systems develops under longitudinal hydraulic and chemical gradients, yet autopsy studies rarely resolve spatial differences between elements and within elements. This study evaluates how heterogeneity between upstream and downstream positions and along the lead-mid-tail direction governs clean-in-place (CIP) response and hydraulic recovery. Two full-scale SWRO elements (1st and 5th positions) were dissected; three sheets per element were cut into nine predefined pieces for spatially resolved fouling analysis using chemical, biological, structural, and rheological diagnostics, combined with permeate flux recovery measurements. Total organic carbon (TOC) was higher in the upstream element (0.45-0.50 g/m & sup2;) than downstream (0.25-0.30 g/m & sup2;) and declined from lead to tail within elements. In contrast, mechanical strength increased downstream, with yield point and elevated storage modulus rising from 6 to 9 kPa in the 1st element to 19-41 kPa in the 5th element, indicating consolidation despite lower TOC. Within the downstream element, the yield point increased toward the tail even as the organic load decreased, demonstrating that mass accumulation and mechanical resistance do not scale proportionally. CIP reduced biological activity by >90 % but restored <20 % the permeate flux. Residual organic-inorganic matrices and elevated yield point persisted after cleaning, confirming that hydraulically dominant fractions were mechanically stabilized and spatially structured. These findings demonstrate that fouling behavior and cleaning response in full-scale SWRO elements are governed by spatially distributed mechanical reinforcement rather than average foulant load. Spatially resolved diagnostics are therefore essential for interpreting autopsy results and evaluating cleaning effectiveness beyond bulk compositional indicators.