Sustainable high-flux ceramic membrane operations for seawater pretreatment under diverse algal bloom intensities

Abstract

Achieving reliable high-flux ultrafiltration (UF) for seawater treatment, particularly during harmful algal blooms (HABs), remains a significant challenge. This study revisited the conventional in-line coagulation followed by ultrafiltration (IC-UF) process and evaluated its performance under high-flux ceramic membrane operation (net flux: 200 L/m²/h) across varying HAB intensities. In addition, we re-evaluated a pretreatment approach combining in-line coagulation and flocculation prior to ultrafiltration (ICF-UF), a well-established yet rarely implemented strategy in seawater desalination, and critically compared its performance with IC-UF. A stepwise strategy was employed to develop a treatment-performance matrix incorporating parameters such as algal organic matter (AOM), turbidity, chlorophyll-a, and modified fouling index (MFI). This matrix, established through jar tests and batch filtration experiments, aimed to guide seawater pretreatment by assessing IC-UF and ICF-UF performance under varying iron doses, flocculation durations, and HAB intensities (10⁴-10⁶ cells/mL). During the transition from batch to continuous UF, AOM emerged as a key predictor of fouling in continuous high-flux UF, as it contributed primarily to physical irreversible fouling. Conversely, elevated MFI values observed in batch UF, mainly driven by microparticles, did not reliably indicate membrane fouling under continuous operation, due to effective routine physical backwashing. Importantly, AOM form stable complexes with Fe, generating synergetic irreversible foulants that substantially reduced the efficacy of maintenance cleaning (MC) during continuous high-flux UF operation. To mitigate this, MC strategies were optimized through sequential chemical cleaning, wherein citric acid preconditioning facilitated subsequent NaOCl cleaning by improving removal of AOM-Fe complexes. In continuous multi-week high-flux seawater pretreatment, IC-UF effectively controlled membrane fouling at lower algal densities (e.g., 10⁴ cells/mL) by promoting microparticles formation and AOM removal. However, at higher algal densities, IC-UF resulted in severe physical irreversible cake layer formation and pore blockage. In contrast, ICF-UF enhanced coagulation kinetics and microparticle aggregation through active iron sites (e.g., η-H2O and η-OH), leading to greater removal of microparticles (2-6 μm), biopolymers, humic substances, and low molecular weight (LMW) compounds. Consequently, ICF-UF demonstrated superior fouling mitigation under severe HAB condition (>3 × 10⁵ cells/mL) by reducing both irreversible cake layer formation and pore blockage (characterized as aromatic-proteinic AOMs), compared to IC-UF. Collectively, this study demonstrates the potential of conventional IC-UF and the newly applied ICF-UF to enhance ceramic membrane sustainability in seawater pretreatment across a range of HAB intensities, achieving substantially higher fluxes than those typically reported in full-scale desalination plants, without requiring dissolved air flotation (DAF).

Keywords: Chemical enhanced backwashing; Coagulation flocculation process; Desalination; Modified fouling index (MFI); Seawater reverse osmosis (SWRO); Ultrafiltration.