Evaluation of multimedia filtration as reverse osmosis pretreatment: Removal of particles, bacteria and organics

Reverse osmosis (RO) desalination mitigates water scarcity, but remains vulnerable to membrane (bio)fouling, demanding effective pretreatment. While conventional mono- or dual-media filtration (DMF) is prevalent, it exhibits limited efficacy against submicron and microbial particles, presenting (bio)fouling risks poorly captured by turbidity and silt density index (SDI), two standard RO feedwater quality metrics. Here, we present a long-term, pilot-scale evaluation of a four-layer multimedia filter (MMF; anthracite, silica, alumina, magnetite) for seawater RO pretreatment under practice-relevant conditions. Red Sea water was treated at hydraulic loading up to 40 m/h and evaluated using conventional metrics, particle counting, flow cytometry, and dissolved organic carbon (DOC) analysis. The MMF achieved 94-99% particle removal, sustaining >90% removal down to 0.5 µm at twice typical filtration velocities. Microbial removal efficiency ranged from 20 to 50%, while DOC removal was minimal (1-7%), indicating predominantly particulate control. SDI, consistently ≤3, showed weak correlations with direct particle and microbial counts, underscoring limited diagnostic sensitivity. After backwash, particle removal recovered within 15 min, whereas microbial retention required ∼48 h, peaking at 8-11 days. Lifecycle analysis projected a ∼30% cost reduction compared to DMF due to reduced capital and chemical costs. Although specific to Red Sea seawater, these results indicate that MMF can operate as a high-rate pretreatment strategy and highlight the value of integrating high-resolution monitoring tools to assess pretreatment performance and foulant loading relevant to RO.